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Is off axis guiding suitable for my setup?

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#1 Douwe79

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Posted 24 May 2020 - 04:02 PM

Hi all, 

 

Beginner here (1 year in AP now). 

 

I am considering switching from a guide scope to an OAG because I suspect my current setup (with guide scope) has severe flexure problems. My stars are never round and when I stack them (undithered and without aligning) the stars have shifted between images. I get elongated / egg shaped stars even with decent guiding (0.6-0.7 RMS arc-sec) and excellent PA. 

 

My focal length is 630 mm, F6.9. My guide camera in an QHY-5II-M. 

My main camera is a Canon 750D. 

My mount is a EQ6 pro.

 

My questions about OAG are: 

 

1. Is an OAG guider a good option for mu setup and do you have specific recommendations ? 

 

2. I heard an OAG can improve your guiding since image scale is the same as main telescope, is that correct? 

 

3. When using an OAG, I cannot use sharpcap for PA anymore, since you need a guide scope to PA in Sharp cap. What are the best alternatives? I was thinking about the Polemaster. I also thought about drift aligning but I don't have a good view to the south/equator. I dont know if its possible to drift align near the zenith/meridian? 

 

Thank in advance! 

 

Douwe 



#2 Cfreerksen

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Posted 24 May 2020 - 04:20 PM

You may not have enough back focus with a DSLR. The camera sucks up 55mm. An OAG will take up about another 16mm or more. What is the back focus required for your scope/flattener?

 

Chris



#3 astrohamp

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Posted 24 May 2020 - 06:19 PM

Regarding polar alignment with SharpCap:  I am easily able to PA through my 990mm fl refractor using the primary camera. No guide scope in use, just the smaller ZWO OAG and an ASI120 camera.  In my case I could remove the OAG focuser (for direct camera connect) accomplishing the OAG focusing task with stalk and washer adjustments.

You do need to work out, (measure) your current back focus situation to see if the 16.5mm thickness will fit (for a ZWO OAG).



#4 Kevin Ross

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Posted 24 May 2020 - 06:42 PM

Before you start throwing money at the problem, let's first determine why you are getting egg shaped stars. You mention the stars have shifted from one frame to the next. This is absolutely not a problem. In fact, we will intentionally induce drift between frames. It's called dithering.

 

You can't look at the final stack to make this determination. You must examine individual subs. Are the stars round in individual subs, or are they egg shaped there?

 

Also, how exactly do you have your guide scope mounted to your rig? Is it mounted a stalk that fits into the place meant to hold a finder scope?

 

Edit:

 

I re-read your original post. I see you are getting egg shapes in individual subs. So this does point back to how is your guide scope mounted.


Edited by Kevin Ross, 24 May 2020 - 06:44 PM.


#5 Douwe79

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Posted 25 May 2020 - 04:12 PM

You may not have enough back focus with a DSLR. The camera sucks up 55mm. An OAG will take up about another 16mm or more. What is the back focus required for your scope/flattener?

 

Chris

Thanks for replying. I dont own a flattener yet, but have ordered one. Dealer says my back focus is 113 mm. The camera indeed sucks up 55 mm, so that leaves 58 mm to cover. I ordered additional spacers. I guess an OAG of 16 mm would be easy to incorporate, i hope you agree with that? 

 

Douwe 



#6 Douwe79

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Posted 25 May 2020 - 04:32 PM

Before you start throwing money at the problem, let's first determine why you are getting egg shaped stars. You mention the stars have shifted from one frame to the next. This is absolutely not a problem. In fact, we will intentionally induce drift between frames. It's called dithering.

 

You can't look at the final stack to make this determination. You must examine individual subs. Are the stars round in individual subs, or are they egg shaped there?

 

Also, how exactly do you have your guide scope mounted to your rig? Is it mounted a stalk that fits into the place meant to hold a finder scope?

 

Edit:

 

I re-read your original post. I see you are getting egg shapes in individual subs. So this does point back to how is your guide scope mounted.

Thanks for looking at it from the money perspective :-). I get your point, but I did mention that I (deliberately) did not dither for testing reasons. Let me explain what I did, based on an advice of a more experienced AP-photographer. I took the first and last sub of my imaging session (undithered), then entered them in the process Pixelmath in Pixinsight, and the combination of the pictures showed shifting of the stars. The other guy who advised me had the same problem (no round stars) with a guide scope, then bought an OAG, did a (undithered) imaging session with it, and entered the first and last sub in Pixelmath: the result showed absolutely no shifting of stars. So he fixed his (flexure) problem with an OAG. 

 

The odd thing is that when i did a second test with Pixelmath today, but now with the first sub of the session and a sub from the middle of my session, again the result showed shifting but this time in a completely different direction (which could not be caused by the telescope position). 

 

My guide scope has 2 guide rings from Primalucelab. Both rings are attached with two bolts that attach to the main scope rings. This look very solid. Possible weak points ar are the plastic contact points on the guide rings, and the soft material on the inside of the rings of the main scope, which might allow some play. 

 

My train of thought was A) with an OAG I could check if it's flexure and B) they are not that expensive. 

 

IMO, with my setup, I should at least be able to get round stars in 1 - 2 min subs. My last imaging session my RA RMS was 0.18 on average and my dec RMS was like 0.13. Is that good enough? So basically  I am really happy with my mount performance, but it doesn't show in my images :-) 

 

Douwe 



#7 xiando

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Posted 25 May 2020 - 04:46 PM

I tried using an oag on my 6" F4 scope (610mm fl). It was a bust. I ended up buying a 240mmfl, F4 "regular" guide scope and secured it to the scope body to avoid flexure. It has served me well for the past few years.


Edited by xiando, 25 May 2020 - 04:46 PM.


#8 Cfreerksen

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Posted 25 May 2020 - 05:21 PM

Thanks for replying. I dont own a flattener yet, but have ordered one. Dealer says my back focus is 113 mm. The camera indeed sucks up 55 mm, so that leaves 58 mm to cover. I ordered additional spacers. I guess an OAG of 16 mm would be easy to incorporate, i hope you agree with that? 

 

Douwe 

Sure, but what kind of scope do you have and can you post a picture of the guide scope attachment?

 

Chris



#9 fewayne

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Posted 25 May 2020 - 05:40 PM

An OAG can improve guiding by reducing (not necessarily eliminating -- see below) differential flexure, and perhaps by guiding at a longer focal length. It will probably also be lighter than a guidescope and easier to balance, since its center of mass is closer to the main scope's optical axis.

 

An OAG can degrade guiding relative to a guidescope if it's peering through a higher focal ratio (dimmer), so that the signal/noise ratio is lower, or if its tiny field of view lacks good guide stars. Or if you have trouble getting it focused well.

 

Differential flexure is a lot less likely, but if the guide camera is waving about on its little stalk, or something isn't tightened down, it could happen. Haven't heard of anyone actually seeing that.

 

I'm still working at getting mine sorted; I haven't jammed enough photons through the thing to establish baseline performance. For the few hours I've spent guiding with it, it is not yet as good as the best performance I got with a guidescope.

As a newb, I'm happy to report that the setup was a lot less tricky than its reputation. One minor perk is that you only need one Bahtinov mask. I got a ZWO OAG used from a Cloudy Nighter, no complaints about it. I went from a finder-shoe mounted scope to an improvised scope-top mount, and then to an OAG largely because I was sick of looking at my improvised mount.




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