DSI LPI and autoguiding

DSI in ETX controlling the mount u got? Cant that be done? Or do you have to magnify so much to be able to track?

I could be wrong but I dont beleive Dave McD autoguides at all. (Its not such a priority when you have a mount like his )

Also Dave has removed his ETX from the mount (Is that permanent Dave?) and very firmly attached it to his scope. For autoguiding you need a reasonably long focal lenght so that even tiny errors in tracking are noticed by the autoguider and adjusted. Many argue over what constitutes an acceptable minimum focal lenght for guiding but I have never encountered anyone who managed decent guiding at 350mm. About 700mm seems to be the minimum and around 1000mm is recommended You might be able to do it by using a barlow lens to double or even triple the focal lenght of the scope however its far from ideal as it significantly dims the view the guiding camera sees and the barlow also introduces flexure into the system that you just dont need. When it comes the time to guide. Look out for one of those Lidl skylux telescopes (700mm focal lenght) which are dirt cheap and seem to work well for guiding.

Eirik just to add to my last comments my setup is as follows.

I have three scopes;

C9.25 Schmidt Cassegrain (2350mm)
Skywatcher 4" Maksutov Cassegrain (1500mm)
Celestron 80ED (600mm)

Celestron CG5 Advanced Series Mount

Canon 300D and Modified Webcam (for guiding)

I usually image in one of the following configurations

Imaging Scope: C9.25 Guiding Scope: 4"Mak
Imaging Scope: 4"Mak Guiding Scope: C9.25
Imaging Scope: 80ED Guiding Scope: C9.25

Note I never guide with the 80ED.

But at least the DSI could be an investment for later, and probably a good start for me. Maybe if someone here got one for sale and wraps it in as a gift i dont have to pay vat or tax hihi

Eirik - Dave G has described my system very well.

I use the DSI in the C8 for narrow field imaging. But I can put the DSI into the ETX70 for widefield imaging.

The ETX *could* be used for autoguiding but as Dave says, the FL is right on the limit of being too short (it could actually be too short a FL).

The ETX OTA was indeed removed from its mount and I built a home-made cradle and attachment to stick it on the C8. Its not terribly good and there is a bit of flexure (though I don't use it for autoguiding). Its still OK for wide-field images though I haven't done any lately. Hmmmm. There's an idea :idea:

I don't autoguide - as Dave says, the mount (an Astro-Physics 1200) is rock solid. By tweaking polar alignment and PE and using a proprietary software routine, AP claim to get 30 minutes of unguided imaging with no discernible trailing. And I believe them. The word on the street is they will release better software this year that will allow as much as 60 minutes of unguided imaging :shock: I have my fingers crossed...

As Dave says, you *could* increase the FL of the ETX by using the Barlow but that will definately add unwanted flexure. Overall, the Skylux is the better guiding OTA of the two (ETX70 and Skylux) - the FL is much better and optically, you wouldn't better it unless you got a sem-apo or apo.

And there are imagers out there who have three scopes on one mount - wide-field imager, narrow-field imager and guidescope. Its very tricky keeping them all flexure free though.

Hope this adds a little more illumination to your questions though...

Cheers

Dave

Might as well join in here...

I'm in the process of developing an astrophotography setup at the mo also. I have a Meade 8" LX90 on a wedge in my observatory. If we ever get a clear night again, I'd like to further fine-tune my polar alignment and tracking rate. I have a 4" mak which will be delivered by ups tomorrow. I plan to use this as a guidescope and also as a portable scope, especially with the eclipse coming up.
I also recently bought a DSI which I will use as an autoguiding camera. I've ordered an adapter to connect the DSI to my Canon lenses so this should give me some nice wide-field shots - in particular my 50mm f/1.8 mkII which should give a field of view of 3.1 x 2.3 degrees. How good this will be is anyone's guess but it sure is cheaper than buying an apo!
All that I need to get now are a mounting rail & rings for the mak. I'll probably get one from ADM as they're alot cheaper than Losmandy.

Question in terms of mounting rings: if I have a 4" OTA and let's assume that the tube outer diameter is 110mm, should I get 125 or 150mm rings?

Michael get the 150mm rings if you can. The 125mm rings will, of course, only allow 7.5mm play on each side, you will often need to offset more than this for acquiring guide stars.