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Off axis or Guide scope?

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Eben Gay
New Member
Joined: 2 years ago
Posts: 4
Topic starter  

I have recently purchased a used 8" EdgeHD Celestron on a Nexstar mount.  I built a wedge for it so I can do astrophotography.  My experience with it is mixed - sometimes I get it aligned just right and it tracks like a rock for 3 min exposures and sometimes it's iffy at 60 seconds.  As I learn the gear I'm finding that the mount itself has some play in it, there are fundamental scope-to-mount alignment issues (that I'm working on), and my expectations are going up (from "so cool!" to "wait, this could be a lot better").  

Of course I have a limited budget so I'm looking at what would be the best next step to improve the stability of long exposures. 

I'm thinking that an auto-guider could work around issues in the mount and be significantly less of an investment than a really good mount (which is on my "someday" list).  

Assuming that's reasonable, I've found diametrically opposed recommendations for a 2000mm scope.  Either an off-axis guider is the only way to go because of the scope's long focal length or a separate guide scope is the only way to go because of the scope's limited field of view.

I'd love to hear your experience and opinons.



Terri Zittritsch
Member - Treasurer
Joined: 3 years ago
Posts: 165

Hi Eben,

My experience with OAG's is not positive.  After fiddling with one for some time, admittedly early on in my AP learning, I put a rail on the top of my SCT and used a cheap ST-80 guidescope from Orion with a Starshoot autoguider and had great success.     While I did use the scope at times at the full 2000mm, which is imaging at F10 and the exposure times are long to get good signal.   More often I usually used a reducer to make the scope F6.3, and this also helped the incredibly long focal length guiding scale.   I know the reducers for the edge are expensive!    Guiding at 1300mm is less demanding than trying to guide at 2000mm focal length.    

So that was when I was just starting.    I am going to try OAG again as it's built into my full frame ASI6200 setup I just purchased.. I'll report back on my success, or frustration.   

I know for really long focal lengths that many advocate using OAG.

I don't know if this is helpful or not as my OAG experience was short and bad.   But I'm going to take another run at it!



Active Member
Joined: 2 years ago
Posts: 7

Hi Eben,

My experience with Off Axis Guiding is kind of similar to Terri's. I have a 8" RC sitting on top of an Atlas Pro mount. A Focal length of 1600 mm and I thought I'd be able to improve my guiding with an OAG. With the camera I'm using (QHY 8L) I have a pretty large sensor at (24.2 x 15.8) and I struggled with the OAG casting a shadow on my sensor, even when I had it positioned to be on the flat side of the image sensor. I went back to a guide scope sitting on top of the main scope. It works pretty well and now with PHD2 offering multi-star guiding, I'm happy with the performance. The other pain point I had was just getting focus to be in synch with the main scope and the OAG.... Lots of hours trying to get that perfect!


Paul Walker
Trusted Member Admin
Joined: 10 years ago
Posts: 59

FYI - for anyone reading the above posts, "AP" is astrophotography and "OAG" is off-axis guider.


Paul Walker
Trusted Member Admin
Joined: 10 years ago
Posts: 59

Hi Eben,

A few thoughts.  First I am an advocate of using what one has.

You may want to consider getting a focal reducer for your Celestron8" EdgeHD regardless of when you get into using an auto guider.  It will shorten the focal length which will be more forgiving of tracking errors and mount stability issues.  It looks like there is a 0.7X reducer available but it is ~$350 (don't know if any others exist).  Along with a shorter focal length and larger field of view, this will give you brighter image for a given exposure time or a shorter exposure time for a given image brightness.  The difference is the ratio of the squares of the f-ratios.  So (10X10/7X7) 100/49 = 2x. It will also increase it's versatility.

This next part may not be useful if you plan on getting into auto-guiding fairly soon. As you may know, all motor drives on mounts have "periodic errors" due inaccuracies in the gears.  It can be helpful to characterize the this periodic error.  It is easy to do. Rather than polar aligning the mount, purposely offset from the pole left or right by 5-10 degrees (with the long f/ratio you may have to offset less than 5 degrees if the star trails (see below) go out of the camera's frame.  Aim the scope toward the south and near the celestial equator.  At f/10 you can probably set your camera to ISO 800 or 1600 (you may have to experiment a little).  Take a carefully timed image of say 10 minutes (may have to experiment a little to see how long it needs to be).  What you will get is wiggly star trails.  They will trail because the scope will be drifting north or south.  The trail will wiggle because of the small errors in the gears.  There will be small amplitude wiggles and large amplitude wiggles.  The exposure needs to be long enough so the big wiggles repeat. Is the main periodic error and (hopefully) the worse of the wiggles.  Knowing the total length of the exposure you can take measurements off the computer monitor and calculate the length of the periodic error in minutes and get an idea of what the maximum exposure you can use and still get a reasonable number of good enough images (when stacking images a little trailing on some of images can still give satisfying results). I'm not sure what reasonable exposure length is relative to the periodic error, maybe 1/2 to 3/4 of the time for one cycle of the error.

Or what may be more useful is you can take sets of images, say 10-20, of 30 sec, 45 sec, 1 minute, etc., and see how many are good enough at each exposure time for stacking.  This is a more direct way of figuring out what you can use for exposure time without using an auto guider.

Paul Walker

Joseph Comeau
Active Member Admin
Joined: 9 years ago
Posts: 8

I use an Orion Starshoot Pro and a Mini External 50 mm guide scope with the program PHD Guiding 2.  I always image with a computer.  Since I image with several scopes, It is easy to switch from one scope to the other.  I have two off axis guiders but have never had much success with either.  It takes too long to set them up and I often cannot find a suitable star at high magnification in the small field of view.

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