Nexstar 11 Guiding

Hi there,
I have been planetary imaging with a Toucam & Nexstar 11 for a while but now I want to do some deepsky imaging and have purchased a EOS350D (unmodified) w/release cable. As regards guiding I have no equipment but I am thinking of using a Celestron 80mm wide view spotting scope (cheaper than Celestron 80 ED)! Can anyone foresee any problems using this scope for guiding and for imaging?
Also, as regards attaching the spotting scope to the NS11, is there an alternative to the Losmandy dovetail and rings?

Thanks!

Hi There,

I've just helped a colleague get autoguiding up and running with an N11. The good news is it can be done. The bad news is it was a little more difficult than one would have hoped.

I'll address your question re-guidescope first. The 102mm wide is perhaps not the first choice for guiding. The focal lenght is just too short. You can use this in conjunction with a barlow lens to increase the focal lenght but best avoid that if you can. The first choice guide scope iMHO is the lidl skylux. Its a lovely 70mm scope with a 700mm focal lenght and its dirt cheap. Bad news is lidl may well be out of them There was one left in lidl on the malahide road last weekend. I dont know if it is still there. This scope makes an excellent guidescope.

As for mounting, I use the losmandy gear, however my friend came up with some kind of a home made mounting arrangement. I havent seen it yet but he says it works well. The important thing with mounting is it must be ROCK solid. I really mean that. Forget about ball and socket heads, slow motion adaptors etc, they just do not cut it. There cannot be so much as a fraction of a mm flexure between the quidescope and the mainscope.

Next question is the guiding camera, Ideally you need a guide camera capable of long exposures. A perfect choice would be a long exposure modified webcam. If your handy with a soldering iron you can do this yourself (I did) however it is NOT easy. You can do autoguiding with a non modified webcam but its not ideal. You are limited to guiding on the brighter stars and also you are more prone to oscillations caused by seeing.

The other thing you need is a cable to connect your computer to the scope mount. You can buy one ore easily make one yourself. See here

www.nexstarsite.com/PCControl/RS232Cable.htm

Lastly is perhaps one of the bigger problems. The N11 is an alt/az scope which means that even if perfectly guided your long exposure photos will exhibit 'field rotation' to varying degrees. This causes the image on your camera to rotate about the center point. The longer the exposure the worse it becomes. The ideal solution is to mount your N11 on the optional equatorial wedge. Unfortunately this adds another level of complexity to an already complex issue! You can deal with field rotation to a degree by taking shorter exposures and stacking the resulting images ontop of one another. In fact with a DSLR you pretty much have to do this to get optimal images (this is the way most imagers work these days)

Lastly you need some software. The good news here is that the best is free see;

www.barkosoftware.com and check out guidedog v1.0.6.

Good luck!

Just to show you what can be done heres my friend (Gary Clarke) first ever deep sky image taken with a nexstar 11 (equatorially mounted on wedge), canon 350D and autoguided.

www.webtreatz.com/index.php?option=com_z...&key=0&hit=1

IMHO thats a fine image for a first effort. I wish my first efforts were this good.

Dave - as you say, ALt-Az gives rotation in images hence you have to limit exposure duration. Doesn't that obviate the need for guiding? Or is there another way to get long exposures (without a wedge) and not get rotation? Just curious...

Of course, the Meade Autostar software corrects rotation on the fly so I presume that's one solution (though I'm not sure if the result stand up to scrutiny).

Cheers

Dave

Dave - as you say, ALt-Az gives rotation in images hence you have to limit exposure duration. Doesn't that obviate the need for guiding? Or is there another way to get long exposures (without a wedge) and not get rotation? Just curious...

No guiding is still required with as from tests i've done, the N11 cant even make 1 min without guiding and this is about the minimum needed for any kind of imaging with a DSLR. You might manage it with something like a meade DSI.

Of course, the Meade Autostar software corrects rotation on the fly so I presume that's one solution (though I'm not sure if the result stand up to scrutiny).

Yes but this poster is using a 350D (a good choice IMHO).

BTW the only other way I know of to get rid of field rotation (aside from stacking shorter images) is to use a hardware based field "derotator", which attaches to the focus drawtube and rotates the camera in the opposite direction to the field rotation. Very pricey AFAIK.