Imaging and guide scopes fixed in parallel..

[Stuart1971]

Hi Guys,

I have moved to a fixed guidescope, so instead of guide scope rings I use a fixed clamp Rings..so this means my imaging scope and guide scope are pretty much  parallel to each other.....they are on a side by side set up, guide scope one side and imaging the other....

so my question, is, how perfect do they have to be in parallel, absolutely perfect or just pretty close....as I guess a small amount off, on my set up will be a hell of a long way off on my target....😮👍

[inFINNity Deck]

Hi Stuart,

You will find the following article of interest:

http://adsabs.harvard.edu/full/1989JBAA...99...19H

Nicolàs

[Skipper Billy]

Reasonably close is usually more than adequate. A good solid flex free mount rather than rings is a great move.

[Stuart1971]

18 minutes ago, inFINNity Deck said:

Hi Stuart,

You will find the following article of interest:

http://adsabs.harvard.edu/full/1989JBAA...99...19H

Nicolàs

Hmmmm, a bit beyond me TBH, too much math....Thanks anyway... 👍

[Stuart1971]

7 minutes ago, Skipper Billy said:

Reasonably close is usually more than adequate. A good solid flex free mount rather than rings is a great move.

Thanks Billy..

So by just fixing the rings to dovetails and sliding the dovetails into clamps, and where ever they point , should be close enough, the only error should be in The engineering of the rings, clamps and rails...I guess.... 👍

[Skipper Billy]

27 minutes ago, Stuart1971 said:

So by just fixing the rings to dovetails and sliding the dovetails into clamps, and where ever they point , should be close enough, the only error should be in The engineering of the rings, clamps and rails...I guess.... 👍

Yep - just do what you can to minimise any flexing. 

[Seelive]

1 hour ago, inFINNity Deck said:

http://adsabs.harvard.edu/full/1989JBAA...99...19H

The numbers quoted in that article are based upon using emulsion film, for example to assumes an error of 30um as being accepable. With todays large CMOS sensors, an accuracy 10 times better would probably be required. But the maths are (mostly) valid and the gist of it is that any misalignment between the imaged target and the guide star will result in some field rotation of the image. How much depends upon the accuracy of the polar alignment, the amount of misalignment, the declination of the target, the focal length of the imaging system, the exposure time etc etc. The polar alignment accuracies assumed in the article are generally much worse than those that can now be readily be achieved, and who needs to have 60 minute exposures these days?

My imaging system and guide scope are not perfectly aligned and PHD2 always seems to pick a guide star near the field edge of the guide camera but I've never (yet) noticed a problem. If you don't get a problem then everything is fine, but if you do, then you at least may have an answer to the possible cause 😆

[inFINNity Deck]

Hi Seelive,

glad you read the article. Indeed it deals with 30micron 'pixels', whereas nowadays we use anything between roughly 3 and 9 micron, so the answers in that article need to be corrected by a factor of roughly 3 (for 9 micron pixels) to 10 (for 3micron pixels). Even then, as you have noticed, the errors are not that significant unless you want to do 60m subs (and get overexposed images). In my region (Bortle 5) I cannot even do 60m luminance: my image goes 100% white well within 20 minutes... 😞 Only in NB I can manage 20m subs, something I only occasionally do to test my unguided system.

Cheers

Nicolàs