Using a F 7.7 scope for auto guiding??

[hotspur (Chris)]

Hi Folks

Looking at getting into auto guiding using the 'Orion Star-shoot auto guider',

Just wondering if a F 7.7 scope could be used as guide scope-I did use it once before,but it seemed to take along time to find a star to guide on.

I have got the impression that these auto guiders work on short F number scopes,no longer than F 6,is that so??

If I get auto guider I might get either a WO megrez 72 mm or the cheap and nasty orion 80 mm,which works well for auto guiding.

I have heard mention 'finder guider' apparently people make a guide scope out of a finder scope,has anyone here done this?,or know any links to 'how too's'

thanks for any pointers,Chris

[Paul Haese]

Auto guiding now can guide at sub pixel level so the f ratio of the scope or focal length is less important that previously. Your image brightness can be controlled easily and one can take longer guide exposures too.

The scope you have is fine and would work well with that camera. I have used that camera on my Tak to guide with and that is an f8 scope.

[hotspur (Chris)]

Ok Paul,

That sounds good,you mention 'image brightness settings',does that mean you can adjust a setting in the auto guider for different length scopes?

My friend had a megrez 66 at the time with his Orion auto guider,we just put in on my Vixen ED 81 F 7.7,and we had a devil of a time trying to find a guide star (we were under a very dark sky),once we found a star all went well.

I heard someone else mention they had issues with longer focal lengths.

But with what you say with yourself having no issues on F 8 Tak,may be its just a matter of learning to use adjustment settings.

I will buy Auto Guider first and play around with it on ED 81

thanks for advice,Chris

[AlexN]

Your Vixen ED81 will make a fantastic guide scope... I've used pretty much everything..
- 8x50 finderscope
- WO ZS66
- WO ZS70
- Orion ST80
- OAG @ 480mm FL
- OAG @ 2800mm FL

etc. etc.

I've found best results come down to your settings rather than your hardware... Focal length and ratio are not as important as they once used to be, although they are still somewhat scalable, Ie, you probably wouldnt use a 8x50 finder (200mm FL) to guide a 2800mm FL scope as the imaging scope could see massive errors before the guider noticed a movement..

too long a focal length guider brings about its own issues.. namely, not being able to find guide stars.

[hotspur (Chris)]

Ok,thanks Alex

This thread is bringing relief to my back pocket!,I was looking at buying a WO megrez 72

As,I thought the F 7.7 vixen was un- useable.

I will be using a Vixen 103 F 7.7 ED for main scope.

When I did use a Orion auto guider we just could not find many guide stars,I suspect the settings had been set for a shorter focal length scope-the owner used a megrez 66,so his auto guider was set for it-it worked fine.

But we did not change any settings on his auto guider whwn we put it on F 7.7 scope,so may be that was why we had issues finding stars.

Thanks for help,saved me $600!

Chris

[AlexN]

yeah mate, have a play with the gamma settings, and increase the exposure duration and you'll have no problems finding guide stars....

[jjjnettie (Jeanette)]

Chris, as I mentioned the other day, a focal reducer will really help in this situation. It doesn't have to be top shelf if you're only going to be guiding through it. Steve Mogg has them for around $75.

[Paul Haese]

Chris,

Alex's comments have gone to the heart of what I meant. You can increase your exposure guide lengths to about 2 seconds or so. You can also use darks on the guide exposures to increase the signal to noise ratio and of course you can change the gamma settings. The guide will find plenty of stars with the focal ratio you are asking about.