C9.25??

garymck (Gary) · #1 04-06-2011, 10:04 AM

Hi,

I'm currently using a Vixen GPD2/ED80/Skywatcher FR/C QHY8 combo for imaging. This setup is proving so effortless to use (after years of fighting EQ6/newt combos) that I'm getting bored doing nothing on the rare nights in Melbourne when imaging is possible these days. I mean, I set it up, start the guiding/imaging process and there's nothing to do until it finishes! I'm thinking of getting a CPC9.25 for doing some visual stuff while the camera is ticking away. A friend has just bought one and it is a VERY nice scope, visually at least, and in Alt/ASz mode viewing is especially comfortable. It had been so long since I actually looked through a telescope that I had forgotten the pleasure that could be obtained.

My main query is related to occasional use as a long FL imaging scope. Anyone know what the field would be like at F10 on a QHY8? What about with an F6.3 reducer? Anyone know if the Allan Gee reducer11 is better than the Celestron one?

Just wondering whether much cropping would be needed to get round stars at the edge?

cheers
Gary

renormalised (Carl) · #2 04-06-2011, 10:26 AM

Grab yourself a copy of CCD Calc, plug the specs for your scope and the CCD into the program and it will allow you to see what the FoV is for various objects. You can also run it for a scope with a reducer as well (just type in a new entry with the amended focal length).

You'll find targets such as most galaxies and PN's etc will be better imaged at the native f length.

renormalised (Carl) · #3 04-06-2011, 10:29 AM

If you're going to use it as an occasional imaging scope, and in alt-az, grab a field derotator. Optec make a good one which will fit most scopes (there are others at Optec apart from the one I have linked to).

I also believe the CPC9.25 is Fastar compatible, so you can get a Hyperstar lens to attach to the scope.

Although, by the time you've bought all the stuff to image through it, you probably won't use it visually!!!!. So forget about the QHY8 attaching to the scope and if you want to do occasional imaging through it, just use a webcam and take happy snaps of the Moon and the planets (however, a cam with long exposure enhanced innards will also work on many DSO's)

garymck (Gary) · #4 04-06-2011, 10:53 AM

I was actually after info about whether the field with a QHY8 would have much coma/curvature at native and reduced f ratios that would need cropping. Not just the size (already checked that out) - sorry if I wasn't clear enough in my original post.....If imaging I'd turn it equatorial......

cheers
Gary

renormalised (Carl) · #5 04-06-2011, 11:08 AM

Since the optics in the CPC are stock standard SCT optics and not the aplanatic EdgeHD optics, you'd get some curvature at f10 and more at f6.3 (so long as the corrector wasn't a field flattener as well). If you're going to image in the equatorial, you'd be better off buying an EdgeHD OTA and sticking that on a EQ mount. Why stuff around removing the CPC from its fork mount just to take piccies???. Or buying the EQ wedge, which is your best option here. However, you'll need a good steady tripod because the scope on the EQ wedge isn't the most stable of configs.

renormalised (Carl) · #6 04-06-2011, 11:26 AM

Gary, here's a review over at Cloudy Nights...

http://www.cloudynights.com/item.php?item_id=2294

garymck (Gary) · #7 04-06-2011, 11:26 AM

Or buying the EQ wedge, which is your best option here. However, you'll need a good steady tripod because the scope on the EQ wedge isn't the most stable of configs.[/QUOTE]

Wedge is how I'd do it, not remove ota, not interested in the edge ota's too much $....!!!!!! This is only for occassional AP use, hence would love to see edge of field samples with an APS sized sensor to see if it's useable. Not expecting perfection, just wondering how good?

cheers
Gary

jenchris (Jennifer) · #8 04-06-2011, 11:32 AM

Sorry to hijack -
If you put a field flattener on an ACF or Edge scope, does it mess it up?

renormalised (Carl) · #9 04-06-2011, 12:07 PM

Quote:

Originally Posted by jenchris

Sorry to hijack -
If you put a field flattener on an ACF or Edge scope, does it mess it up?

Yep....use the Meade .33 FF on any ACF scope and you'll be instantly moving at Warp 9

The stars trail out like you've just jumped to hyperspace. The 0.67 corrector is better suited but still not the best. I think Meade didn't consider that anyone would use the ACF scopes at anything but the native FL. They should've developed a corrector for them, but haven't. Optec apparently make a corrector for aplanatic scopes (both the ACF and EdgeHD) but I'm not sure if it's any good though.

renormalised (Carl) · #10 04-06-2011, 12:13 PM

Quote:

Originally Posted by garymck

Quote:

Or buying the EQ wedge, which is your best option here. However, you'll need a good steady tripod because the scope on the EQ wedge isn't the most stable of configs.

Wedge is how I'd do it, not remove ota, not interested in the edge ota's too much $....!!!!!! This is only for occassional AP use, hence would love to see edge of field samples with an APS sized sensor to see if it's useable. Not expecting perfection, just wondering how good?

cheers
Gary

Yeah, the EdgeHD's are a tad on the dear side!!!. Not for want of them being any more expensive than the ACF's. You can thank the importer for that little debacle.

I don't have any cameras with APS sized sensors, but I'll see if there are piccies around using that sized sensor and scope.

The_bluester (Paul) · #11 04-06-2011, 12:20 PM

Can not comment on it for AP use for anything beyond the happy snap level as I really only use mine for visual observation, but I have a CPC925 and have been very happy with it (Although time and recent skies near Melbourne mean that I have not put eye to eyepiece for weeks

)

Looking at the link posted earlier about the derotator, my initial thought was "What a good idea, maybe I should buy one" but it looks like they would cost about the same as the Celestron wedge for the scope in any case which would mean one less thing to go wrong in the dark.

renormalised (Carl) · #12 04-06-2011, 12:35 PM

Yeah Paul, they do cost about as much as a wedge. However, you don't have to faff around trying to align your scope with the pole, you don't have to try a drift align, it's much more stable in the alt-az config anyway, the derotator will take care of any field rotation in your images, and setting up in alt-az is a lot faster.

The_bluester (Paul) · #13 04-06-2011, 01:20 PM

I did wonder about that. If a derotator is used, would that play hob with autoguiding as the camera axis would not be fixed in relation to the mount?

renormalised (Carl) · #14 04-06-2011, 01:48 PM

It shouldn't because the FoV will stay in a constant position as you rotate c.f. the camera axis. That's why it rotates in the first place, to compensate for field rotation in azimuth/RA because that axis isn't actually aligned with the rotation of the planet. An OAG should stay aligned with the guide star as it rotates across the FoV.

garymck (Gary) · #15 04-06-2011, 01:58 PM

You'll be guiding on a star that doesn't rotate, while the field of the imaging camera does rotate, won't work. You need both camera and guide scope to rotate for imaging......

check out:

http://www.mapug-astronomy.net/astro...G/Derotatr.htm

cheers
Gary

renormalised (Carl) · #16 04-06-2011, 02:29 PM

Quote:

Originally Posted by garymck

You'll be guiding on a star that doesn't rotate, while the field of the imaging camera does rotate, won't work. You need both camera and guide scope to rotate for imaging......

check out:

http://www.mapug-astronomy.net/astro...G/Derotatr.htm

cheers
Gary

That's why you can't use a guidescope with a derotator, only an OAG. Unless you're using it as a field rotator....basically a convenient way of changing the FoV perspective.