Guiding Tips With an Autoguider on a Schmidt-Cassegrain

Guiding Tips With an Autoguider on a Schmidt-Cassegrain
By Chris Vedeler
Copyright 1997

1- Introduction

This page is an outline of some of what I have learned about the tricky art of autoguiding using a CCD autoguider with my Meade 10" LX200. I want to share what I learned so hopefully others don't need to go though the expensive and frustrating learning experience I did.

These tips are geared toward fork mounted Schmidt-Cassegrain telescopes like the Meade LX200 or the Celestron C series. Some of these tips (and the problems these tips are designed to correct) may not be relevant for people using Newtonian's or Refractors for astrophotography.

If you are into astrophotography you know about guiding. And you probably know that manual guiding is the most boring and fatiguing thing about astrophotography. But next to focus (Click here to read about tips with focusing) it is about the most important thing to getting great astrophotographs.

After successfully using an autoguider, you will probably never want to manually guide again. Once you get an autoguider working it is a wonderful way to take great shots, and have a relaxing and enjoyable night under the stars at the same time. However, unlike the marketing claims, using an autoguider is not a plug and play scenario. It takes a little know-how and some patience to figure these little wonders out. Unfortunately the marketing and the documentation for these autoguiders does not tell the whole story and it is left up to the customer to figure it out. The following is some of what I figured out using my autoguider on my telescope.

Thanks to all my friends on APLM and MAPUG two astronomy news groups on the Internet for feedback on this article.

2- Off Axis Guiding Nightmares

I own an Orion off axis guider, and have manually guided quite successfully with it many times. Typically I would center or frame the object I want to photograph through my camera. Then spend some time to carefully focus. Once focused I would put in my 9mm Meade reticle eyepiece into the off axis guider and start the arduous hunt for a suitable guide star. This is done by moving the eyepiece holder forward, backward, left or right the tiny amount possible, and/or rotating the entire assembly around. Quite often the act of moving the assembly would require that I re-frame the picture, which would make me start from square one all over again. Other times I would look for the guide star first, and then try to frame my shot as best I could while keeping the guide star within reach of the off axis guider. Sometimes the only guide star I could find would be in such an awkward position, it was worthless even trying. Now, when you add the additional complexity of an autoguider this processes is almost too frustrating to even bother.

I tried the top of the line Lumicon Giant Easy Guider once and found it to be even worse for using my autoguider than my Orion off axis guider. This is a fine piece of equipment, don't get me wrong, but it was clearly not designed for use with an autoguider. The main problem is how far out you must put the autoguider to reach focus. I had to use an extension tube AND slide my autoguider out. This left my 2 lb autoguider attached to this spindly little tube about 7 inches from the off axis guider. Talk about a flexure problem!

Another problem is actually a designed feature of the Giant Easy Guider. With the Giant Easy Guider, you can be photographing at f/6 (using a built in focal reducer) while guiding at f/10. This is great for manual guiding as any star movement is easily seen before it is significant enough to show up on the film. Autoguiders are extremely sensitive and do not need the benefit of guiding at f/10. With the flexure problems I was having, I could not get it to lock on to a very nice guide star for more than a minute or two because at f/10 it was too sensitive and the star would drift off the CCD chip before it could correct for it.

The other problem I found seems to be a problem with all off axis guiders by design. It turns out that CCD autoguiders seem to like pin point star images to guide with. Off axis guiders by design do not give very good star images for several reasons. First, the prism is rectangular and thus gives a terrible diffraction pattern. Second, they are taking light from the edge of the field which is the most prone to coma and other distortions from the main scope.

Take a look at this illustration to see how the different star images appear.

3- The Benefits of a Guide Scope

Given all the inherent problems with an off axis guider, the only other solution is to use a separate guide scope. Using a guide scope solves many of the problems that plague off axis guiders. But it introduces some others that can require some engineering and can limit how you photograph in other ways.

Lets look at some of the advantages:
First of all, finding a guide star is very easy and is completely independent of the main scope. This means that you can frame your shot and carefully focus once instead of many times as you disturb your prize shot in the hunt for a decent guide star with an off axis guider.

Second, you can find a guide star a degree or two away from where you are photographing instead of being limited to a very narrow ring of sky around where you are photographing. Another advantage of this is that you can often pick a guide star that is fairly bright. A bright guide star means that you can set the exposure time very short on the autoguider, which translates to quicker corrections and thus better guiding.

Third, a guide scope (whether it is a cheap 60mm f/15 refractor, or a high quality 90mm Maksutov like the Meade ETX) will give a much better star image than the best off axis guider. Better star images means that the autoguider will lock on and stay on.

Fourth, with a guide scope it is much easier to guide on comets as they are moving relative to the background stars.

Finally, a guide scope can be used as a secondary photographic instrument, often giving a wider field of view than the main scope. The main scope can the act as the guide scope giving you near perfect tracking.
(I think it makes the telescope look cool too.)

Lets look at some of the disadvantages:
The First is the biggest, and that is differential flexure. One nice thing about off axis guiding is that you are guiding using the same optics that you are using to photograph with. Flexure is not a concern because you only have one optical system (except for the problems I had with that Giant Easy Guider). Flexure is when one optical system moves relative to the other. If you have too much flexure, your autoguider may seem to work perfectly and keep it's guide star perfectly centered with the field of the guide scope, but because the guide scope has flexed slightly relative to the photographing scope the stars will trail and ruin the picture. The only fix for this is a very ridged mount for the guide scope. If it flexes as much as 1/1200th of a degree (3 arc seconds), it can show up on the film as oblong or trailed stars.

The second is a problem with Schmidt-Cassegrains in that that the optics can shift slightly depending on the orientation of the scope. This translates to the stars appearing to move as the orientation of the scope moves, and thus your photograph being ruined. With an off axis guider the autoguider will compensate for this movement, but with a guide scope the mirror movement can not be compensated for. I measured as much as 70 arc seconds of mirror shift, so this can be a serious problem. The only fix is to lock the mirror down so it does not move at all relative to the rest of the telescope. See my attempted fix for this problem by using a mirror stabilizer . Based on my tests however, locking the mirror down completely is the only way to get the tracking error down enough for the demanding needs of astrophotography. See my test results using the mirror lock down bolt, the mirror stabilizer and using and nothing at all.

Finally, with all the added weight of a guide scope your scope will require the use of counterweights on any fork mounted Schmidt-Cassegrain telescope. This added weight is rotational weight and thus puts more strain on the Declination motors. This extra weight will also make the entire telescope less stable and more prone to mount flexure. See my mount stabilizer article on how I helped reduce this.

4- My Setup and Fixes

I think that the advantages of using a separate guide scope far out weighs the trouble it takes to make it work. Here I will describe in some detail some of the steps I went though and the equipment I use to get around the problems associated with using a guide scope.

First, lets take a look at my guide scope. I picked up an 80mm f/11 Celestron refractor optical tube assembly at a star party for $125. It is a wonderful guide scope, with excellent star images and what turns out to be an acceptably long focal length (910mm) for autoguiding my Meade LX200 10" f/10 (2500mm). Adding a barlow may help a bit in guiding accuracy, but I found that my shots taken at f/10 (2500mm prime focus) were guided perfectly well using only the prime focus of the 910mm guide scope. One essential feature of any guide scope is the ability to lock down the focus with a screw. This keeps the guide star in focus but also makes the whole assembly more ridged reducing flexure. A cheap department store refractor may not have a locking screw.

Here you see my guide scope mounted to my LX200 with the autoguider attached at the prime focus.

4.1- The Battle of Flexure
I made my first mount using PVC plastic for the rings. PVC is easy to cut, easy to work with and easy to find at almost any hardware store. I put together my mounting system with aluminum flats and angles that can also be found at most any hardware store. I epoxied the PVC rings on the rest of the assembly and waited for dark. Once the epoxy had cured, I eagerly took the scope out to test for flexure.

I tested the flexure by first centering a bright star in both the guide scope and the LX200. I focused the guide scope with my parafocal Meade 9mm reticle eyepiece and then put in my Meade Pictor 208XT autoguider. I then moved the reticle eyepiece to the LX200 and centered the crosshairs on the guide star (I did this by moving the cross hairs and not the telescope). With the star perfectly centered in the LX200, I started the autoguiding and watched the apparent motion of the star. After 45 minutes of guiding, the star had drifted about 30 seconds of arc. Way to much flexure for photography.

Upon inspection, I found that the PVC rings were visibly flexing as I tried to tighten the guide scope to the mount. I suspected that if such flexure was visible it must be the source of the 30 second drift I witnessed. I was then on the hunt for some aluminum rings. I wanted to get 5" diameter 1/4" wall rings, but I could not find any for less than $100 (the minimum order from a place in Phoenix). I did find 5" diameter 1/8" wall tubing at a junk yard here in Tucson. I got 6 lbs worth for only $5. I cut a 1 1/2" wide test ring and put my weight on it to test the relative flexure from the 1 1/2" PVC. It flexed about half as much as the PVC under the same weight. Better, but not good enough. I then cut a ring 3" wide and did the same test. This time there was hardly any detectable flexure. I decided to give myself a little room for error and cut two rings 3 1/2"s wide. Once the holes were drilled, threaded and the epoxy had cured I was off to a dark site to test the flexure again. This time the star drifted about 5 seconds of arc in 1 hour. Not perfect, but good enough for most color films at 2500mm prime focus and certainly good enough for color film at f/6.3 or 1600mm.

My guide scope mount assembly.

4.2- The Battle of Weight
A problem one runs into after putting a four pound guide scope, two pounds of guide scope mount, and another two pounds of autoguider on the top of a telescope is balancing it.

You could go out and spend hundreds of dollars for a Losmandy counter weight system, or spend about $30 and a few hours of work and make one that works just about as well.

I made my counterweight system using a single 1" aluminum channel with seven 1/2" holes drilled along the entire length. I then I J.B. Welded seven 1/2" nuts to match the seven holes on the inside of the channel (they fit perfect inside the channel). I then J.B. Welded washers and 1/2" nuts to a two and a half pound and a five pound steel weight I bought at a sporting goods store (the kind used in free weights). Then using a 6" section of 1/2" bolt, I can screw the weights onto one end, and screw the 1/2" bolt into the channel. I use a 1/2" wingnut to tighten the counterweight assembly to the channel. To move the weight closer or further away from the OTA you simply screw it one way or the other. I use another 1/2" wingnut to lock the weight in position on the 1/2" bolt. Being able to move the weight away from the OTA lets you use less counterweight because you can take advantage of leveraging. The whole assembly mounts to the telescope with two extra long stainless steel screws. It has worked beautifully with up to 7 1/2 lbs of counterweight.

My counter weight system disassembed.

My counter weight system attached to the scope.

5- Real World Results and Impressions

I have found my guide scope a joy to use. I can comfortably take six 1 hour exposures in a night. Before having an autoguider or a guide scope I was unable to take more than about 30 minute exposures due to fatigue and boredom. I would spend 20 to 30 minutes each shot framing, focusing, looking for a guide star, re-framing, finding the guide star again, re-framing and so on before I ever opened the shutter. With my autoguider and the off axis guider, I could only get one or two shots in a night because of all the problems trying to get the autoguider and the off axis guider to cooperate together. Now I can frame, focus, find a guide star, start the autoguider and open the shutter all in about 5 to 10 minutes. Then while the telescope tracks I can take a nap or be sociable at the star party.

I am still experiencing some intermittent problems with differential flexure. It will take more experimentation to iron those out and I will post my discoveries on this page as I figure them out. Most of my shots with my autoguider and my guide scope are well guided even up to an hour at f/10. All in all I am very happy with this new setup, and I hope to quickly build a portfolio of great astrophotography.

Update 11-5-97: Most of my differential flexure problems seem to be resolved. By tightening everything down very soundly, I seem to have eliminated most of my flexure problems. On occasion I still get some star trailing that my be due to differential flexure. But when about two shots per 24 exposure roll has slightly trailed stars, I'm pretty happy with the results.

Comments: ckvedeler@access4less.net

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