A Web Compendium by Paul B. Van Slyke, Founder & Director, Black Forest Observatory (BFO)
Black Forest Observatory (pictured above) was Colorado's largest observatory, period! It had been in existence for over 15 years (see User Installations link, The Story of BFO for more info). The telescope pictured above is BFO's computer controlled 30-inch Cassegrain telescope, with 20" Byers drive, set up for astroimaging. The imaging train equipment, shown in the above telescope picture from left to right, is an old SBIG ST-6 CCD camera, a VSE remote controlled 2-inch [dual wheel] filter wheel, an early custom 2.75-inch Flipper (with giant 2.75-inch format eyepiece, shown on right side, and real-time CCD autoguiding camera in the off-axis side port, shown on top of Flipper), and finally a massive 2.75-inch remote controlled rack & pinion focuser. The second picture is BFO's control center (warm room), with three computers networked together; one for telescope positioning, one for image downloading, and the one on the far right is for general reference (included in this database is the Palomar Sky Survey on many CD-ROM's, over 100 professional astronomy books and sky catalogs, including the Hubble Guide Star Catalog). Note BFO's 30" Cassegrain through the warm room's viewing window.
Printing this compendium results in a 13 page document
INTRODUCTION
PROPERLY INSTALLING AN IMAGING TRAIN ON YOUR TELESCOPE
Installing a Secondary Focuser on your Telescope
Installing a Focal Reducer on your Secondary Focuser
Installing your Slider
Installing your Sidewinder
Installing your CCD or 35mm Camera on your Slider
PREPARING YOUR IMAGING TRAIN FOR ASTROIMAGING
Paul's Pictorial Parfocusing Primer
Focal Plane Focusing Devices for your 35mm Camera (under the knife's edge)
FINALLY, TAKING THE FIRST EXPOSURE THROUGH YOUR IMAGING TRAIN
Eyepiece Guiding on a Guide Star
CAMERAS & FILMS FOR ASTROPHOTOGRAPHY
FILTERS FOR 35mm & CCD ASTROIMAGING
A BRIEF HISTORY OF TIME (my time, not Stephen Hawking's)
CONCLUSION
Note: all caps above indicate a main section, cap on first letters [only] indicate a sub-section
For many experienced astroimagers, correctly installing an imaging train on a scope and using the correct procedure to capture high quality images, has become common sense. But we must also consider the novice, who has no experience whatsoever in astroimaging. Therefore, this basic [quick and dirty] introduction to astroimaging is provided for the beginner, who is very interested in becoming an experienced astroimager, but has no idea where to begin to learn the art [and subtleties] of astroimaging, or doesn't have the time [or inclination] to read through all those "hieroglyphic" astrophotography books with all those graphs and heavy math.
There are various pieces of equipment necessary to construct an imaging train and, when properly stacked together (similar to railroad cars), give you the capability to produce a quality astroimage in the least amount of time, and with as little frustration as possible. There are many [adaptive] ways to attach various accessories to different types of telescopes. We are only going to discuss one simple, basic imaging train, because all the different models and imaging accessories operate similarly. Once you've learned the basics, they will apply to all the other imaging systems on the market. Some will have more features, and be easier to operate, and others will be awkward and frustrating but, after reading this compendium, you will be better equipped to discern the "good stuff" from the bad.
One of the most common coupling formats is the 2"-24tpi male threaded found on most SCT visual backs. This format was first introduced by Celestron, and Meade Instruments soon followed with the same fitting (almost), which has now become an industry standard in Schmidt-Cassegrain telescopes. Another very common telescope coupling is the 2" female slide-fit [usually a focuser input] that accepts 2" barrel-nose eyepieces and other accessories, and is mounted on most Newtonian, refractor, and some types of Cassegrain telescopes with their myriad of common optical configurations (i.e. Schmidt, classical, Ritchey-Chretian, Maksutov, Dall-Kirkham, etc.). The equipment's female threaded slip-ring attaches to its 2"-24tpi male counterpart on the telescope's visual back, and offers a very short profile and excellent security. It was primarily designed to support extended astroimaging trains in the days of 35mm astrophotography, but has been easily adapted to CCD astroimaging. This format does not easily allow for field rotation. It was primarily designed for visual eyepiece observing, but has been extensively utilized to secure imaging trains because it was simply there at the time. In hindsight, no barrel-nose format should have ever been used for imaging train applications because it wastes profile (except when inserted in a focuser) and can't be properly secured with one or even two thumb screws, thus the necessity to develop VSE's new Quad-Lock system which allows the barrel-nose format to be as rigid and secure as any threaded format. However, this barrel nose format does allow for easy field rotation (addressed later). The 2" slip-fit barrel-nose format can be made more secure (but not more rigid) by providing a 1/2" long [or so] machined security indent in the male barrel tube. This provides a recessed section in the barrel in which the thumb screw(s) can reside. Even if the thumb screw(s) come loose, the equipment cannot detach itself from the telescope, and cause a catastrophy.
The novice will think that a secondary focuser is an option he really can do without. He may be right, but not usually. With rare exceptions, most commercial Schmidt-Cassegrain telescopes (SCTs) require a secondary focuser. Secondary focusers are designed for the current generation of Meade/Celestron Schmidt-Cassegrain telescopes that focus by moving their primary mirrors and accept accessories via their standardized threaded visual backs. The four SCT standards are 2"-24tpi for both Meade and Celestron (using a reducer for the 10" and larger models), 3.25"-16tpi for Meade (10" and 12" models), 3.29"-16tpi for Celestron (11" and 14" models), and 4"-16tpi for the Meade 16" SCT.
Without a secondary focuser, a serious problem arises when you try to focus using the focusing knob protruding from the SCT's visual back. Your image will begin to shift laterally around the field of view - very frustrating indeed. Small amounts of image shift are acceptable when casually viewing the celestial sky through even a moderately high-power eyepiece, but when you attach an imaging train to your SCT and try to center a guide star in your eyepiece's illuminated reticle crosshairs, etc., you find that this mild irritation has become intolerable. The image jumps all over your image plane and field of view. You have just encountered unacceptable lateral image shift. Until Meade and Celestron start building a better SCT (yes, you can eliminate the lateral image shift by simply designing and implementing a better fork-type primary focusing arm), the astroimager must use a secondary focuser docked to the SCT's visual back to eliminate the focusing wiggles. Even if there was no lateral image shift, SCT's move their primary mirrors, which provides a focusing ratio that is approximately 6 times as coarse as other types of common secondary docking focusers. Therefore, rock-solid micro-focusing becomes a very useful upgrade even without lateral image shift.
Since your SCT's primary focusing system manages all your large focusing position changes, your secondary focuser need not have more than about 0.1" of actual overall travel. In reality, most secondary focusers have much more focus range (all VSE MasterGlide focusers provide a full inch of redundant travel and our MicroGlide focusers provide 1/2-inch). You attach a secondary focuser to your SCT by simply screwing it onto your visual back (Note that MasterGlide focusers utilize the larger, more rigid 3.25"-16tpi large format threaded visual back for Meade 10" & 12" SCTs with a 4" adapter available for Meade 16" SCTs, and a 3.29"-16tpi adapter for Celestron C11, C14). You then insert an eyepiece or diagonal, etc. into the focuser's input as you would with any other standard focuser. VSE's 2" Threaded Barrel Adapter (item #A2LT) can easily be inserted in a MasterGlide or MicroGlide focuser's moving tube and rigidly affixed using Quad-Lock. This converts the output to a rock-solid 2"-24tpi male format so you can utilize all the standard, commercially available accessories and extended imaging trains.
The upper right picture illustrates an imaging train composed of VSE components. The straight-through imaging train consists of a [now discontinued] VSE Filter Wheel (FW4) + Super Power Focuser [also discontinued, but replaced by the MasterGlide M5] + Slider 2 + 35mm SLR camera (which could easily be substituted for a CCD camera in a matter of seconds since both camera formats now use the same T-thread fittings). A 40mm Televue wide-field eyepiece has been inserted in the Slider's top slide-mirror port, and a VSE 25mm illuminated reticle guiding eyepiece (also discontinued because we finally ran out of military-surplus glass reticles) has been inserted in the side pick-off port. It doesn't get much better, or easier, than this! And you thought I forgot expensive! Nope. You could cut the expense to a small fraction of the above imaging train by utilizing one of VSE's less expensive focusers (i.e. M1, M2 or MGF, etc.) and a Slider using it's [FREE] built-in 2" format filter slot and accomplish the same end imaging result without all those high-dollar "bells & whistles" which are typically used for total remote operation from observatory warm rooms, etc. VSE also offers in-line Filter Slot Adapters, or our Filter Slide System (see Filter Adapters link).
The second imaging train accessory we're going to discuss is the focal reducer (pictured at right to scale, full size, depending on your monitor's resolution of course). The focal reducer used to be a necessity when imaging with CCD cameras with relatively small imaging areas, compared to 35mm formats. A focal reducer and CCD camera combination can approach the larger field offered by 35mm cameras without a focal reducer. When the CCD imaging industry first came on board, the chip size wasn't much larger than a gnat's butt, and it was almost impossible to acquire a bright star on the [microscopic] chip area, let alone a faint deep-sky object. Today, CCD chips have dropped dramatically in price, and increased [literally exponentially] in area. These new larger chips are now approaching the size of 35mm format, negating the need for a focal reducer altogether, and the optical distortions caused by shortening your light cone and passing it through all that glass. Simple prime focus is always the best way to acheive the best images, but focal reducers can still be used effectively for wider-field imaging.
If you have a Meade/Celestron Schmidt-Cassegrain telescope, you just screw their respective f/6.3 focal reducer onto the 2" threaded male visual back of your SCT. If you have a telescope with a standard 2" slide-fit focuser that accepts 2" barrel-nose stuff, you need to purchase one of VSE's 2" Threaded Barrel Adapters (see ADAPTERS link). You then screw the 2" Threaded Barrel Adapter (Item #A2LT) into the female end of your focal reducer, and insert that combination into your 2" slide-fit focuser and lock down the thumb screw(s) into the recessed indent in the male tube. Now you have a 2" male threaded fitting available on which to screw a Slider (with the 2" threaded female slip-ring input). Of note, using a focal reducer should be avoided if at all possible, unless you like to immerse yourself in a "bag of worms."
Consider VSE's exclusive focal reducer "hidden cavity" super feature located inside the moving tube of all MasterGlide focusers. Instead of installing a focal reducer (FR) conventionally, and consuming all that valuable profile, you install it inside the focuser's moving tube and save your profile for other imaging train accessories. It also moves the FR closer to the SCT's visual back where it was originally intended to be installed. All Meade/Celestron f/6.3 focal reducers fit inside the MasterGlide focuser's moving tube. The f/3.3 focal reducers, utilized in the MasterGlide's "hidden cavity," are too fast to be practical in most extended imaging train applications, unless you are simply inserting an eyepiece or camera directly into the focuser itself.
Next, screw your focal reducer onto the Slider's 2" female slip-ring input or, if you decided to install your FR inside the MasterGlide focuser's "hidden cavity," just screw a VSE 2" Threaded Adapter (Item #A2LT) directly onto the end of your Slider. Then insert the Slider's [now] 2" barrel nose into your MasterGlide focuser's 2" format moving tube and apply VSE's Quad-Lock system to the barrel-nose. With your Slider rigidly affixed to your imaging train, you will [next] need to install your 35mm or CCD imaging camera, to the rear output port of the Slider (discussed below). For additional info, also see VSE's Imaging Train Configuration link.
You have, or are considering the purchase of, a CCD camera with a built-in guiding chip, or a CCD camera that has an imaging chip with an area that can be dedicated to guiding, so you don't need a Slider. Think again! You think it's easy to find a suitable guide star with a fixed separate guide chip, or a fixed chip area dedicated to guiding? Are you, or would you like to be, a masochist that likes guide star frustration? Since your guide chip [area] is integral and fixed, your ability to scan for an adequate guide star is diminished by a factor of ten, compared to the Slider's easily adjustable independent pick-off mirror. Even with your fancy dual [fixed] chip guiding/imaging CCD camera, guide star acquisition can still be a literal nightmare if you don't have a way of positioning a guide star independent of your imaging target. This is the capability a Slider offers. Of course, you need to use a separate guiding CCD camera, or guiding eyepiece, allowing you to simply scan for a guide star and pick & choose any usable star in or around your target object. This is accomplished by simply rotating the Slider itself (Z-axis). Yes, you can also rotate a CCD camera, but you are still limited to a fixed area of rotation without the necessary independent X-Y axis adjustable pick-off mirror controls of the Slider. Once you've acquired a usable guide star, it's quick and easy to center, or place, any star in the Slider's pick-off field in 2 seconds or less. Consider the new Starlight Express SXV-H9C with the separate guiding camera that slips right into the Slider 2's [1.25" format] off-axis guiding port - a marriage literally made for heavenly imaging!
Understand that a standard focal reducer will not install on the Sidewinder's front input port. You need to install it on the rear port just before your camera. If you do this, you will need to extend out all the lateral ports about 6" to achieve parfocus. If you have a few extension drawtubes or diagonals, they will do the job. This means that any focal reducer is not recommended. If you have a chip that is larger than 35mm format, you really don't need a focal reducer that adds glass and other imperfections. Your scope's light cone should remain pure to achieve the best imaging results. This is an opinion that others may disagree with. Also, Sidewinders will not work with any 2" format focuser, including VSE MasterGlide and MicroGlide focusers. However, the Sidewinder will install on the output of our MicroGlide Coupling Focuser (Item #MGFCO), which has a larger input and output thread, with a 2.25" internal clear aperture. Next, screw your Sidewinder (using the proper docking port ring) onto your scope. You can screw the docking port ring onto your scope, then slip the Sidewinder over the ring and tighten the three set screws, if you wish. With your Sidewinder rigidly affixed to your scope, you will [next] need to install your 35mm or CCD imaging camera, to the rear output port of the Sidewinder (discussed below). For additional info, also see VSE's Imaging Train Configuration link.
You have, or are considering the purchase of, a CCD camera with a built-in guiding chip, or a CCD camera that has an imaging chip with an area that can be dedicated to guiding, so you don't need a Slider or Sidewinder. Think again! You think it's easy to find a suitable guide star with a fixed separate guide chip, or a fixed chip area dedicated to guiding? Are you, or would you like to be, a masochist that likes guide star frustration? Since your guide chip [area] is integral and fixed, your ability to scan for an adequate guide star is diminished by a factor of ten, compared to the Slider or Sidewinder's easily adjustable independent pick-off mirror. Even with your fancy dual [fixed] chip guiding/imaging CCD camera, guide star acquisition can still be a literal nightmare if you don't have a way of positioning a guide star independent of your imaging target. This is the capability a Slider or Sidewinder offers. Of course, you need to use a separate guiding CCD camera, or guiding eyepiece, allowing you to simply scan for a guide star and pick & choose any usable star in or around your target object. This is accomplished by simply rotating the Slider or Sidewinder itself (Z-axis). Yes, you can also rotate a CCD camera, but you are still limited to a fixed area of rotation without the necessary independent X-Y axis adjustable pick-off mirror controls of the Slider and Sidewinder. Once you've acquired a usable guide star, it's quick and easy to center, or place, any star in the Slider or Sidewinder's pick-off field in 2 seconds or less. Consider the new SBIG STL-11000, 11 Megapixel CCD camera. In fact, SBIG's STL was the catalyst for VSE's Mega-Port Sidewinder - the time had come! Use the STL and Sidewinder in combination with SBIG's STV and you've assembled a dream machine that will easily create "frustration-free" professional astroimages worthy of the largest observatory on Planet Earth and beyond.
on your Slider
First we will discuss the installation of a CCD camera as your final item in your imaging train. All modern CCD cameras, such as the SBIG, Starlight Express, and Apogee lines of excellent CCD imaging cameras, now have a standard 42mm female T-thread input coupling, not like the old-style 1.25" barrel-nose ST-6 CCD camera pictured at right. Providing your CCD camera's input has female T-adapter threads, you will use VSE's Zero Profile T-Adapter (ZPTA, item #AZP2T) between the Slider's rear port and your imaging camera. If you are going to use a 35mm camera for imaging, just screw a T-ring adapter, that is specifically designed for your particular brand of 35mm camera (i.e. Nikon, Minolta, Olympus, Pentax, etc.), onto the Slider's ZPTA in the rear imaging port. If you have a Slider 2 with the side pick-off port, you should position your camera in the vertical position which will put the long [skinny] side of the rectangular film further away from your pick-off mirror and lessen the vignetting on your film caused by the pick-off mirror's protrusion into your telescope's light cone. If you have a rectangular CCD chip, this procedure also applies.
Since your 35mm camera focuses about 1.5" further in than a CCD camera, you'll need an extension drawtube out the top slide-mirror port and the side pick-off mirror port to parfocus your system. You may think that this 35mm camera procedure is awkward, but it's much better than extending/wasting your profile for CCD cameras. To explain, most other flip-mirror devices on the market extend the rear port to parfocus for CCD cameras. The superior Slider method of parfocusing always maintains the shortest straight-through profile whether you're using CCD or 35mm cameras. Just remember, CCD = no extension drawtubes, 35mm = extension drawtubes with Sliders. We'll talk further about parfocusing your Slider between 35mm and CCD in "Paul's Pictorial Parfocusing Primer" below, and the subject is also discussed at the Questions & Answers (FAQ) link on VSE's home page.
Illustrated at right is a partially assembled imaging train (less secondary focuser to simplify), as discussed above, docked to a Meade 8" LX200's visual back, a good astroimaging entry level scope. You will also need to insert a very low power 2" format eyepiece (pictured is a 2" format 40mm eyepiece, see FYI below) in the Slider's top slide mirror port, and a higher power 1.25" format illuminated reticle guiding eyepiece in the side pick-off port. You can use any of the commercially available guiding eyepieces on the market.
FYI, a field lens (sky side lens) that is the full internal diameter of the 1.25" tube is the widest true field achievable in that specific format. Example: A 1.25" 40mm eyepiece doesn't have a wider true field than a 32mm eyepiece. A 1.25" 40mm eyepiece will achieve a lower magnification, but achieve the same true field as the 32mm eyepiece with no increase in true field. This causes the 40mm eyepiece to seem like it is losing apparent field compared to the 32mm eyepiece, creating a "tunnel vision" view through the eyepiece. This field relationship applies to 2" format eyepieces too. The maximum true field tops out at about 55mm in 2" format.
First you need a good brisk, clear night with above average "seeing" conditions. You can test "seeing" by imaging a finite solar system object or a bright star (splitting double stars is a good test too). If the object looks like it is boiling, and/or appears to have a lot of chromatic scintillation (color shifting), I would wait for an atmospherically calmer night, or you can use a bad "seeing" night to practice.
After you've leveled and polar aligned your scope (explained in the manual that came with your scope), you need to decide which object you would like to image first. I would start with relatively bright summer deep sky objects, such as the Ring Nebula (M57) in Lyra, or the globular cluster in Hercules (M13). If it's winter, try imaging the Orion Nebula at the tip of Orion's sword, although it may be a little too large for CCD cameras with smaller chips. You can also use an open cluster, or a brighter star field, to test your imaging system. But before you try to locate an object through your scope, push the Slider's slide mirror knob all the way in. That puts the angled 45 degree slide mirror in the path of your incoming light cone, and sends the cone to the top slide mirror wide-field eyepiece. Do not use high power eyepieces in the Slider's top port when trying to image. That port is specifically designed for lower power field-finding eyepieces only (explained in-depth later). Of course, any power eyepiece will work in the top port. The Slider or Sidewinder make great diagonals for visual observing, when you're not trying to acquire an object to image.
Focus your telescope, by looking through the Slider or Sidewinder's top port LOW power eyepiece, on a low to medium brightness star (10th to 5th magnitude) near the object you are going to image. Do not focus on a star halfway across the sky from where you plan to image. Also, re-focus your imaging camera when moving to another object that is [let's say] more than 20 degrees from where you are currently imaging. You need to do this because all telescopes have mechanical flexure, caused simply by gravity, that can de-focus your telescope and shift your telescope's field center by many arc seconds, to possibly a few arc minutes, as you move your telescope across the sky. Once you have that star in focus, and centered in your eyepiece's field of view, you need to focus your imaging camera. Don't forget to pull the Slider's mirror knob out so your imaging cone is redirected straight-through to your imaging chip. With a CCD camera, and the specific software that came with your camera, you can manually focus using the software's focusing sub-routine, or use your autofocus sub-routine in conjunction with an autofocus equipped focuser. When you have the smallest pixel count possible, you're CCD camera is focused and ready to take a time exposure of your deep-sky object. Never try to focus any imaging camera by looking through an adjacent eyepiece! More on this important focusing subject latter.
Say that three times real quick. Betcha can't without spitting all over yourself? Anyway, I hope this section will provide you with a better understanding of parfocusing any imaging train, not just the Slider used in this compendium. I selected the [long] discontinued Micro-Slider (MS) and Flipper because they are designed to parfocus "in-reverse" of each other, which should provide you with a better concept of parfocusing principles (oops! there's another P-word). The MS parfocuses, from 35mm to CCD camera, by adding length to the straight-through imaging port (noted in red, upper left) which is the most inefficient method of parfocusing (like the Meade flip-mirror devices) because you are increasing your imaging train's profile - that's a NO-NO! The Flipper and Slider parfocus, from CCD to 35mm camera, by adding drawtubes to the top port (noted in red, lower right) and side pick-off ports (not shown), which is the most effective method to parfocus your imaging train. Because you are not adding length to your existing straight-through profile to parfocus your system, it remains the same for either CCD or 35mm cameras. Remember that a shorter imaging train is always preferred because it's simply more solid, eliminating system flexure problems as you move from object to object.
Typically, with a CCD camera, you will not need an extension drawtube using a Slider or Sidewinder (see lower left Flipper), because most CCD cameras are roughly parfocus with most newer standardized eyepieces. However, with a 35mm camera, that focuses about 1.5" further out from a standard CCD camera, you will need an extension drawtube (Item#AD22S or AD21S for Sliders) between the top port and your eyepiece (see lower right Flipper, area in red). Of note, Meade has adopted the inferior reverse design on their flip-mirrors that extends the imaging train profile instead of the top, bottom or side mirror port profile like the Sliders and Sidewinders. To further reiterate, note the two lower Flippers above. The red area noted on the lower right Flipper illustrates an inserted drawtube that is needed to parfocalize your system when using a 35mm camera, and is not needed when imaging with a CCD camera (see lower left Flipper). Conversely, the upper two Micro-Sliders obtain parfocus by extending the CCD camera's straight-through profile (again, a no-no) noted in the red area in the upper left Micro-Slider image.
A 35mm camera is a little more difficult to focus because what you think is in focus, when you look through your SLR camera's viewfinder (onto the ground glass image plane), is usually out-of-focus just enough to give you a soft or slightly fuzzy picture. The best way to make absolutely sure your 35mm camera is in perfect focus is to use an after-market focal plane focusing device (i.e knife-edge, grating, etc.), like the one pictured at left. There have been different types available that incorporate knife edges, split images, double images, Ronchi gratings, etc. Celestron offered, what they called an MFFT-55 (Multi-Functional Focal Tester-55), that would evaluate a telescope's focal plane for focus, collimation, and curvature of field, but have discontinued it long ago. With the decline of 35mm imaging and the increase of CCD imaging, these devices are becoming less and less common. There is still one very high-quality device currently on the market. Stiletto Image Focusers from STI offers many 35mm and CCD focusing devices.
The illustration at right represents
a too far in, too far out, and perfectly focused image as seen through a
knife edge type, focal plane focusing device. Illustration 1 represents
a shallow focused gradient image produced by the knife edge that moves across
the field from left to right. Illustration 2 represents a deep focused
gradient image produced by the knife edge that moves across the field from
right to left. Illustration 3 illustrates a perfectly focused image,
as the field instantaneously goes to dark across the entire field of view.
Don't be fooled by claims that you can obtain quick, accurate, pin-point camera focus from a flip-mirror device using a secondary high powered, helical focusing eyepiece, because you can't. And don't ever try to achieve critical focus through your SLR's viewfinder (it's real hard on your neck, too). There are similar looking devices (like Sliders) out there that indirectly imply you can achieve critical camera focus with a high powered eyepiece in the top flip-mirror port. Not possible! First, you can't achieve critical [sharp] focus using a high powered eyepiece in any device's top flip-mirror port (explained in the next paragraph). Second, how are you going to locate your imaging target through a high powered eyepiece with no field of view? This type of device looks spectacular to the novice, but in reality is quite dysfunctional when trying to acquire your sky object and achieving final critical focus.
You should always use a "non-eyepiece" focusing method to achieve final critical focus, like an after-market focal plane focusing device (such as the ones discussed a couple of paragraphs up), or your 35mm astroimages will typically turn out fuzzy and/or soft. Oh, you might "get lucky" with an image, from time to time, but is it really worth all that effort to get lucky, or would you rather spend a little more time using an aftermarket focusing device, and be guaranteed tack sharp super astroimages every time? To reiterate, helical focusing using multi-imaging portals (MIPs) is completely unnecessary. The standard distance, from the front face of your T-ring to your SLR's film plane, is precisely 55mm. The human eye simply can't perceive and focus on that critical distance by looking at an image directly through the eyepiece. It is impossible, especially when that accuracy must be precisely 55mm, not 55mm plus or minus 1mm (which is 0.04").
Think about this simple lesson in common sense. Your eyes are never perfectly "in-focus." Even if you are diagnosed with 20-20 vision, critical camera focus is not achievable with your eye, because there is no such thing as perfect 20-20 vision. Many people wear glasses to correct focus imperfections, and from year to year an eyeglass wearer needs to get his eyes examined and new glasses with a different Rx, because your eye's focus point on your retina has changed. Your eyes will be changing for the rest of your life. Every eyeglass wearer has tried looking through a telescope with, and without, his glasses. The eyeglass wearer had to drastically change the telescope's focus each time. So how can you possibly try to achieve critical camera focus with your eye - it is literally impossible! Especially with an astigmatism, which afflicts half the people in the world. The above has been confirmed by conversations with Dr. Dale Anderson, MD, PC, who is an ophthalmologist and noted eye surgeon.
There is one particular [current] misleading ad in S&T magazine that says, and I quote, "Focus through your eyepiece." Focus what? Your eyepiece for your eye's sake? Yes, you can do that. Your camera? Definitely not! If this statement is telling us that all telescopes focus through their eyepieces, then I think even the novice has figured out that basic concept long ago. If not, then the true implication can only be interpreted to deceive. This deceptive advertising is deliberately leading the novice [want-to-be] astroimager to believe that he can simply focus his CCD or 35mm camera, using his eye and an eyepiece, at the focus of their flip-mirror device. If you are one of those people who have been lead to think this, then I hope you appreciate this basic reality check. Designing an imaging train around this misconception could have [or may already have] lead you down the wrong path.
If it isn't exact, your images will always turn out to be mediocre, instead of celestial masterpieces worthy of enlargement and framing. You will sacrifice those special moments in time, when you can proudly say to your friends, "I took that picture!" And they'll say, "WOW! That picture looks like it was taken at one of the big research observatories." Sure, it's a little more effort to remove your 35mm camera from the telescope, and screw a focal plane focusing device on to the Slider or Sidewinder's rear T-adapter port. Then, after you have achieved optimum focus, unscrewing the focusing device, and carefully reinstalling your 35mm camera onto your scope. But you don't have to go through that procedure for every image, unless you move your scope to a completely different area of sky. Remember the flexure problem discussed a couple of sections up. And, that extra effort is a lot less time consuming and frustrating than trying to switch between high powered [no-field] and low powered [wide-field] eyepieces to find your target object.
From over a decade of imaging experience at Black Forest Observatory, we've found that using wide-field eyepieces for image locating in your top port, with simple push/pull secondary focusing (on all Sliders and Sidewinders) is much quicker and more efficient than any redundant helical focusing system (all Sliders offer full aperture top port mirrors for those wide-field eyepieces). You're actually wasting that top port, on a function that can't be achieved, when you could be using that top port for other purposes. Your secondary eyepieces (top port wide-field and side port guiding) can simply be "ball park" focused, because your eye doesn't need [and can't see] that critical focusing difference anyway. The only critical focus is at the camera. With these procedures, your camera's image will always be guaranteed tack sharp, and that's the bottom line, period! BTW, VSE does offer a helical focusing drawtube insert (see ADAPTERS link) as a convenience accessory if you want one.
Let us assume that you have already gone through the effort to pin-point focus your CCD or 35mm (using a focal plane focusing device discussed above) camera on a nearby star, and have aimed your telescope at a neighboring deep sky object, after parfocusing your top port's wide-field eyepiece with your imaging camera (Oh, if I forgot to mention this earlier, parfocus simply means that your camera and eyepieces are focused at the same point). Once this object, let's say the Ring Nebula in Lyra (M57), is centered in your parfocused wide-field eyepiece, go to your Slider or Sidewinder's side pick-off port's guiding eyepiece and parfocus that next, on an appropriately bright guide star. If you're lucky, you'll have a guide star in your guide eyepiece's field of view, and you can just center the star using the techniques explained below. If you don't see a suitable guide star in your guide eyepiece, you will need to acquire a guide star out-of-field. You do this by simply turning the Slider's little black knob (loosen the knurled lock knob first), or Push/Pulling the Sidewinder's insert in and out, across your light cone's lateral X-axis. If this doesn't acquire an appropriate guide star, then move the Y-axis lever protruding from the top of the Slider's side port, or rotate the Sidewinder's insert. This action will rotate the pick-off mirror effectively providing a perpendicular axis of adjustment. These two controls provide X/Y-axis, 360 degree micro-positioning of the internal pick-off mirror and, 9 chances out of 10, this simple [exclusive to VSE] procedure will acquire a usable guide star. If not, then you can obtain a third axis adjustment (I call it the Z-axis) by rotating your Slider or Sidewinder in a circular motion, either by loosening the thumb screws in your 2" focuser and rotating your Slider, or rotating the Sidewinder on its threaded input docking port. It is advisable to rotate the Slider or Sidewinder body only a few degrees at a time, since your guiding eyepiece usually has less than a one degree field of view. Then do your X/Y-axis "scan" again. Once you've acquired and centered a suitable guide star in your guiding eyepiece, you're ready to start the exposure (either 35mm or CCD camera). For 35mm camera imaging, you'll need one of those extension shutter release cables that have a built-in lock. For CCD imaging, just go to your computer to start the exposure. Don't forget to pull the main mirror out slowly and carefully, to remove the diagonal mirror that is blocking the light cone's straight-through path to your imaging camera. Hopefully, your image will already be perfectly focused on the film plane in your SLR camera (see above for camera focusing procedure).
We're only going to discuss manual eyepiece guiding, except for the following brief note, because all the information you should need is usually in the CCD guiding camera's instruction manual. When using a CCD guiding camera, the set-up procedure is almost identical except, when you've acquired and centered a guide star in your guiding eyepiece, you carefully remove that eyepiece, and replace it with the CCD guiding camera. Guiding on a boiling stellar dot, in a guide eyepiece, for long periods of time, can get quite physically fatiguing, not to mention eye strain. Bilberry Extract, available at your local health food store, is said to help your night vision, and help relieve eye strain from long guiding sessions. Try to keep your guide star in the corner of two perpendicularly opposed illuminated lines, and use a comfortable chair/stool. Don't guide standing up, because when you lose your equilibrium, and fall over in the dark, you really don't know what your going to fall on in those farmers' fields. It could get really ugly and messy down there. That way you can catch a drift almost instantly, and correct for it with your drive controls, before your imaged stars become "squiggly things." Eventually, you will learn your local sky's exposure duration, before your film/chip begins to fog from background light pollution. You will find that a properly aligned mount will nearly eliminate any drift in declination, so correcting in DEC will not be as frequent as correcting in right ascension. You see, worms (the small diameter helical cut gear, with the drive motor attached to one end of its shaft, that drives your RA shaft's larger diameter gear, at an angle that is perpendicular to itself) can't be manufactured to be perfectly concentric or symmetrical. They always end up with eccentric or asymmetric rotational errors, because even the most accurate lathes or milling machines in the world still can't eliminate the worm's imperfections. Ed Byers made the lowest periodical error RA drive systems available for commercial telescopes, because he had those very expensive, and accurate, "tenth" milling machines ("tenth" is machine shop slang for 1/10,000th of an inch). You will always have to deal with periodic errors induced by the worm, unless you don't drive with a worm.
If you have a PEC (periodic error correction) system on your scope, don't forget to calibrate that system. It can reduce your guiding to a minimum. It's also a great "bandaid" for all those low accuracy, high periodic error worm gears that Celestron/Meade put in their scopes. Some even install spur gears on their RA shafts in conjunction with helical cut worm gears. These "non-meshing" worm/spur gear systems are designed only for casual eyepiece viewing, not astroimaging. Make sure your scope has a true worm gear drive, not a sloppy fitting spur gear system.
If you process your images in a computer, using Adobe Photoshop, etc., you can usually go past your fogging limit without damaging results, because you can process the image faults out of existence. I use Photoshop 7 to create VSE's display ads for Sky & Telescope magazine, and all those gaudy graphics you see at VSE's web pages. The software's capabilities are near unlimited, and the unsharp mask feature is very valuable for sharpening sky images. What you can create in your mind, you can transfer to screen/paper. Your only limitation is Photoshop's steep learning curve. I've been using Photoshop since version 2 (that's many years), and I haven't touched on many of its features.
If you have one of those fancy "automatic" SLR cameras, leave it in the closet. You need to have an old "totally manual" SLR camera with a bulb setting, and definitely without auto-film advance. Could you imagine using an auto-advance camera for astrophotography? Your shutter lock slips, and your camera advances to the next frame. Bzzzzzzzzz! It could even wake you from your celestial nap! The best camera I've found, and use at BFO, is the Nikon F2 with the removable top, prism viewfinder. You could also easily remove the old ground glass focal plane, from underneath the prism viewfinder, and replace it with a Beattie Intense Screen (4 times the brightness of conventional ground glass focal planes). We use a Beattie Intense Screen (available through the camera merchants who advertise in the back of S&T) in BFO's Nikon F2. You can insert a homemade block (made of soft aluminum or hardwood) in the camera's empty rectangular viewfinder hole. The homemade block should have a 1.25" hole in the center, for the insertion of a low power eyepiece, that has a primary focus about a half to 1-inch below the 1.25" format bottom cylinder (either a push/pull fit or set screw would hold the eyepiece secure in the plate). This installation would provide comfortable right-angle viewing and coarse focusing through your SLR's viewfinder, with or without a Beattie Intense Screen. It could even have interesting applications beyond astroimaging. At minimum, it would be a useful, and fun, construction project.
Some good films for astrophotography are Fuji print film or Fujichrome (slides) 400ASA, or higher. Kodak's 1000ASA films are quite grain-free compared to a few years ago. If you would like to experiment with gas hypered films, which increase the film's sensitivity to light and lower the film's reciprocity failure, you can purchase that special film through Lumicon, listed in the advertiser's index in the back of any S&T magazine. They even have a gas hypering kit, so you can create your own gas hypered film.
There are many different filters available that will drastically improve your astroimages. Besides the standard RGB (red, green, blue) filters, there are some exotic filters available for 35mm black & white films, like fine grain Tri-X and Plus-X, for color films (discussed above), and even CCD imaging cameras. Again, most of these special filters are available from Lumicon. For all types of astroimaging, you can get improved results by using Lumicon's exclusive Deep-Sky filter, that will dramatically darken the stellar background, and help to block light pollution. For B&W films, a minus violet filter will give you pin-point stars. An H-alpha pass filter will increase the visual range of your film into the infrared end of the spectrum. However, this filter has a very severe application limitation; it only works with B&W 2415 film. Since CCD cameras are very sensitive in the infrared, you can purchase an IR Block filter to obtain more natural looking images in the visible part of the spectrum. To obtain good 35mm color images, you can just use color film, but if you want your colors to decisively stand out, you can use a set of RGB filters, and take three separate images with B&W film. You could even obtain a 4th separate image layer with an H-alpha pass filter, to extend your red into the infrared, but you could only use 2415 high resolution film to take advantage of this 4th image layer. The RGB multiple image filter array can just as easily be used with any CCD camera too. Just name your images redm57, grnm57, blum57, etc. With B&W films, you scan your processsed prints (or negatives, with a slide scanner) to create a computer file of each color. Then create layers in Photoshop of each color, combine them and flatten the image into one image, and you've got a superior color image using B&W film.
My recommendation is to purchase all 2" filters. They may cost a little more, but you can use them with larger CCD chips and 35mm films. 1.25" filters have become obsolete because larger CCD chips and 35mm format films are too large for the smaller filters and will vignette your images. All Sliders are equipped with a built-in, very low profile 2" filter slot.
In the early days of astroimaging, guide scopes were extensively used to track stars for astrophotography, but they found that guide scopes inherently produced unacceptable system flexure that was very difficult to eliminate. To reduce this flexure, heavier tubes, truss assemblies and mounting brackets needed to be used, adding great expense and weight to the telescope. FYI, guide scopes are still extensively used in Japan and the Orient, one of the world leaders in amateur astrophotography per capita. That's why Takahashi (Japanese) refractors, you purchase in the US, had no back focus. Takahashi is just now "Americanizing" the refractors they import to the US (see the new TOA-130 at the Installations link). Then came off-axis guiding, but it was a frustrating literal nightmare to find a suitable guide star, because the off-axis prisms were very small and fixed. Adjustable off-axis aids were then introduced, but they too were very difficult to adjust, requiring you to loosen thumb screws and move various mechanisms back and forth.
Then came Sliders and Flippers (now discontinued). They evolved from a simple beginning. The need for practical, efficient, and effective ways to acquire astroimages. There was nothing but awkward, unusable, devices on the market back then. The only alternative was to manufacture our own multi-imaging portals (MIPs) at BFO's machine shop. After many years of beta-testing and experimentation, the Slider and Flipper design was finally perfected, but for many more years, it was only used internally at BFO. Then, after ten years, and much coaxing from my astronomical friends, I decided to manufacture and market a production model at BFO's machine shop. And today, VSE creates a revolutionary product line that is "second-to-none." Only the "pocket book" or the erroneous purchasing choices of the novice provide VSE with any competition.
After reading the above, these procedures
may sound like a lot of work and a very steep learning curve for the beginner,
but after you've purchased and set up your equipment and captured a few images,
it'll seem like second nature. And, most important, you will have bypassed
most of that astroimaging frustration, and put the fun back in this wonderful
celestial hobby. Unfortunately, many "would be" astroimagers have wasted
thousands of dollars because they simply don't research the subject, or take
the time to find [or purchase] the best products. People think that because
a product is advertised in a reputable magazine, it has to be a good, functional
product. Not true! You'll eventually thank yourself for taking the initial
time to find, and read, this compendium. Remember, VSE products are based
on functional simplicity, not a bunch of dysfunctional baubles. Examples
are: 1) separate filter wheels that waste important profile and add flexure
to your imaging train; 2) critical focusing devices where none are needed;
and 3) claims of pin-point focusing with the human eye which can't be done.
Now, you know better after reading the above, don't you. The simpler
[and shorter] your imaging train, the more efficient it will be, and then
you're on your "skyway" to becoming a reputable, world-class astroimager
(see Astro Links to visit some of these web sites).
I have tried to simply present "tried and true" logical imaging facts that, I think you will agree, are justifiably stated. I want to use my expertise to save my astronomical friends and colleagues the time, expense, and frustration that can be found in astroimaging. Following the wrong path [to an end] can disappoint many people, and discourage them from pursuing their astronomy, space exploration, and astroimaging goals. We need all the [what I call] pro-space citizens we can acquire, if we are to save the Planet for future generations of yet unborn billions that need to survive on this speck of dust in the universe (see Why Space? for more "soapbox" stuff).
So, print this compendium, find a comfortable chair to go over the finer points, and enjoy the "read." Keep this reference with your imaging equipment until the astroimaging experience becomes a conditioned response, and don't forget to make copies available to your astronomy club members who are interested in astroimaging. Or reprint it in your club's newsletter, in sections (part 1, part 2, etc.) of course. I wish I 'd had this kind of basic information when I began my astronomy efforts, but back then, the wheel hadn't been invented yet, either.
And, THANK YOU for considering VSE products!
Telescopically,
Paul Van Slyke, Director, Black Forest Observatory
For more detailed information on specific subjects, regarding the above equipment and it's specific operations, reference the Questions & Answers (FAQ) and the big metal question mark (?) link.
NOTE: This compendium is under continuous construction/revision, so check back once in a while. You might find some new and useful imaging hints. This is revision 2.6
To help promote and advance astroimaging, this how-to compendium can be copied, reprinted, and distributed freely.
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