Astrophotography - Celestron 925 Alt/Az Wedge & Autoguider

[alienshores]

Hi - I'm just starting out on the Astrophotography journey .... have purchased a Canon 7D as my choice of weapon. From there my options I guess are to do a prime T-Ring/Connector arrangement and obtain the Celestron wedge to correctly track a DSO .... or "piggy back" the camera with lens to do wide field work - With either option do I also need an Auto-guider to obtain reasonable images ?

[ollypenrice]

I'd start by piggybacking once you have the wedge. (I'll say up front that personally I don't like wedge and fork mounts and find them hard to work with. However...) Once you have a polar aligned mount and camera piggybacked you should be able to do long exposures at short focal lengths. The longer the lens you use the more critical the guiding becomes and at some point in the focal length hierarchy you are going to need to start guiding or reducing your sub exposure lengths - which is a pity if you have a dark site.

You can put a guider in an SCT but it makes a poor guidescope with not enough FOV and too much focal length. However, it can be done. I did it regularly at one time with a small apo piggybacked.

Imaging through the SCT will certainly require an autoguider. That is beyond doubt. Your best bet woud probably be an off axis guider though you can usually get away with a guidescope on the smaller SCTs. You will certainly want the F6.3 reducer unless you have the Edge model. The standard model optics need the flattener-reducer to cover your chip. Besides, F6.3 is a way better idea than F10, which is too slow especially for a DSLR.

Olly

[Herrman]

Hi Olly,

I hope you don't mind me butting in; but I was very interested to read your comments about imaging using a SCT. I have access to a Meade 10" f10 LX200, which I've mounted on a Losmanday plate. I was thinking of plonking this onto my NEQ6 together with a piggyback 80 mm SW Startravel/Orion Autoguiding combination. Imaging with Canon 40D and 383L+ cameras. D 'you think this setup is fraught with potential problems? I guess one answer is try it and see, but with so few clear evenings here at present I'd like to use the time available wisely, also if obtaining good results will require investing in OAG and focal reducers then I may need to think again as I don't actually own the scope (although I may put in an offer one day!).

I'd be very interested to hear your thoughts on this.

Again, apologises for highjacking the topic!

Regards, Herrman

[pete_l]

I'd start by piggybacking once you have the wedge. ... The longer the lens you use the more critical the guiding becomes and at some point in the focal length hierarchy you are going to need to start guiding ...

You can put a guider in an SCT but it makes a poor guidescope

To guide your DSLR for piggy backed imaging, you may also be able put a guide-cam on the SCT's finder scope. Depending on they type you have on your C925, it may be as simple as whipping out it's eyepiece and inserting a small camera like a QHY5.

No matter when you start to guide you'll need a guidecam so buying one before you start imaging from the SCT is not extra expense, just earlier buying.

[alienshores]

Thanks Pete & Olly for your advice - You've confirmed that doing it right the first time is better than getting poor results by messing around .... will go with the guider as an optimum solution. Olly - without even having the wedge yet to assess, I think you're bound to be right about it being an awkward arrangement - it just looks like that from the photos I've seen. The 925 is heavy enough just mounting normally let alone on the wedge at an angle.

[ollypenrice]

Thanks Pete & Olly for your advice - You've confirmed that doing it right the first time is better than getting poor results by messing around .... will go with the guider as an optimum solution. Olly - without even having the wedge yet to assess, I think you're bound to be right about it being an awkward arrangement - it just looks like that from the photos I've seen. The 925 is heavy enough just mounting normally let alone on the wedge at an angle.

What I found with the Meade Superwedge ( a few rough unmachined castings tossed into a pile) is that in loosening and tighteneng any of the adjusters all the others lost their position so I went round and round in circles.

Here's the thing; there are millions and millions of fork SCTs in circulation. Go to any imaging forum, though, and compare the number of images posted with these with the number posted on GEMs. There has to be a reason for fact that GEMs produce 95% of the images.

Olly

[Merlin66]

You can guide a Al/Az mounted MEADE SCT....but you still have the issues of field rotation due to the alt/az.

I used a Meade 10" Alt/Az with a beamsplitter guider to get long exposures (up to 10 mins) on a spectroscope.

Unfortunately the Celestrons don't seem to be able to do this...

I now use a C9.25 and a C11 on a NEQ6pro mount for spectroscopy - holding a target star on a 20 micron slit indefinately.

The only reason I dropped the fork mount was lack of space behind the OTA's to fit the spectroscope etc.

(The ol' 12' Lx200 was certainly capable of holding the same guiding tolerance on the HD wedge and tripod)