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Old 06-08-2014, 12:57 PM
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Amaranthus (Barry)
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Differential flexure on C8?

I've been doing some small FOV imaging using my C8, guided by an 80mm guidescope attached on piggyback, and I suspect I'm getting major differential flexure.

Attached is:

(1) An average stack of 50 x 1 min subs, which shows classic trailing and some wiggle in the trails, but no real curvature. To me that suggests DF, not field rotation is occuring - correct? The FOV is about 18 x 13 arcmin, so quite narrow (effective magnification of ~200X).

(2) An animated GIF of 11 x 2 min subs taken at 5 minute intervals. As you can see the stars are quite round in each individual 2 min sub frame, but the whole set of images marches sedately from left to right across the screen during the >1 hour session!

The AZ-EQ6 GT mount seems to be well polar aligned, after I did a long DA session using Metaguide (also my guiding software, guiding camera is a ASI120MM-S). Overall, the setup is rigid and the mount is well underweight (about 11 kg vs rated 25 kg). The guidescope is pretty solidly held using a Vixen-style bar attached (securely) to the C8 OTA, and held with Losmandy rings.

The CCD on the C8 has a f/6.3 FR and filter wheel in front of it, and sits about 125 mm from the back of the FR. The guide camera is attached straight onto the back focusser of the guidescope.

However, the C8 is a basic model with no mirror lock. For those who've been here before, is my diagnosis of DF/mirror flop correct, or am I missing something? If I'm right, I suspect I'll have to wait for when I get an OAG before using the C8 for any more imaging (apart from planetary)!
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Old 06-08-2014, 03:17 PM
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multiweb (Marc)
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I don't think it's mirror flop. That's a lot for a small SCT. I think it is more likely that your guider is moving in reference to the scope OTA or vice versa.
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Old 06-08-2014, 05:10 PM
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Amaranthus (Barry)
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Right - I'm pretty sure it's flex. It probably stems from the guidescope being mounted in adjustable rings (among other things). I'm going to seriously look at getting an OAG for all my imaging. I recently bought an ASI120MM-S camera, which is extremely sensitive, so OAG is at last feasible...
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Old 06-08-2014, 05:25 PM
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multiweb (Marc)
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Quote:
Originally Posted by Amaranthus View Post
Right - I'm pretty sure it's flex. It probably stems from the guidescope being mounted in adjustable rings (among other things). I'm going to seriously look at getting an OAG for all my imaging. I recently bought an ASI120MM-S camera, which is extremely sensitive, so OAG is at last feasible...
Yes - that makes sense. An OAG with an SCT is the best thing to do.
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Old 15-08-2014, 02:34 PM
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I ended up getting an OAG (the Orion Thin Off-Axis Guider), and after the usual frustrations in getting the darned thing focused, I'm having success. No more star marching!

Attached is a single B&W 10 min sub (of NGC 6362 - a somewhat random target that was nearby to my test zone at the time). Round stars! (and no marching across subs). Yes, I know this image is noisy as hell (as I said, only 1 test sub), but it shows that I'm going to achieve what I wanted with the OAG -- long exposures (I kept it to 10 min here due to skyglow + moonlight).

So, I'm happy!
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Old 15-08-2014, 02:37 PM
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Looks great Barry

Soooo....looks like I'm on a hiding to nowhere if I try it with my scope then it's what I've got laying around, no cost option
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Old 15-08-2014, 02:42 PM
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Dunk, the OAG is not all that expensive, and my goodness, it works! I'll never got back to a guidescope now.
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Old 15-08-2014, 02:57 PM
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I hear you, but I'm supposed to be sensible at the moment does it come with enough adapters to achieve the correct back focus with our beloved little glass topped paper baskets?

The Celestron OAG looks really nice and comes with all the right bits... but works out about $400 landed
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Old 15-08-2014, 02:59 PM
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One of the reasons I chose the TOAG was that it only adds 10.5mm to backfocus, and so I could still readily keep all the correct spacings, on both the C8 with the f/6.3 FF/FR and 'fracs with the 0.8 FF/FR. It's only $225 delivered from Bintel. It comes with one 15mm spacer (and a bunch of attachments), but I already had a kit with 5, 10 & 15 so was able to get the right combos.
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Old 15-08-2014, 03:07 PM
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Hmmm...not sure how well it'd go with my scopes Celestron says +/- 0.5mm spacing for the back focus on the Edge HD scopes

Thanks for your answers Barry, sorry for the hijack
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Old 15-08-2014, 03:20 PM
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Amaranthus (Barry)
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No worries about hijacking - it's a threat about the need for OAG!

What is the total backfocus distance for your Edge? I could get the spacing from my FR to chip perfect using the OAG, filter wheel, camera and some spacers. According to this you get 105 mm with the 8" Edge FR:
http://www.celestron.com/browse-shop...-7x-edgehd-800

This is similar to the distance for my non-edge C8. http://www.celestron.com/c3/support3...articleid=2243
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Old 15-08-2014, 04:50 PM
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Yeah it's 105mm with the 8 and 146 for the 11. Then I have my little frac to consider....don't want to leave it out in the cold...well I do, if it's imaging

It's the general accessories I have trouble justifying...the OAG is $209, then another $49 for a few spacers...it's the unknown more than anything else, because before I realise it I've spent as much as I could on the one from big C, which I saw a few chaps using at AF, serious quality piece of kit...and the big C OAG is US$249 all-in, at least as far as my cats go

Sigh...I know it's futile fighting it

Edit: just saw you mention filter wheel too...which one are you using Barry?
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Old 15-08-2014, 05:43 PM
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Amaranthus (Barry)
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Manual one from ZWO (it's quite smooth): http://www.zwoptical.com/Eng/Cameras/filters

Biding my time to an upgrade like a QSI 690-wsg8!
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Old 15-08-2014, 05:46 PM
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Nice one, that is reasonably priced too

I'd need to go for a 2" version though, as all my filters are that size
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