Guidescopes

[Rossco72]

Hi all,

Quick question, is there any advantage to gettting a larger objective guidescope?

The Skywatcher ST80, ST102, ST120 and ST150 are all f5 but is the extra light grasp an advantage?

Weight isn't really a consideration so I am more interested in whether or not the guiding would be easier / better with brighter stars?

Cheers

[jgs001]

The larger apertures, have longer focal lengths, and therefore narrower fields of view. Given they are all f/5, and that's the key to imaging, then you want the widest FOV you can get, the typical guidecam has a pretty small sensor, and therefore a pretty narrow FOV anyway...

[dmahon]

Longer focal lengths may mean more accurate guiding, but the narrower field may mean less chance of finding a guide star (without moving the scope in the rings).

I think there used to be a formula suggested for the minimum focal length of a guidescope compared to that of the imaging scope, although it may no longer apply with sub-pixel guiding calculations.

[Byz1453]

Quick question from me on the topic of guidescopes (apologies if I am hijacking another point).

On another thread I was asking about ideal grab & go scopes. Various options were put such as a Megrez 72, TS 80, Lomo 80, etc.

Would I be able to use these as a guidescope for my FLT 98 or am I better sticking to the standard suggestions.

Just trying to keep the amount of kit down to a minimum.

[Merlin66]

IMHO the ST80 is a good compromise between limiting magnitude, FOV and guide capabilities. I'd recommend it.

[Dangerous-Dave]

The ST80 is spot on in terms of aperture and focal length, just a shame the focuser is cack. There is a lot of movement when you lock up and it is common to lose the guide star in the process.

Before the ST80, I used a ZS80FD with a 0.63 SCT reducer and it gave an even wider FOV with the benefit of a decent focuser, and no need for extension tubes.

I currently use a Megrez 72FD which lacks the light grab and short focal length of the ST80 but makes up for it with a quality focuser.

[ollypenrice]

The ST80 is great but needs to have the focuser bolted down. Once the whole lot is tight I never touch it.Focus is not critical for guiding. Some even say being a bit soft helps the centroid calculation. I haven't touched mine in a year.

Olly

[Merlin66]

I replaced the focuser on the ST80 with the TS 2" Crayford... no more issues!!

[michael.h.f.wilkinson]

Another hijack: I have a 70mm F/5 I use as a finder scope on my C8. am I right in assuming that could be used as a guider for my 80mm F/6 APO, once I get a autoguider port on my GP-mount?

[Merlin66]

If you can get the guide camera to focus on the 70mm OTA, then yes it should do a good job.

[michael.h.f.wilkinson]

If you can get the guide camera to focus on the 70mm OTA, then yes it should do a good job.

It may need an extension tube, but I have modified the focuser so it has no play anymore, so if I rip the guts out of a "barlow" that came with a cheap scope I should be OK.