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Old 04-10-2015, 12:06 PM
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LewisM
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Matching FL guidescope

OK, if OAG is considered the bee's knees because it uses the main scope's optics and focal length, would second best be using a guidescope that matches the main scope's focal length and approximating aperture?

I currently use a converted 9x50 finder with a helical focuser that is a little faster in focal ratio than the FSQ106N. Was thinking if I went and used the ST80 which is f/5 and closer in aperture (therefore more precise guiding anyway), would I see any advantage?

Below is a 100% crop of my test image on NGC 246 - 45 minutes stacked worth. Slight elongation in the stars which I am attributing to consistent passing clouds leading to very slight tracking errors while the guiding stops while the cloud passes (the mount tracks extremelly well, but of course will likely show minute trailing, especially considering my polar alignment was off by a couple minutes). Regardless, using PHD2 I was getting guiding/tracking continually HALF of the 0.25 inner circle (1/8th of a pixel guiding?)

Just wondering if I can further enhance guiding, WITHOUT using an OAG (I have tried many times using an OAG and just don't like them - just my preference.
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  #2  
Old 04-10-2015, 12:31 PM
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Atmos (Colin)
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I imagine that it has a lot more to do with pixel scale than it does focal length or aperture. One of the major advantages of guiding with an OAG is that you can guide at a pixel scale smaller than what you're imaging at. Therefore an 1/8th of a pixel guiding movement becomes even less in the imaging camera, especially when dealing with the larger sensors like the 11002 or 16803.
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Old 04-10-2015, 12:35 PM
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My 2c would be guidescope performance is more dependent on the stability of the OTA, guide optics and mountings. For 2 refractors, hopefully you won't have any issues with optics moving (as is possible in Newts and SCTs). That leaves the quality of rings and attachments as well as optical train for both refractors.

I guided using a WO ZenithStar 80mm piggybacked to my FSQ for 18 months with good quality (WO) rings and dovetails. Changed to OAG 9 months ago and stars are way better.
Prior to using the ZS80 I used a 50mm guidescope in good quality rings - can't really say there was any benefit with the longer FL - although it did look nicer


Your experience my differ from mine......
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Old 04-10-2015, 01:09 PM
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RickS (Rick)
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Most guiding software does a pretty good job of measuring the guide star centroid to a small fraction of a pixel, so it's fine to have a guide image scale that's less than your imaging scale. The benefit of an OAG is that it sees through the main scope optics so flexure is eliminated, or at least greatly reduced, and mirror movement (assuming it's not a refractor) affects both guide and imaging fields and guiding can compensate for it.

At short focal lengths, especially with refractors, a guide scope should be fine so long as some care is taken with the mechanical linkage between guide scope and main scope.

Cheers,
Rick.
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Old 04-10-2015, 05:48 PM
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I'm just going to second what Rob and Rick have said; OAG is all about eradicating flexure. In my experience it's worth it even with small refractors.

Not sure what your dislike for OAG is, maybe you've just used some bad ones? There's a lot out there now and they can be fiddly.

I've used a few OAG now and believe it or not, the Celestron is great: helical focuser for the guide cam makes focusing a breeze, 360 degree rotation makes finding a guidestar easy, comes with heaps of adapters and spacers in the box. Get a decent guide cam and I don't know what's not to like.

I went from OAG, to guidescope and back to OAG and with a decent OAG I'd not consider going back to guidescopes.
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Old 04-10-2015, 08:02 PM
mymoon (Aziz)
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This article by SBIG reasses the focal length requirements of guide scope. It says that focal ratio is more important.

http://www.sbig.com/site/assets/file...ersion3_mb.pdf


Using their advice I have been using the SBIG Sti guider set piggy back on the Edge C11 with great results.

Cheers
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Old 05-10-2015, 01:22 AM
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Eden (Brett)
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Quote:
Just wondering if I can further enhance guiding, WITHOUT using an OAG (I have tried many times using an OAG and just don't like them - just my preference.
You're welcome to borrow my ONAG Lewis, if you're interested in seeing whether or not it gives you any improvement. PM me if you'd like to have a go with it.
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Old 05-10-2015, 11:56 AM
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I've done everything from self guided adaptive optics sbig cameras through to a humble 50mm finder with a Qhy5, I've used an ST80, zs71, I've used camera lenses as guide scopes too. By far, no exceptions, OAG (and self guided/AO) setups work the best. I think self guided with ao had the edge, but with a good quality OAG and a sensitive guider like a qhy5 2 or a Lodestar beats self guiding without AO. The reason I say this is because with my st10xme that I used to have, the imaging sensor had 6.3um pixels and the guider had 7.4um. The difference is slight, but I noticed when going to a guider with 3.7um pixels I found guiding to be more accurate. Could have just been in my head.

Using an st80 was nothing but heartache. Guide rings that flexed, the focuser was wobbly as all get up. I ended up needing 3 rings, 2 solid rings to mount the tube and a small guide ring securing the focuser drawtube. I still think there was a margin of flex in the rings somewhere because I went from that to a solid mounted zs71 piggyback on my tmb80/480 and that provided very good results but when I planked a 2.5kg camera on the back of the TMB guiding went up the wazoo again. I moved to an OAG and never looked back.

Now I'm using a finder guider but it is solidly mounted to the imaging dovetail through a home made bracket, and I'm imaging at 300mm focal length so I assume when I get it running it will be fine. Only reason I'm not using an OAG for this is because it's a dslr lens on a dslr.. 0 available backfocus.
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Old 05-10-2015, 12:16 PM
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Quote:
Originally Posted by mymoon View Post
This article by SBIG reasses the focal length requirements of guide scope. It says that focal ratio is more important.

http://www.sbig.com/site/assets/file...ersion3_mb.pdf


Using their advice I have been using the SBIG Sti guider set piggy back on the Edge C11 with great results.

Cheers
I found that article to be false.

I used the SBIG Lens kit guider and it gave poor results with an AP refractor 1050mm focal length and a good mount that was well polar aligned.

AP have a nice modified Baader vario finder scope autoguider that some are using with great results on Riccardi Honders scopes which are very precise.

Worth a look.

Greg.
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Old 05-10-2015, 03:05 PM
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Lewis if you want to eliminate flexure and do long subs, then OAG is the way through. Which OAG's have you used in the past? What has been the issues that you found with an OAG? Maybe we can help you sort those issues.
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Old 05-10-2015, 03:26 PM
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Seeing I am using an FSQ, I should have known better, but I TRIED using the TS 9mm thin OAG - narp, no go. I KNOW I will need to go to an Astrodon MOAG or bigger IF I want to pursue the OAG route.
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Old 05-10-2015, 03:27 PM
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LewisM
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Quote:
Originally Posted by Eden View Post
You're welcome to borrow my ONAG Lewis, if you're interested in seeing whether or not it gives you any improvement. PM me if you'd like to have a go with it.
Very gracious offer, but considering I know I would never afford one (with pure conscience anyway, considering how infrequently I image), I will decline. Thanks all the same.
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Old 05-10-2015, 06:09 PM
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Eden (Brett)
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Originally Posted by LewisM View Post
Very gracious offer, but considering I know I would never afford one (with pure conscience anyway, considering how infrequently I image), I will decline. Thanks all the same.
You're most welcome. The offer remains open if you have a change of heart at some point.

If you can eliminate flexure completely, using a guide scope of the same or similar focal length to the main scope should theoretically give you exactly the same results as the OAG, minus the star choice limitations.

I don't know how much your FSQ weighs, but one thing to consider would be whether the weight of a second scope of around that focal length would negatively impact the good guiding you are currently getting on that mount. By the looks of it, the combination of your current auto-guiding setup and the custom tweaks you have implemented is really getting the most out of your mount hardware.

But as others have pointed out, OAG is king for a flexure-free setup.

MMOAG is probably the only other option but at $875 USD as new the baseline model (minus any adapters/spacers which might be needed), you might as well consider ONAG which about $30 cheaper and has NIR guiding and Sharplock to boot.

Whichever way you go I hope you find a solution which yields some improvement.
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Old 05-10-2015, 06:38 PM
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LewisM
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Thanks again.

I am FAIRLY satisfied with the resultant star shapes, but still not focus... still need to get the Sharpsky system again (every time I go to buy, SOMETHING comes up1). You THINK it is nailed, then you look at the 3D model in MaxIM to see a flat top, and the first image has slightly bloated stars.... it's frustrating at f/5
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Old 05-10-2015, 08:45 PM
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You should try focusing my lens at f/2.8
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