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C9.25 SCT - Guide Scope versus OAG

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#1 KTAZ

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Posted 26 May 2020 - 11:21 AM

Hello all. I am working on a decision for a guiding solution for my C9.25 SCT. I intend to buy the ASI294MC Pro camera. I now need your advice helping me to decide between a guide scope or OAG.

The Celestron OAG has a mix of good and bad reviews as expected; however, I was leaning in this direction since it is complimentary to my Celestron SCT and has a large prism. My primary concern is flexibility. Based on seeing conditions and target selection, there will be times when a corrector is, or is not, warranted. While the OAG may reduce errors from differential flexure and mirror flop, I seem to lose flexibility as it appears that using a corrector/reducer or barlow does not work with an OAG.

With respect to a guide scope, I was looking at the Stellarvue F50 50mm Guidescope W/Straight-Through Helical Focuser. I am wondering if the 210mm FL of this guidescope is sufficient for a 2350mm FL SCT (or 1480mm if using the C/R).

 

Any feedback or thoughts would be appreciated.



#2 nimitz69

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Posted 26 May 2020 - 11:24 AM

I’ll defer to others for specific brand advise but for long focal length imaging like with your 9.25” SCT an OAG is always the best solution

Edited by nimitz69, 26 May 2020 - 11:25 AM.

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#3 wrnchhead

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Posted 26 May 2020 - 11:29 AM

Funny, I was just watching James Lamb's video on this very thing https://www.youtube....h?v=8VaEncw9RMw

He has tons of other videos about the 9.25 and guiding, well worth watching. 


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#4 Alex McConahay

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Posted 26 May 2020 - 11:29 AM

The one job of a guide camera is to determine if the main camera is still pointing to the same place as it was in the last guide shot. If the guide camera is not in a relatively fixed position, it cannot do its job. "Relatively Fixed" does not mean "pretty near fixed." It means "Fixed in RELATION to" the main camera. 

 

One needs an unflexing, unchanging mounting, focuser, and all that between the main camera and the guide camera to pull this off. 

 

It is possible to have this with some telescopes. It is even possible to have this some of the time in an SCT. But since most SCT's are built with a mirror that flops, it it highly unlikely. 

 

Therefore, in the long run, you will find that an OAG or an ONAG will give you better guiding. 

 

Which OAG I shall leave it up to the equipment geeks. 

 

I do not see why you say that OAG's cannot work with a focal reducer. They take some adjusting of the prism position, but they should work. 

 

Alex



#5 MikiSJ

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Posted 26 May 2020 - 11:33 AM

I am currently learning how to use my new kit which is a C11EdgeHD. I read as much as I could regarding guiding as using my scope visually is a no-go.

 

I think the difference comes down to ratios - the ratio of the guide scope focal length to the imaging scope focal length. With the C9.25's f/l of 2350mm it would need a guide scope of at least 1/3 or 1/2 or 750mm or 1,200mm. This would be hard to accomplish with a guide scope.

 

On other hand, I bought the Celestron OAG which will allow me to guide at a ratio of 1/1. I also am imaging with an ASI294MC-Pro. I found that with the available connectors supplied with the Celestron OAG I was able to get to a focus easily.

 

Another plus about the Celestron OAG is that it has 12.5 x 12.5mm pickoff mirror. Most of the more affordable OAGs on the market do not have a pick-off mirror this size. The sensor of the ASI174MM-mini nearly fills the horizontal width of the Celestron OAG.

 

I would go with the OAG and forget the separate guide scope. Also, the Celestron OAG is designed to work with the Celestron F/L reducer.


Edited by MikiSJ, 26 May 2020 - 11:35 AM.


#6 nitegeezer

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Posted 26 May 2020 - 11:40 AM

Your concern about a reducer is a valid one. When I added a reducer, my guiding did suffer a bit. The reducer shrinks the imaging circle, and since an OAG works at the edge, the stars were very distorted. Fortunately my scope has a mirror lock so I don't have an issue with mirror flop. Without a lock, you may not have a choice other than an OAG. Hopefully, recent software may deal better with the distortion. I am using an older version of PHD2 but with an older laptop I don't want to change things that aren't broke!

#7 AhBok

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Posted 26 May 2020 - 11:43 AM

A guide scope can work with a C9.25 if it is rigidly mounted. (See my Astrobin page listed in my sig). However, I have used a ZWO OAG and have recently moved to it full time. However, my camera is a 183, so my stars are round and pretty tight. With the 294 and the C9.25 and r/c, you will need every advantage you can to get good stars. The ZWO OAG works for me because my main imaging chip is relatively small and I can lower it’s stalk a good bit before intruding on the optical axis. With the 294, you will benefit from the larger prism of the COAG if you use a guide camera that has a ledge enough chip to take advantage of the larger prism (such as the 174).

#8 KTAZ

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Posted 26 May 2020 - 01:21 PM

I do not see why you say that OAG's cannot work with a focal reducer. They take some adjusting of the prism position, but they should work. 

As a newb to this topic and the OAG in general, I should include the disclaimer that anything I say might be wrong! I read somewhere on the Internet that Internet research can be less than 100% accurate...cool.gif

 

As I read through some replies, it sounds as if the R/C should go directly on the back of the scope, then the OAG, and then the camera, not vice-versa. This would keep the distance between the OAG and both cameras minimized, correct?

 

I guess that would also apply to filter wheels, or just filters, as well? Place them upstream of the OAG, not downstream.



#9 Alex McConahay

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Posted 26 May 2020 - 01:52 PM

Filter wheels go behind the OAG. Otherwise you will have light passing through the filters before going to the guide camera. This dims the light, and is something to avoid. 

 

Usually (from the back) it is main sensor, filters, pickoff mirror (OAG), spacers as necessary, adjusting optic (flattener, reducer, Barlow, etc.), and focuser.  In some rigs (that omit a rear mounted focuser) you may be attaching directly to the back of the telescope, and adjusting focus internally. 

 

Spacing is critical for some of these placements. Remember, the light from infinity is being refracted or reflected into a cone converging on perfect focus. The sensor is at the point of perfect focus. Since you are dealing with a cone, the further away from the main sensor you are, the larger it is. So, filters have to be bigger than the cross-section of the imaging plane. The further they are from the imaging plane, the bigger they have to be. (If they were a focal length away, for instance, they would have to be the full aperture of the main scope!!!).  A focal reducer broadens the cone more quickly while a barlow makes it less steep.  

 

Spacing for most correctors, and some other optics are critical because they are engineered to work with a specific back focus---a distance at which all the adjustments are made to come together. 

 

With focal reducer, which is often a flattener, the image circle can be reduced. Therefore, there is not as much light on the periphery. You can push the pickoff mirror further down the stalk, thus compensating for the smaller image circle. 

 

Alex


Edited by Alex McConahay, 26 May 2020 - 01:54 PM.


#10 KTAZ

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Posted 31 May 2020 - 11:01 AM

Thanks to all for the information. A few more questions regarding back focus.

 

I will be using the Celestron .63 R/C which does not seem to have any published dimensional specs as far as I can see (pretty dumb). As I try to configure the OAG setup, I have the following information:

 

Stated backfocus of the 9.25 is 139mm. I have an assumed depth for the R/C of 54mm (based on a measurement from another person), SCT Adapter to OAG 25.3mm, OAG 29mm, Male 42mm T Adapter 12.5mm, and ZWO stated BF to camera of 6.5mm. That leaves 11.7mm that I need to make up in spacers (after OAG), assuming no filters or filter wheel.

 

I've read many posts on these forums stating that the A) back focus calculation does not have to be precise (you just get a slightly different reduction factor); and B) Back focus must be perfect. It obviously cannot be both.

 

Am I ok with either 10mm, 11mm or 12mm of spacers? Or must I go and find some exotic combination that will give me precisely 11.7mm?



#11 CapnRon

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Posted 31 May 2020 - 07:54 PM

The Celestron .63 Focal Reducer has a 105 mm back-focus requirement on the SCT 9.25.




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