Last updated on
4/11/
2006

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Astrophotography Accessories
[Polar Align] [Wedges] [Focus Options] [Guiding an SCT]

This page is under construction - my astrophotography work is evolving (it probably always will...)  But on this page I’ve described a number of astrophotography accessories that you may find useful, based on my experience and/or that of other astrophotographers much better at it than I am.  (Note that there is a separate discussion of useful accessories for visual observing under the SCT Tips section of this site.)  I go into more detail in separate pages for Polar Alignment accessories, Equatorial Wedges,  better focus options, and Guiding an SCT during exposures, as indicated in the navigator above.

I do describe below a lot of accessories that help you take better astrophotographs, and in total these accessories can amount to a very large financial investment. Please don’t be discouraged by this - you can take very satisfying pictures with little more than a simple consumer digicam. But if you find that you want to take astrophotographs that compete with the work done by the very best photographers, you’ll find that you will need to achieve very precise control over all the variables in a very exacting process.  (After all , your subjects are tough - very far away and thus very dim and difficult to shoot.) Don’t be surprised if over time you find yourself investing a lot of effort and money to achieve better results - the best astrophotographers have invested a LOT of time and money to achieve their results. But they’ve still invested orders of magnitude less than NASA has in the Hubble telescope, and you can go as far as you want with this hobby and still have a lot of fun along the way.  If you find you’re not having fun along the way, then what you’ve chosen isn’t really a hobby...

Here is what my U2K looked like (my NexStar11GPS looks about the same as this, albeit with a larger diameter <grin>), outfitted with many of the astrophotography accessories I describe below or in the sub -pages under this page (click on the thumbnails to view larger pictures).  Atop the scope are three finders: the standard U2K finder scope on the Celestron Quick-Release Bracket and the Celestron 60mm Guidescope (and while not an astrophotography accessory per se, the Telrad finder with the Kendrick Sun Finder mounted on it, is shown as well).  The finder scope has a Celestron Illuminator attached, and the 60mm Guidescope’s eyepiece has the Meade Illuminating Reticle eyepiece.  The Celestron Lens Shade (see below) is mounted on the front of the scope. The rear cell has the Meade Flip-Mirror mounted with the SBIG ST-237 CCD camera attached to it, and a JMI MFC8 Motofocus with the DRO encoder.  (Note that the 60mm Guidescope and the Lens Shade provide enough forward weight that the scope only needs a little additional balancing weight despite all the items mounted on the rear cell.) The scope is mounted on a Mettler Wedge, providing a solid platform for astrophotography (and a convenient tray to hold the scope and JMI hand control units).

Useful Astrophotography Accessories:

Counter Balance Weights When you hang astrophotography gear on your scope’s rear cell, the standard Celestron counter balance weight is insufficient to balance the scope.  See the Balancing An SCT page on this site for a description of ways to balance your scope.

Flip Mirror I described a flip-mirror on the main Astrophotography Basics page on this site.  I mention it here only so that this page represents a more complete list.

Equatorial Wedge I mentioned the advantages of an equatorial wedge for visual observing in the “Nice to Have Accessories” under the “SCT Tips” section of this site. A wedge is critical for most astrophotography - click on the Wedges button above for a detailed discussion on the wedge options available to you for your scope, specifically for astrophotography.

Polar Alignment For information on what accessories can simplify polar alignment (absolutely critical for most astrophotography) click on the Polar Align button above.

Focus Aids
Once you have a good wedge and good polar alignment with that wedge, your next job is to achieve good focus with your scope. This is because unlike cameras, your scope’s optical system will be used at maximum aperture so you essentially have no depth-of-focus; the focus of the scope on the “film plane” (whether a 35mm film or a CCD chip) must be exact to achieve crisp images.  There are two accessories that can help you achieve good focus: a focus aid on the front of the scope, and a better “focus knob”.  They accomplish different things - the focus aid tells you when you have achieved focus, and the electric focus motor allows you to change the focus in much finer increments than you can do by manually turning an SCT’s focus knob.
The Kendrick Kwik Focus is a Hartmann Mask that mounts on the front of your scope and has three holes in it.  This aid “splits” the image when you are out of focus - you see three images of a star, which converge to one image when you are in focus (as shown on the right here, going from out-of-focus to in-focus to out-of-focus again).  Note that you can make a focus aid like this from  a piece of cardboard with three holes in it but you would need to figure out a way to mount it on your scope.  The Kendrick aid fastens to your scope with nylon screws so it is easy to mount and you may find it is worth the $60 price. Note that the three holes in the Kendrick come with caps so the KwikFocus can serve as a dust cover for the front of your scope - you don’t need to carry it around as a separate accessory, and you may find (as I have) that with its nylon screws it is a lot easier to remove and install than prying off and pushing on the friction-fit dust cover that came with your scope.

To fine-tune the focus on your SCT when doing astrophotography it helps a lot to have an electric focus motor mounted on your scope. Celestron indicates one available, but when you check it out you will find that they refer you to JMI (Jim’s Mobile Incorporated) who makes “Motofocus” accessories for Celestron SCTs.  When you go to the JMI Web site to try to figure this out, you may find it confusing (I did at first).  So I’ve sorted out your options  - click on the Focus Options button at the top of this page to open the page that describes some focus motor options for SCTs.

Focal Reducers A focal reducer is a lens system that mounts on the rear cell of a scope in front of a flip-mirror, camera, or diagonal and eyepiece. It acts as a “wide-angle converter” and changes the focal ratio, or if you prefer, the effective focal length of a scope. It is particularly useful when using affordable CCD cameras because they have small CCD chips and thus do not view a large area of sky. Most deep-sky objects (nebulae and galaxies) are larger than this view and you need a focal reducer to fit them on the CCD chip.  Note that a focal reducer is actually the worse thing you can do for planetary photography - planets are small and need magnification (increasing the focal ratio of a scope, for example with a barlow lens), not reduction. But if you plan to do deep-sky photography with a CCD you probably will need to get a focal reducer, especially if you don’t have (or can’t use, in the case of Meade SCTs) the Celestron Fastar system. Because they increase the effective viewing area of a scope, focal reducers have the benefit of increasing the scope’s optical (and thus photographic) speed which is beneficial for dim deep-sky objects especially if you are using 35mm film. Both Celestron and Meade sell an f/6.3 focal reducer, which they claim also flattens the field of view for better resolution at the edges of the field of view.  The Celestron f/6.3 focal reducer sells for about $120 and the Meade for about $130. Although it appears that they both get the unit from the same manufacturer I have been told that the multicoating on the Celestron is much better than on the Meade.

Meade also sells an even wider-angle f/3.3 focal reducer for $150; Meade says this one should only be used for CCD photography, not for visual work. Note that if you calculate the optimum pixel scale for CCD cameras (see the Comparison of CCD Cameras for more information), for 8” f/10 SCTs the best match between the scope and the CCD camera is with an f/3.3 focal reducer. Optec also sells the Maxfield f/3.3 focal reducer, which is considerably more expensive than the Meade unit ($200 plus $50 to $125 for an adapter for your CCD camera).  Because an f/3.3 focal reducer can be an important piece of equipment for CCD photography with an f/10 scope, Chris Vedeler has posted a comparison of the Optec Maxfield and the Meade  f/3.3 reducers that you should review.

Update (2/26/04): One of the potential problems with hanging a CCD camera on the rear cell of a fork -mounted SCT can be that the camera, plus an electric focuser and a filter wheel (if your CCD camera requires one for color photography) adds a lot of length to the rear cell. This makes balancing the scope more difficult and worse, can add enough length that the scope’s rear cell cannot swing through the fork base.  This prevents astrophotography in the direction in the sky that the scope’s mount is aimed towards (i.e. the celestial pole in an equatorially-mounted scope or the zenith in an Alt-Az-mounted scope). And adding a focal reducer exacerbates this problem. If you find that this is your situation, Ted Agos sells an alternative on his Acorn Hollow Observatory web site. His “6.3 Universal Focal Reducer Solution” is a tube into which the optics of a focal reducer are inserted.  He sells various tubes for different scope/camera configurations, and they allow you to connect your CCD camera to your scope with a focal reducer while minimizing the increased length of the resulting optical system. You can purchase various tubes into which you insert the lens system of a focal reducer you already own, or if you want to keep your focal reducer intact for visual observing you can purchase a tube with the Celestron f/6.3 focal reducer optics installed (see that web site for more information).  Many astrophotographers with fork-mounted scopes have found that this system solves the problem of the scope’s rear cell swinging through the fork base when using a focal reducer.

Lens Shade Celestron makes a rigid plastic Lens Shade/Dew Cap for the U2K or C8 scopes (model #94019, shown on the right).  This is useful for a number of reasons. It prevents stray light from entering the scope during photography, it acts as a dew shield if you live in a humid area (see the Dew Prevention page), it adds weight to the front of the scope to help counterbalance any photographic gear you add to the rear, and it protects the U2K or C8 corrector lens when you use the Fastar option. (See Christopher Anderson’s Darklight Imagery site for an example of what can happen if you don’t protect the corrector lens while using the Fastar assembly.) It sells for $38 and it is very well made - it is light-weight but fastens to the front of the U2K or C8 solidly. Astrozap also makes rigid dew shields that can protect the corrector lens when using a Fastar, but these are substantially more expensive than the Celestron version (although they probably do work better for dew prevention).

Framing Eyepiece The size of a CCD chip in affordable CCD cameras is rather small, and it is often difficult (without a lot of experience) to determine how an object you plan to photograph will fit on it. (For example, deep-sky objects like nebulae and galaxies will likely need a focal reducer to fit on the chip at all.)  One accessory that can make this process easier is the Meade 25mm Illuminated Reticle CCD Framing Ocular. In outward appearance it looks like a Meade Illuminated Reticle eyepiece (discussed on the Guiding an SCT page).  However the reticle is very different, as shown on the right; the reticle has rectangular frames for many of the common CCD chips. Unfortunately the SBIG ST-237 isn’t one of them, but the ST-237 would fit between the smallest frame in the reticle (the TC-255 frame) and the next larger frame (the ST-7 frame).  This eyepiece sells for $130.

One last note: at some point you may consider building a permanent pier and even an observatory enclosure for your scope, to eliminate the need to move and set up your equipment each night.  See the Items You Can Make page for more information on these.


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