Considering a Guide Scope - Never used one - Forums

I am very new to astronomy and astrophotography. For just over a year, I have been trying my hand at this astrophotography "hobby". Getting sucked deeper and deeper into this ever-growing desire to get better and it is rapidly developing into an obsession. Everyone on here knows what I mean. There is always something new or something to make the product better. Anyway, I am considering a guide scope or maybe OAG. Never used one. Usually keep my exposure time at 120 seconds so I am not sure I even need to pursue this. However, I decided to post this because of the wealth of knowledge on this site. Knowledge is a very valuable thing.

So, these are the questions or things that makes me consider this. Maybe others need this information too:

1. Do longer exposures typically yield more detail or is it truly the total exposure time that matters?
2. Would a guide camera help with 120 second exposures or maybe the exposure time is not long enough to matter? (Considering going to longer exposure time).
3. I am considering a guide scope for an AT115EDT. I don't want to go 'CHEAP", but I want to have a dependable system. Considering a 60mm guide scope. Not sure of the camera yet. If you have any experience with guiding, I will certainly value your input on this one.

Thanks for any help or information you may have.

Tom

Tom Behel:
1. Do longer exposures typically yield more detail or is it truly the total exposure time that matters?
2. Would a guide camera help with 120 second exposures or maybe the exposure time is not long enough to matter? (Considering going to longer exposure time).
3. I am considering a guide scope for an AT115EDT. I don't want to go 'CHEAP", but I want to have a dependable system. Considering a 60mm guide scope. Not sure of the camera yet. If you have any experience with guiding, I will certainly value your input on this one.

1. Mathematically yes. Practically, sometimes, it depends on the target and your sky conditions. You don't want to clip the stars unless you're doing separate star exposures and blending.
2. Absolutely you would benefit from guiding, even if you don't increase your sub exposure duration. Your AM5 mount has very significant periodic error and you will achieve substantially better results if you correct for it with guiding.
3. A guide scope or an OAG would both generally work fine with the AT115EDT. The quality of the optics of a guide scope are largely immaterial. A 60mm scope is plenty. Spend your time/attention/money on making sure the guide scope is attached very rigidly. Differential flexure (the scope and the guide scope flexing differently due to gravity or other forces at a sub-arcsecond scale) is the main challenge of using a guide scope successfully. You have a relatively short focal length, so it's unlikely to be a problem, but rigidity is always best.

For a small guide scope most guide cameras will be fine. There will be plenty of guide stars even on a small chip camera because the view is quite wide. You'd prefer a guide camera with small pixels since you'll have a short focal length on the guide scope. Since you seem to like the ZWO ecosystem, a used ASI290MM mini (discontinued) would be a good and inexpensive choice. This isn't going to be too critical with a small guide scope, whatever you decide on will probably be fine, with a bias towards smaller pixels.

I also have an AT115EDT, I originally tried to use my smaller 30mm and 40mm scopes and my guiding was mediocre, I moved to a 60mm guide scope and it works very well when paired with the 115,

I image similar to you, I only take 90-120 second images, I do get better images when I guide, and I get to keep more subs vs non guided. I would suggest making guiding part of your setup, it will allow you to go longer too. I have taken 5 minute subs before, but I am not experiences enough to see a difference between 12x5 min subs vs 30x2 min subs, I am sure there is a difference but I am not experienced enough to see it yet.

To answer your questions with my opinions

1. Do longer exposures typically yield more detail or is it truly the total exposure time that matters?
I have not noticed a difference, for me total integration time seems to be more important that longer exposures.

2. Would a guide camera help with 120 second exposures or maybe the exposure time is not long enough to matter? (Considering going to longer exposure time).
I have noticed improvement in my images when I guide, even for short 30 second exposures. I recommend guiding.

3. I am considering a guide scope for an AT115EDT. I don't want to go 'CHEAP", but I want to have a dependable system. Considering a 60mm guide scope. Not sure of the camera yet. If you have any experience with guiding, I will certainly value your input on this one
I have purchased the PrimaLuce 60mm guide scope, its OK, my biggest hang up is that my guide scope and camera backspacing seems excessive my camera hangs off the back further than I would like, but its really just a minor complaint.

Unrelated to your questions, but worth sharing,

I upgraded from the asi120mm mini to the asi220mm mini when I got this new guide scope, I was super concerned about the pixel scale, because the 220 has larger pixels. My guiding RMS actually improved with the 220, this is because I have shorter exposures due to sensitivity, this may or may be your experience, I am using an AM5 mount which loves short guide exposures for performance, but other mounts may not get the same benefits.

Just my experience

Generally, an OAG will produce better results than a guidescope. This is because you are guiding on the same light path that the camera is experiencing. Guidescopes experience differential flexures that can cause the guide corrections to be different from the correction actually needed for the image. If your image scale is small enough the difference in results between the two options will not matter.

For some reason, the thinking is that using a guidescope is the simpler/easier option. I disagree with that thinking. Here's why:

1) Guidescopes add more new weight than OAGs and that weight is farther from the RA gear giving it more impact.
2) Guidescopes must be separately focused. The OAG is refocused automatically with each main camera focus. You focus it once to match the main camera and you are done.
3) Differential flexures in the mounting of the scope and the focuser sag need to be considered with Guidescopes

The additional complexities of using and OAG are:

1) The OAG takes up backfocus so you will need to alter your spacers to maintain the correct backfocus distance after adding the OAG.
2) You need to set the pick-off prism distance to be outside the main camera FOV but inside the light cone.

Both of these are one time setup tasks.

I agree with everything Kyle noted above. Just wanted to promote the idea of OAG since many of us tried the guidescope first and could have saved money if we just went straight to OAG.

Kevin

I agree with Kevin and with Kyle,

You will laugh but the reason I stopped using my OAG and sold it, was that it didn't fit on my scope while I stored it in the case, so I have to re-focus everytime I setup. (I know that silly), but I can confirm I got better guiding with an OAG too.

Clayton Ostler:
I agree with Kevin and with Kyle,

You will laugh but the reason I stopped using my OAG and sold it, was that it didn't fit on my scope while I stored it in the case, so I have to re-focus everytime I setup. (I know that silly), but I can confirm I got better guiding with an OAG too.

Why didn't you just get a bigger case.

Bob Lockwood:
Clayton Ostler:
I agree with Kevin and with Kyle,

You will laugh but the reason I stopped using my OAG and sold it, was that it didn't fit on my scope while I stored it in the case, so I have to re-focus everytime I setup. (I know that silly), but I can confirm I got better guiding with an OAG too.

Why didn't you just get a bigger case.

That would be practical.

Kevin Morefield:
Generally, an OAG will produce better results than a guidescope. This is because you are guiding on the same light path that the camera is experiencing. Guidescopes experience differential flexures that can cause the guide corrections to be different from the correction actually needed for the image. If your image scale is small enough the difference in results between the two options will not matter.

For some reason, the thinking is that using a guidescope is the simpler/easier option. I disagree with that thinking. Here's why:

1) Guidescopes add more new weight than OAGs and that weight is farther from the RA gear giving it more impact.
2) Guidescopes must be separately focused. The OAG is refocused automatically with each main camera focus. You focus it once to match the main camera and you are done.
3) Differential flexures in the mounting of the scope and the focuser sag need to be considered with Guidescopes

The additional complexities of using and OAG are:

1) The OAG takes up backfocus so you will need to alter your spacers to maintain the correct backfocus distance after adding the OAG.
2) You need to set the pick-off prism distance to be outside the main camera FOV but inside the light cone.

Both of these are one time setup tasks.

I agree with everything Kyle noted above. Just wanted to promote the idea of OAG since many of us tried the guidescope first and could have saved money if we just went straight to OAG.

I agree with Kevin, for the record, that an OAG is superior. For short focal length the two tend not to differ much in their results, though. I'm neutral on whether I think a guide scope or an OAG is simpler/easier. If an OAG will fit in your backfocus easily enough and you don't already have a guide scope then the OAG may well be the simpler course.

Thanks to everyone that has responded. Great information. I was considering the OAG route. I have read where some people discuss an issue of fitting it in the imaging train. I agree that maybe the right way to go.

I leave my imaging train set up most of the time. Some have mentioned an AM5 mount in this discussion. I have an AM5 mount but mostly use this telescope EQ6R Pro mounted on a pier.

I think maybe I need to look at the OAGs out there and just got that route to begin with.

This has been a lot of good info.

Thanks.

Tom Behel:
I am very new to astronomy and astrophotography. For just over a year, I have been trying my hand at this astrophotography "hobby". Getting sucked deeper and deeper into this ever-growing desire to get better and it is rapidly developing into an obsession. Everyone on here knows what I mean. There is always something new or something to make the product better. Anyway, I am considering a guide scope or maybe OAG. Never used one. Usually keep my exposure time at 120 seconds so I am not sure I even need to pursue this. However, I decided to post this because of the wealth of knowledge on this site. Knowledge is a very valuable thing.

So, these are the questions or things that makes me consider this. Maybe others need this information too:

1. Do longer exposures typically yield more detail or is it truly the total exposure time that matters?
2. Would a guide camera help with 120 second exposures or maybe the exposure time is not long enough to matter? (Considering going to longer exposure time).
3. I am considering a guide scope for an AT115EDT. I don't want to go 'CHEAP", but I want to have a dependable system. Considering a 60mm guide scope. Not sure of the camera yet. If you have any experience with guiding, I will certainly value your input on this one.

Thanks for any help or information you may have.

Tom

1. No, as long as the sky noise is larger than read-out noise. If not, it depends. In general I can hardly find any difference.
2. It depends on your mount. If your tracking is good enough as it is then don't bother with autoguiding.
3. 60mm will be fine. keep a pixel scale no less than 5x that of imaging camera.

Tom Behel:
Thanks to everyone that has responded. Great information. I was considering the OAG route. I have read where some people discuss an issue of fitting it in the imaging train. I agree that maybe the right way to go.

I leave my imaging train set up most of the time. Some have mentioned an AM5 mount in this discussion. I have an AM5 mount but mostly use this telescope EQ6R Pro mounted on a pier.

I think maybe I need to look at the OAGs out there and just got that route to begin with.

This has been a lot of good info.

Thanks.

The guiding parameters would vary a bit with the EQ6R instead of the AM5, but the bottom line of guiding getting you a better result is no different, so I would continue pursuing this path.

I didn't see what main camera you are using. One issue to consider with an OAG is that for larger sensors (i.e. full-frame), if you don't have a pretty large illumination circle, the pick-off prism be so far off the optical axis that the light received by the guide camera is significantly reduced, potential making it hard to find guide stars. This can be an issue with the AM5 since it generally works best using fast exposures (0.5s). Also, the stars that far off-axis might not be perfect; that in itself might not be a killer, but it does exasperate the situation. If you are using a smaller sensor (APSC or less) these issues are typically not a problem. FWIW, I have a 115mm APO and use a 60mm guide scope with a ASI290mm-mini and it works great (using an AM3). I have a OAG on my Edge-11 with a full frame camera, and have to deal with these issues I described constanty, which is how I know about them!

Clayton Ostler:
I agree with Kevin and with Kyle,

You will laugh but the reason I stopped using my OAG and sold it, was that it didn't fit on my scope while I stored it in the case, so I have to re-focus everytime I setup. (I know that silly), but I can confirm I got better guiding with an OAG too.

Ha! Well I may make some folks queasy, but the way I always transported my refractor was to slide it off the mount completely wired and assembled, then set it on a pillow inside a large tray with 4 inch walls. I'd use toilet paper (needed for the camping) and shipping Styrofoam to stabilize everything for the drive. I'd do that in reverse the next month to the dark site. I only had to plug in and out one power cable and one USB to get going. The think even held its focus spot!

Honestly, breaking the thing down enough to get it in the cases would have really been some wear and tear over the 10 years I used the scope in the field.

Kevin Morefield:
Clayton Ostler:
I agree with Kevin and with Kyle,

You will laugh but the reason I stopped using my OAG and sold it, was that it didn't fit on my scope while I stored it in the case, so I have to re-focus everytime I setup. (I know that silly), but I can confirm I got better guiding with an OAG too.

Ha! Well I may make some folks queasy, but the way I always transported my refractor was to slide it off the mount completely wired and assembled, then set it on a pillow inside a large tray with 4 inch walls. I'd use toilet paper (needed for the camping) and shipping Styrofoam to stabilize everything for the drive. I'd do that in reverse the next month to the dark site. I only had to plug in and out one power cable and one USB to get going. The think even held its focus spot!

Honestly, breaking the thing down enough to get it in the cases would have really been some wear and tear over the 10 years I used the scope in the field.

I can picture you telling your significant other on a camping trip to stop using so much Toilet paper so your scope is better protected.

Rick Krejci:
Kevin Morefield:
Clayton Ostler:
I agree with Kevin and with Kyle,

You will laugh but the reason I stopped using my OAG and sold it, was that it didn't fit on my scope while I stored it in the case, so I have to re-focus everytime I setup. (I know that silly), but I can confirm I got better guiding with an OAG too.

Ha! Well I may make some folks queasy, but the way I always transported my refractor was to slide it off the mount completely wired and assembled, then set it on a pillow inside a large tray with 4 inch walls. I'd use toilet paper (needed for the camping) and shipping Styrofoam to stabilize everything for the drive. I'd do that in reverse the next month to the dark site. I only had to plug in and out one power cable and one USB to get going. The think even held its focus spot!

Honestly, breaking the thing down enough to get it in the cases would have really been some wear and tear over the 10 years I used the scope in the field.

I can picture you telling your significant other on a camping trip to stop using so much Toilet paper so your scope is better protected.

That's funny right there. "Hey Babe, just use leaves my scope needs that paper"

Richard Carande:
I didn't see what main camera you are using. One issue to consider with an OAG is that for larger sensors (i.e. full-frame), if you don't have a pretty large illumination circle, the pick-off prism be so far off the optical axis that the light received by the guide camera is significantly reduced, potential making it hard to find guide stars. This can be an issue with the AM5 since it generally works best using fast exposures (0.5s). Also, the stars that far off-axis might not be perfect; that in itself might not be a killer, but it does exasperate the situation. If you are using a smaller sensor (APSC or less) these issues are typically not a problem. FWIW, I have a 115mm APO and use a 60mm guide scope with a ASI290mm-mini and it works great (using an AM3). I have a OAG on my Edge-11 with a full frame camera, and have to deal with these issues I described constanty, which is how I know about them!

Thanks Richard,

I use either ASI385MC or ASI294MC for main camera, depending on the target. I am thinking I will try an OAG with ASI220MM mini for guiding.

I have not tried OAG sofar and am using a 60mm Guide Scope with ASI 120 MC as Guide camera along with my AM5N mount.

While initially the guiding with ASI Air was bit of any issue but through trial and error was able to get this resolved. Now guiding is decent all across and am trending at overall error of 0.5". This is pretty decent and am not sure if an OAG will significantly improve the guide performance. Would request feedback on comparison of performance using Guide scope+Camera vx OAG.

Kevin Morefield:
2) Guidescopes must be separately focused. The OAG is refocused automatically with each main camera focus. You focus it once to match the main camera and you are done.

Kevin

Im a solid OAG advocate, so I agree to 90% of what you said... This however, I have found that I can set a guide scope with reasonably low (ie, sub 400mm) focal lengths once, and not touch it for MONTHS with out issue.

The OAG on the other hand, I sometimes get issues because my OAG focus point is matched to my L filter. My OIII filter, however, focused 52 focuser steps further inward than the L filter. Under most circumstances, this is fine, but sometimes, this is just far enough out that the OAG needs a slight tweak.

For focusing, especially if the OP decides in the future to go down the mono route, certainly does not consititute a pro or con for either a guide scope or OAG. I find it to be something that affects both setups in different ways.

Alex.

Aloke Palsikar:
I have not tried OAG sofar and am using a 60mm Guide Scope with ASI 120 MC as Guide camera along with my AM5N mount.

While initially the guiding with ASI Air was bit of any issue but through trial and error was able to get this resolved. Now guiding is decent all across and am trending at overall error of 0.5". This is pretty decent and am not sure if an OAG will significantly improve the guide performance. Would request feedback on comparison of performance using Guide scope+Camera vx OAG.

The difference in using a guide scope vs an OAG is not to be seen in the guiding stats. In either case that will depend generally on the seeing and the mount quite a bit. The difference is in the images. If you're guiding with a guide scope and the FWHM in the subs seems higher than it should be or especially if the guiding graph looks good, but the eccentricity in the subs is high (> 0.4) then you may have differential flexure, which could be improved by using an OAG rather than a guide scope. With a guide scope what you're guiding is the guide scope itself. If there's flex between the guide scope and the main scope then the main scope isn't doing exactly what the guide scope is. So you can have excellent guiding on your guide scope, but not get those results in the final images. The OAG approach has you guiding the main scope directly. Whatever the OAG see is the same thing the imaging camera sees (just a different part of the field), so it's inherently more robust as you're guiding the thing that's imaging directly.

Considering a Guide Scope - Never used one

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