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SLIDER Imaging Train Configurations

Many have said that VSI has too many confusing port rings, adapters and couplings. This is probably true, because VSI has dozens of different adapters that can be installed in various locations along your imaging train. VSI also has the best selection, by far, on Planet Earth (now where have I heard that before?). So your confusion is well justified, but I do believe that a good selection is important, simply because there are so many different types of telescopes and imaging equipment available to the modern astroimager. I hope the following will make your final imaging train decisions easier and less frustrating. After all, that's what VSI components are all about - creating an ultra-low profile, rock-solid imaging train, and removing the frustration from astroimaging. - PBVS

Exploded Pictorial of Typical SLIDER Imaging Train


A Quick & Dirty SLIDER Imaging Train Selection Guide


*Note that all VSI 2" TOADLOADERS have our exclusive focal reducer "hidden cavity" allowing you to insert your f/6.3 Meade or Celestron focal reducer securely inside the moving tube of your VSI focuser effectively rendering your focal reducer's profile to zero. This unique feature is not included in the above imaging train examples, which illustrate the focal reducers mounted externally, gobbling up that valuable profile.

An In-Depth Selection Discussion to Help You Make the Right Decision

A. Your primary concern will be the type, and number of telescopes you have now, and/or plan on purchasing in the future.

  1. If you have a commercial Schmidt-Cassegrain telescope, your back focus is more than adequate for any VSI imaging train, but don't get too involved with add-on accessories (especially in-line filter wheels) that increase your profile and flexure. Your decision is then based on the secondary consideration (B) below.
  2. If you are going to use a Newtonian telescope for imaging, the best way to gain back focus is to move your primary mirror forward. Do not move it too far or you will cut off the edge of your primary light cone by vignetting your secondary mirror. If you do cut off your primary light cone, you can install a larger minor axis diagonal flat, but you will also lose a small amount of resolution, because you will then have a larger center obstruction. This small clarity loss will probably not be noticeable, even when imaging. By moving your primary forward, this will allow you gain valubable back focus and provide enough profile to use a Slider in your imaging train. If moving your primary forward is not possible, you can install a low profile focuser such as the helical models that are available from Orion and other telescope accessory dealers. This should provide enough back focus to use a Slider, which I consider ideal for a Newtonian telescope (see Unsolicited User Comments link, Steve Bell's comments).
  3. If you are using a refractor, good luck, because refractors are notorious for their extremely limited back focus, but there is hope. Probably the refractors with the most back focus are the Astrophysics, Meade and now the new Takahashi TOA-130 (see Installations link). At least that's what I've heard from my customers. You should be able to use a Slider with just about any of the today's newer model commercial refractors, using a basic imaging train.
  4. If you are using a different type of telescope that is not listed above (i.e. catadioptric-not SC, off-axis, folded, etc.), then the best way to choose a Slider is to perform the back focus test noted below and follow the guidelines that apply to your imaging train configuration above.

B. A secondary consideration is the number of imaging accessories (i.e. focal reducers, filter holders, secondary focusers, etc.) that you are going to use in your imaging train. In other words, how much back focus does your telescope have (see "Do you have enough Back Focus? below)?

  1. If you are installing a focal reducer (FR) to reduce your magnification and increase your field of view, it will actually shorten the length of your available light cone, so your overall available profile will be much shorter than without a FR. If you're going to use a Meade/Celestron f/6.3 FR, purchase a Slider and just screw it onto the input port of the Slider. These FRs will only consume about 1.5" of actual profile when installed in this manner. Note that if you want to cut your FRs back focus to zero, then use one of VSI's 2" TOADLOADERS and install your FR in the Glider's exclusive, built-in focal reducer "hidden cavity" (see FOCUSERS link for more info). Note that Meade's f/3.3 focal reducer mounts on the rear of the Slider only. Remember, you'll need a long drawtube out the top flip-mirror port and the side pick-off port to parfocus your eyepieces with your CCD camera when using a Meade f/3.3 FR on the Slider's rear port. You can use standard 2" or 1.25" diagonals in combination with various drawtubes to take up some of that distance. You'll gain a lot of versatility by using diagonals to absorb some of that back focus length because you can rotate them to a more comfortable viewing angle.
  2. Will you be using a filter wheel? I don't recommend any type of add-on filter wheel, because they add too much profile to your imaging train, and also add an unacceptable amount of flexure. If you need to use filters, use the Slider's built-in filter slot. Note that some CCD cameras have an excellent built-in filter wheel in the camera's head. Any filter insertion device that doesn't add flexure to your imaging train, like our Filter Adapters, is a good idea.
  3. Are you inserting an adaptive optics device? If so, you will need to sacrifice about 2 inches of your back focus to the profile god. Usually these types of devices don't radically change your focal point, like a focal reducer/extender, but they do consume your normal light cone's valuable profile. Always check with your specific AO manufacturer for installation details, profile requirements, etc.
  4. If you are installing a secondary focuser on your SCT, a 2" TOADLOADER is always recommended for imaging with your Slider simply because all Glider models induce zero flexure into your extended imaging train. This is a perfect "marriage" for the SCT.
  5. Will you be using eyepiece or Barlow projection to increase your magnification for finite imaging (i.e. lunar, planetary, double stars, etc.)? This unique imaging train configuration is not used very often, because most of the time you're trying to decrease magnification and increase field of view, but I have had some customer interest. A Slider will accept all the various adapters, couplings and accessories necessary for eyepiece projection, again, because of it's short profile and new "collimate-able" top slide-mirror port. With this new super feature, the Slider's top port can be "dead-on" collimated with the rear imaging port using high magnification eyepieces with extremely narrow fields of view.You can then attach a Meade or Celestron Tele-Extender to the Slider's rear port using a A2ST adapter, or to the front port using the AT2 adapter.
  6. Will I need my imaging train to clear between my forks for imaging around the North Celestial Pole (NCP)? Although this question is not really pertinent to the discussion at hand, I am asked this "interesting" question more often than I would expect from would-be astroimagers with multitudes of supposed common sense. My answer is simple. It don't matter! Clearing the fork with your imaging train is a situation that would not manifest itself in a dozen blue moons. There just isn't that much "good ol' stuff" to observe or image around the NCP. There's M81, M82 and a handful of other fainter galaxies in that circumpolar area, but none of them are really "through the fork" objects, so don't worry about it. Sure, you're computerized scopes may try to take a journey through that "forbidden region" from time to time, but you do have an abort button, don't you? If you see your imaging train heading for an eminent collision, abort, self-destruct, pull the plug, scream, do something fast, but don't concern yourself before it happens, because it may never happen. Then again...........     Just get on with it. Life's too short. Nonsequitur!

C. A tertiary concern might be size and weight. Not a single VSI product has ever been accused of being too small or too light. On the contrary, VSI products are quite "manageable" for their size, and built like a tank [to the extreme], because quality imaging equipment needs to be normally built to other people's standard of overbuilt, which I consider to be a prerequisite for any instrument used for astroimaging. Unfortunately, this overbuilt standard is not a standard at all in other domains (see specs for individual product weights).

D. A minor consideration might even be too much back focus. Every astroimager would like to have that "problem." As I've always said, "You can use a simple drawtube to extend your profile to meet a long back focus, but you're only alternative to an inadequate amount of back focus is usually too drastic to consider." Cut off your tube, move your primary, move your secondary, install a lower profile focuser, re-design your tube assembly, etc. Telescope designers should always consider the astroimager, but will they?

If you don't have a commercial SCT, you need to check your back focus before purchasing a Slider or Sidewinder. I'm finding that many seemingly experienced astrophotographers know very little about their own telescope's back focus (BF). They order a Slider or Sidewinder and find that it won't work on their telescope. This wastes my time and yours. Back focus is something that you need in abundance if you're going to do astroimaging. Unfortunately, many telescopes, like refractors and Newtonians, have very limited back focus and are designed for visual observing only. The only telescopes that have near infinite back focus are Cassegrains that move their primary mirrors, like the commercial Meade/Celestron Schmidt-Cassegrain telescopes. For every inch that you move your primary mirror, you get an equivalent of 6 inches of back focus change.

How to check your back focus

If you have a commercial Schmidt-Cassegrain telescope (Meade/Celestron, etc.) you don't need to perform this test. Any Glider, Slider or Sidewinder will work with your scope. If you don't, then Rack [or move] your focuser all the way in. Then rack [or move] the focuser out about a quarter of an inch. Move the imaging camera or eyepiece out from the focuser, without moving the focuser position (a drawtube would be very useful at this point, but not always available), until you achieve focus at the camera/eyepiece. The distance from the outer edge/lip of the focuser's rack [or moving] tube, to the OD T-ring butt plate (not the end of a 2-inch adapter tube) of the camera is the profile (in inches) that you need (see BF diagram above left). To install a Slider or Sidewinder you will need 3.5 inches of profile. If you don't have 3.5 inches of profile, then you can't use a Slider or Sidewinder, or anything else for that matter, until you extend your telescope's back focus. There are many ways of modifying a telescope's back focus. Some of these methods are briefly discussed above. If you need advice, give me [Paul Van Slyke] a call and we'll talk about modifications to your telescope.

Paul's Pictorial Parfocusing Primer

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 or Sidewinder. I selected the [now] 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. The Flipper, Slider and Sidewinder parfocuses [from CCD to 35mm camera] by adding drawtubes to the top flip or slide port (noted in red, lower right) and side pick-off ports (not shown), which is the most effective method to parfocus your system because you are not adding any length to your straight-through imaging train 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. After attaching your new Slider to your scope (addressed above), attach your camera, using the appropriate adapter, to the rear imaging port. Adjust the telescope's focus on your 35mm camera's focal plane, or CCD chip of your imaging camera. Then, insert a 2" or 1.25" eyepiece in the top flip/slide mirror port and focus the eyepiece by push/pulling the eyepiece in and out until adequate focus is achieved.

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 flip or slide mirror port and your eyepiece (see lower right Flipper, area in red). Although not shown in the pictures at left, side pick-off ports will need to be extended with a simple drawtube on Slider 2 models, but not on the discontinued Micro-Slider models because of the inferior "reverse" design that Meade has now "adopted" that extends the imaging train profile instead of the top or side mirror port profile. To further reiterate, note the two lower Flippers above. The red area noted on the lower right Flipper is 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.

NOTE: Flippers were beta tested for nearly 10 years at Black Forest Observatory before a production model was finally offered. We found that secondary helical focusing, offered on other brands is time consuming, awkward and literally useless when attempting to focus secondary eyepieces, since your eye can't observe perfect focus anyway. The push/pull method is easier to adjust, fast, accurate and functional.

Also reference The Art of Astroimaging for more info on parfocusing your imaging train.

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