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Rookie (dreamer) Question

Astrophotography Beginner Equipment Imaging Optics SCT
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#1 Young Padawan

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Posted 29 November 2023 - 04:53 PM



I'm asking this question because I'm assuming that it's been done successfully by someone somewhere. 


As I continue to struggle with the OAG learning curve, I've begun asking myself "why not at least try a conventional guide scope?" Yes I know that there are countless posts advising against it. Most are centered around Flexure, and the general rule that the Guide Scope' focal length should be at least 1/3 of the focal length of your main Telescope. That's a real problem, because my Celestron EdgeHD 11" has a native focal length of 2800mm. One third of that, is a whopping 933.33mm.


My preference is to not add a reducer. I mean, the main reason why I purchased the EdgeHD 11" in the first place was for it's Focal Length. 


My understanding is that flexure can be mitigated, to a certain extent, by ensuring that the guide scope is mounted in a very very solid manner, and keeping the weight down as much as possible. That part is doable. But the issue of a 933mm guide scope remains. 


My Questions are:

a. Is there anything less than 933mm that would work successfully as a guide scope for the EdgeHD 11"?

b. If so, how many mm? 

c. What type of scope should I try? A Refractor? Can you recommend one?

d. Could something like the scope in the following link work? It has a focal length of 1000mm and weighs just 4.4lbs. Link: https://www.celestro...specifications 


Please don't waste your time or mine with sarcastic, rude or useless replies; or replies stating what is already known.  


Lastly, the learning curve with the OAG is starting to see slow but consistent progress. And, I haven't waved the surrender flag yet. I'm just a sucker simplicity and ease of use.


Thanks in advance.

#2 DeepSky Di

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Posted 29 November 2023 - 05:18 PM

I always keep a guide scope mounted while getting an OAG working. I can easily switch the guide camera into the guide scope although it needs an adapter on the front of the guide camera.

At first I only had a 30mm guide scope and used that on my C6. The issues I had with the C6 and OAG are probably due to the small image circle. With my Edge HD 8 plus reducer it has been much easier.

As for the reducer, it’s really about the F number rather than focal length. It will take much longer to get a decent exposure without the FR.

#3 blazek


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Posted 29 November 2023 - 05:18 PM

I believe that the key point is the image scale , rather than focal length. You should in generally be OK if your guiding image scale is around 4* imaging image scale. So, you can up to some extent compensate guiding focal length by smaller pixel of guiding camera. However, I am curious what your struggle is with OAG. If you have an OAG with large prism like Celestron OAG , and reasonably sensitive mono guiding camera, you should be OK. I have Celestron OAG and ASI 174 mini camera, and it worked from very first time . On 8" SCT though, also no reducer. For such focal length like yours, OAG is always better choice. 

#4 BucketDave



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Posted 29 November 2023 - 06:40 PM

These days with modern software, you can get decent results with a larger focal length ratio than 3.

I started with an 8" SCT, a mini guidescope and an ASI174MM camera. The focal length ratio was 16 ! It would guide down to about 1/6 of a guide camera pixel, or 1.5 arc seconds. Replacing the cheapo guidescope with an FMA180 (FL=180mm) reduced the guiding to an arc-second. Partially through longer FL and partially due to better focus.

Admittedly, the guidescope was producing guiding errors twice those of an OAG but it wasn't a disaster by any means. This was done using multi-star guiding in PHD2.

Edited by BucketDave, 29 November 2023 - 08:37 PM.

#5 kathyastro


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Posted 29 November 2023 - 06:45 PM

Believe it or not, I have successfully used an old orange-tube C-90, reduced to 630 mm FL as a guide scope for my C-11.  The reason for the reducer on the C-90 was to bring the focal ratio down from f/11 to f/7.


It was just an experiment to see it it would work and it did.  I plan to guide the C-11 with an OAG in the future.

#6 Alex McConahay

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Posted 29 November 2023 - 08:23 PM

>>>>>flexure can be mitigated, to a certain extent, by ensuring that the guide scope is mounted in a very very solid manner,


Certainly that helps when the issue is a shifting guidescope. 


But, with an SCT or other telescope that moves the main mirror, the real problem is making the main mirror secure from moving. This is much more difficult than holding a small guide scope still.


Most of all-----as long as you are not buying new stuff (or if you don't care about money-----go ahead and buy the new stuff)------just try it. See if it makes you happy through the year. 


All those guideline things  (one third the main focal length) are just guidelines. They are a place to get started. Give whatever you got a try.



#7 michael8554



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Posted 30 November 2023 - 05:47 AM

To an extent the ratio of FL's is not important.


The guiding pixel scale is.


Up to about 3arcsec/pixel is good, up to about 6 with mini guidescopes works, more than that becomes challenging.


Flexture is measured in microns, and flex of less than the thickness of a human hair is going to impact on 2800mm images.


Explain the problems you are having with the OAG.


The ASI174MM Mini Binned 2X and a 12mm prism OAG should work.


But you may need to be selective in the framing to get good guide stars:



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