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Old 10-10-2018, 12:06 PM
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LewisM
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A serious discussion on OAG vs gudescope

I have been using a mini-guidescope for many years. I currently use a Takahashi 7x50 finder with the visual back removed and a non-rotating Borg helical focuser installed instead. I use a Lodestar (original version), and I use Baader finder rings as my rings (allows movement, and VERY precise fit so no slopping or looseness in the rings for the screw jacks). I use a Vixen SXP mount.

I get perfectly round stars each and every time, irrespective of sub-length - a testament not only to the impressive precision of the mount, but also shows - in my opinion - no movement in the guidescope arrangement.

I recently bought a mono CCD. I also bought a Starlight Xpress filter wheel, with integral OAG to go with it. First time I used the OAG I got PLENTY of stars without even having to fiddle or move the prism, just a slight focus tweak. Tracking and imaging with it ellicited precisely the same result as the guidescope image - round stars, time after time. Dithered by 7 pixels per shot now, for both the OAG and guidescope.

So, is there any GENUINE benefit to using exclusively OAG if the system as is produces the same results with a guidescope as with an OAG? Is the hold-over of "OAG is better" only relevant in terms of the "ancient" style of using a large guidescope between widely spaced guide rings?

I know guiding at f/5 is easier than f/8 and above, but is there genuinely any benefit to using OAG in a sagless guidescope system?

For now, my filterwheel OAG has a C thread cap on it as I continue using the finder-guider
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Old 10-10-2018, 12:15 PM
Imme (Jon)
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I'm in a similar situation as you Lewis. I've used guidescope for a while and recently put on an OAG.....I saw no improvement at all.
My stars were just as round.
My tracking was just as good.
My finished product didn't change.

I guess I could of stayed with the OAG but for reasons of 'comfortableness' (I swear it's a word) I reverted back to the guidescope......and the fact I liked the look of the smaller/rounder stars in PHD using the guidescope.

I'm with you.....I think as long as the guidescope is securely fixed to the imaging scope then there is no discernible improvement using an OAG

My setup is an F7 imaging scope and 280mm ZWO guidescope
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Old 10-10-2018, 12:21 PM
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multiweb (Marc)
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When you use a quide scope even if it's bolted and has zero flexure you still can get flexure between the guidescope and the main imaging telescope. If it's a refractor then it's less likely but if it has a mirror and depending on its mounting, size and weight YMMV. Now the only way to get rid of flexure 100% is to guide through the same light path you're imaging.

If you use a guidescope on a celestron SCT you will likely get flexure because the primary will move. If you do the same on a big newt same thing. If you image with a scope that is very tight mechanically then you might get away with it.
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Old 10-10-2018, 12:23 PM
Imme (Jon)
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Quote:
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When you use a quide scope even if it's bolted and has zero flexure you still can get flexure between the guidescope and the main imaging telescope. If it's a refractor then it's less likely but if it has a mirror and depending on its mounting, size and weight YMMV. Now the only way to get rid of flexure 100% is to guide through the same light path you're imaging.

If you use a guidescope on a celestron SCT you will likely get flexure because the primary will move. If you do the same on a big newt same thing. If you image with a scope that is very tight mechanically then you might get away with it.
Good point Marc....definitely see the benefit if you are using something with a mirror in it and things that can flex/move.
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Old 10-10-2018, 12:33 PM
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Merlin66 (Ken)
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In spectroscopy we use a reflective slit for guiding on the target star. This is equivalent to an on-axis guider.
Iíve used this slit guiding on telescopes from the Genesis (100mm f5) to the C11 at f10.
I use the Lodestar and never have any issues.
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Old 10-10-2018, 12:58 PM
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Quote:
Originally Posted by Imme View Post
I'm in a similar situation as you Lewis. I've used guidescope for a while and recently put on an OAG.....I saw no improvement at all.
My stars were just as round.
My tracking was just as good.
My finished product didn't change.

I guess I could of stayed with the OAG but for reasons of 'comfortableness' (I swear it's a word) I reverted back to the guidescope......and the fact I liked the look of the smaller/rounder stars in PHD using the guidescope.

I'm with you.....I think as long as the guidescope is securely fixed to the imaging scope then there is no discernible improvement using an OAG

My setup is an F7 imaging scope and 280mm ZWO guidescope
Definitely comfortableness Whilst the OAG with my system yields quite a lot of stars even in a seemingly dim field, I just prefer the guidescope that gives me an absolute wealth of stars to pick from, even using a 50mm finder as the guidescope. Plus I like being able to use the guidescope as an electronic finder too - negates the need for an optical one (saves even more weight).
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Old 10-10-2018, 03:07 PM
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multiweb (Marc)
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I'm going to start a change.org page for all the neglected OAGs out there.
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Old 10-10-2018, 04:38 PM
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codemonkey (Lee)
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Technically speaking you can still get differential flexure when using an OAG, but it's due to either user error (prism stalk not secured), or manufacturing defect. I've noticed the former before.

Anyway, don't fix what ain't broken, Lewis. It's the results that count in this case and you should do whatever works for you and you're most comfortable with.

Just be aware that with different kit you'll have better success... as had been said, guide scopes and floppy mirrors don't tend to mix well. Another consideration is the sampling rate of the guide scope... yes, guiding software does do subpixel centroiding, but how well is it really going to work if you're getting 10"/px through a tiny guide scope?

Again though, if it works, keep doing it. This hobby is difficult enough without "fixing" things that work for the sake of dogma.
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Old 10-10-2018, 05:35 PM
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I am also concerned that, seeing I am strictly portable setup only, that I won't get a good enough FOV with the OAG to use Sharpcap. Keeping the guidescope seems the most sensible and feasible option.

I'll keep the OAG capped
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Old 10-10-2018, 05:44 PM
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Atmos (Colin)
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Long exposures with a SCT and guide scope is just asking for trouble. Mirror flop and the differing cooling flexure of the guide scope/telescope tube against the optical path will cause problems.
On your FSQ with everything bolted together it is far less of an issue
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Old 10-10-2018, 06:09 PM
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Quote:
Originally Posted by Atmos View Post
Long exposures with a SCT and guide scope is just asking for trouble. Mirror flop and the differing cooling flexure of the guide scope/telescope tube against the optical path will cause problems.
On your FSQ with everything bolted together it is far less of an issue
Ah! So THIS is why there are no good images from an SCT!




[it's a joke...don't have a brain spasm]
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Old 10-10-2018, 06:36 PM
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Ultimate test would be measuring FWHM and eccentricity for stars in data sets acquired with OAG and a GS*. But as others have already said, if you are happy with the guidescope then the Newton's 1st Law applies - keep doing what you have already been doing.


* To be politically correct, I feel that Guide Scopes also deserve an abbreviation, same as Off Axis Guiders.
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Old 10-10-2018, 06:55 PM
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I'm getting noticably better performance from my OAG than I was from my GSG. I think the SNR and pixel scale plays a big part in that. I also no longer see the long term creep of the image due to differential flexure.
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Old 10-10-2018, 08:05 PM
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At long focal lengths a guide scope is much less likely to perform well. It's not impossible to make one work, but it is difficult to reduce flexure sufficiently for good performance. Large primary mirrors that allow for a degree of movement, as has been mentioned, also don't mix well with guide scopes. If you lock the mirror in place then you are likely to cause other optical problems.

Cheers,
Rick.
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Old 10-10-2018, 10:03 PM
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I would have loved to use an OAG on my Newt years ago, but backfocus becomes a killer without high quality thin OAG and correct fittings. You also have more weight coming perpendicular from the scope which can be an issue depending on focuser and tube strength around the focuser.

For refractors, you usually have closer in centre of gravity with OAG. For my chinese mounts (EQ6, HEQ5) get far better stars (eccentricity, FWHM) than through a guidescope (which I thought was reasonably well attached). Pain not being able to easily use Sharpcap though.

I'd hate to be having to reliably find guidestars through OAG over 1.5m FL - unless you put a rotator into the mix - then more weight, fittings, complexity.

Big variations in quality/tightness of OAGs too between models/brands.
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Old 11-10-2018, 01:01 PM
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I think it depends on focal length and the type of scope. A mirrored scope may be more problematic than a refractor.

A short focal length scope like an FSQ is a lot easier but still can have problems.

A guide scope on a 3 metre CDK can work but now round stars are not guaranteed and results vary from elongated stars to slightly eggy and occasionally round.


So if you are planning on upgrading to a longer focal length scope you may get different results.

But of course if what you are using is working there is no need to do anything.

I have used a Vixen 95 VMC scope as a guide scope on my 17 inch CDK when I was unable to use an OAG due to using the reducer with its practically no backfocus. I got round stars often and other times they were not perfectly round.


Greg.
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Old 14-10-2018, 03:18 PM
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I have been using OAG's for years now. I moved from a guide scope and have never looked back. It has worked with long focus scopes and very short focus ones too. Examples of both are, the RC12 and FSQ106.

Guiding is made so much easier by having one in place. What I found troublesome with a guide scope was not so much the actual means of fixing it to the imaging scope but the draw tube of the guide scope's focuser. This would flex just a little and that would cause differential flexure.

I only changed because I wanted to solve a problem. If you don't have that problem then you don't need to change. However, a mobile setup will benefit from using an OAG as you don't need to carry a guide scope. With the FSQ I would often see 30 or more stars with a 4 second guide exposure. So I can't see why you would not use one if you wanted to reduce setup time and not carry a guide scope.
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Old 14-10-2018, 05:58 PM
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Paul, my guidescope is 20cm long, with a non-rotating helical focuser - no sag anywhere. No issue demounting and remounting the guidescope either - see picture (finder shoes with a long finder dovetail secured to the finder rings). I have demounted and remounted the guidescope probably 20 or 30 times, and I have never had to readjust the alignment - it maintains alignment each and every time.

Main reason I keep it now is for polar alignment with Sharpcap seeing it is a portable setup (and a genuine dislike of the QHY Polemaster). I will try to see if the OAG will be useable for polar aligning, but I have my doubts. The FOV last time I used it was so small as to not show Canopus in the OAG frame at all, despite being perfectly centred in the main camera and guidescope. Plenty of stars - more than enough for guiding - just not for PA
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Old 14-10-2018, 06:36 PM
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multiweb (Marc)
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How's your tak finder mounted? Is that some kind of quick release?
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Old 14-10-2018, 06:40 PM
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Lewis you can use the OAG for Drift Alignment, using the PHD2 drift alignment wizard. This is what I do and its accurate, but probably takes a bit longer than what you are used to with the Sharpcap procedure.
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