Optimal Guidescope size.

[Terry B]

I currently use a 120mm achromat f5 as my guidescope and find no problems finding a guidestar with my QHY guider. Lots of you seem to use 80mm scopes instead.
I have tried guideing through my 200mm f8 main scope but with it's smaller field it can be harder to find a guidestar. When one is present however it is still just as easy to use.
In skywinty's thread (http://www.iceinspace.com.au/forum/showthread.php?t=30643) he is looking at an 80mm high f ratio scope as a guidescope.
So what is more important?
The larger the scope the more light collected and for point sources (guidestars:)) the aperture shoud be more important than f ratio to find lower mag guide stars. A low f ratio will give a wider field and hence more area to find a star but shouldn't make any difference to the limiting mag.
Which comes out ahead? Field size or limiting mag?

[whmacs]

Hi Terry,
I'm using a Orion short tube 80 F5 400mm from Bintel with my Qguider and PHD to autoguide. This works well with both my HEQ5PRO with Skywatcher 80ED F7.5 600mm and my Nexstar 11 GPS SCT at F6.3 1764mm. I use 2.5 sec adjustments with PHD. The short tube 80 makes it very easy to find guide stars. You can normally choose from 5 or 6 in the field of view.

Regards,
Stephen

[Zuts]

Hi,

Someone please correct me if i am wrong but with guiding while aperture is good its not that necesary as with a smaller guide scope you can just increase the shot time on the guider to find more stars.

What is important is matching the FL of the imaging scope with the FL of the guidescope. For example if you tried to guide a Celestron C11 at 2700 ml with an ED80 at 600 ml you would get very poor photos because the guide scope would not see the movement being seen by the C11. In this case you would need to Barlow the ED80 significantly to at least get closer to the C11 FL and increase the shot time on the ED80.

Paul

[zipdrive]

I concur with Zuts :thumbsup:

[Terry B]

This is true and the focal length has to be taken into consideration. With modern guiding programmes able to compute a centroid to sub pixel size the old need for the guide scope to have a longer focal length than the imaging scope is no longer valid.
You can increase the the exposure time of the guider to find more stars but there is a limit to this. I find going over about 5 sec exposure reduces my guiding accuracy. Having said this I am yet to find an area of sky that I couldn't find a guidestar with a 3 sec exposure using my current set up with a QHY guider and 120mm scope.

[Alchemy]

not going into the theoretical math etc, i tried using a f11 achromat for guiding with a 840k and guidedog and found it hard to do( i use 1 sec updates)

i went for the ed80 and found i could do it,.. just practical experience it works with 1500mm focal length imaging scope, whether it works for you is for you to consider accordingly. I have since upgraded my guider to the imaging source mono camera and now use phd and am happy with its performance.... if i find a better option i would change as we all would.