Aperture vs. FL for guidescope?

[Kirkus]

I'm in the market for a light-weight small guide scope. I'll be mounting this on my 10" reflector, which is already pushing the limits of my mount, so weight is definitely a consideration.

Which would be more important when selecting a guide scope, aperture or focal length. I'm assuming aperture, but what I know so far wouldn't fill a thimble. :)

[iceman]

Hi Kirkus,

I use a cheapo 80mm f/5 refractor as a guidescope, and it works perfectly with my DMK21AU04 as a guide camera. At only 400mm it's not long in focal length, but the f/5 is good for getting guide stars in the FOV and not needing excessive exposure.

[Kirkus]

Perfect! Thank you! :)

[Garyh]

Also the 70mm f/7 skywatcher is a good little guidescope also the f/9 version...thou lighter than the 80mm f/5 doesn`t have as much light gathering and the focuser housing isn`t as well made, only plastic.
Andrews sell the OTA rather cheaply too.

[Kirkus]

Cool! Thanks Gary.

Would I be correct in assuming a smaller F/Ratio and/or larger aperture would increase the chances of finding a suitable star for tracking in the FOV?

[Terry B]

Smaller f ratio gives a larger field of view and not much else. An f5 80mm scope will have dimmer stars than a f5 120mm scope and hence less chance to find a guide staras it doesnt collect as much light. There has to be a compromise as the scope becomes bigger it gets heavier and more expensive. My compromise was to buy a 120mm F5 achromat and using my QHY guider I can always get a guide star with a 2-3 sec exposure no matter where I aim the scope.

[Ian Robinson]

I've been using my trusty old Tasco 60mm (f/15) refractor as a guidescope with my 10" newt. It's all metal , not like those junky later and moden Tasco refractors , mine actually focuses and has pretty good rack and pinion focusor with an extendable draw-tube that is all metal) and had a hard life .... isn't pretty to look at but still works a treat.

I've just ordered a Lumicon Coma Corrector (took ages to find someone who still had some) and have placed my order directly on Lumicon for a 2" Newt Easyguider + 2" Canon T - ring suiting the 40D, so I wont be using a guidescope for prime focus photography from much longer .

I will continue to use my old guidescope on "little" CG5 to guide, just need to rig up a home brewed Side-Side gizmo to do that.


I think for good guiding with a guidescope, the focal length should be near the focal length of the prime telescope or lens , preferably longer than the same, if you can't see a suitable guidestar in the fov , then it's easy enough to offset the guidescope a tad until one become available.

[Kirkus]

Thanks to you all for the information -- it's a huge help having friends who share their knowledge. I just learned much that I didn't know before.

[Dennis]

Hi Kirkus

The guide scope should have a good, quality, rigid focuser to ensure that the guiding CCD camera does not flex the focuser on the guide scope when the “weight” changes as the mount tracks. The draw tube should ideally have a focus lock screw to prevent the focuser slipping under the weight of the guide camera (if it is reasonably heavy).

Any flexure in the guide scope system may still give you nice, round guide stars in the guider CCD camera, but your imaging camera on the imaging telescope will not know about this and may result in trailed stars.

Cheers

Dennis

[rogerg]

I think it depends a bit on what camera you're using for guiding. If you have a sensitive camera then your aperture is less important. I place more emphasis on focal length than I think many other people do. I use a 70x700 skywatcher which works well when imaging through my 80x480 megrez, and I use the 80x480 megrez for guiding when doing widefield photography. I don't have any problem with the F/10 of the 70x700 skywatcher - stars are easy enough to find with my Q-Guider. It was hard with a un-modified webcam, and a modified toUcam was inbetween those two in ease of use.

I think it largely comes down to what your scope/mount can handle without becoming unsteady, and how sensitive the camera is that you're guiding with.

Roger.

[pvelez]

I'll leave it to the gurus for the technical stuff.

I use a Stellarvue finder (details below) with a x2 barlow. With my DSI and PHD at 2 seconds I don't have any issues finding a guidestar. The optics are great and its very, very light.

Pete