Guidescope calculation?

[04Stefan07 (Stefan)]

I have an 8” 2032mm focal length SCT and wanted to ask if there was some sort of calculation to work out a suitable guidescope for it.

I have the Celesteon Guidescope pack but is quite heavy for my setup so looking for something smaller. Current guidescope specs below.

Celestron Guidescope
Ap: 80mm
FL: 600mm

ZWO guidescope
Ap: 60mm
FL: 280mm

Would smaller focal length be suitable?

[kens (Ken)]

You also need the pixel size. Recommendation is for the guiding pixel scale to be no more than about 5x the imaging pixel scale, give or take.
from the PHD2 forum:
Rule of thumb: Guider focal length > 0.2*(GP/IP)*IF
Where:
GP=Guider pixl size in um
IP=Imaging pixel size in um
IF=Imaging focal length

e.g. My guide cam and imaging cam are both 3.8um and imaging scope is 1600mm focal length. So guide scope FL should be at least 0.2x1600 or 330mm FL. The ST80 is 400mm so that is ok.
Other literature also places importance on the f/ratio where a low f/ratio on the guide scope is beneficial.

[04Stefan07 (Stefan)]

Excellent thanks for that information.

Worked out mine

GP=5.2um (QHY5)
IP=4.3um (Canon 1200D)
IF=2032mm (C8)

GFL > 0.2*(5.2/4.3)*2032

= 491

Looks like the ZWO focal length is too small!

However I have a f6.3 reducer for my SCT, not sure how much the fl of my scope comes down though after using that filter.

The ZWO is ideal purely based on weight. The Celestron is way too heavy!

Originally Posted by kens

You also need the pixel size. Recommendation is for the guiding pixel scale to be no more than about 5x the imaging pixel scale, give or take.
from the PHD2 forum:
Rule of thumb: Guider focal length > 0.2*(GP/IP)*IF
Where:
GP=Guider pixl size in um
IP=Imaging pixel size in um
IF=Imaging focal length

e.g. My guide cam and imaging cam are both 3.8um and imaging scope is 1600mm focal length. So guide scope FL should be at least 0.2x1600 or 330mm FL. The ST80 is 400mm so that is ok.
Other literature also places importance on the f/ratio where a low f/ratio on the guide scope is beneficial.

[kens (Ken)]

At 2032mm focal length you should probably use an OAG. If you go that way you'll want big pixels on the guide cam (e.g. by binning) or you could have other guiding issues

[SuperG]

I have the zwo. But I use it with either two imaging scopes with an FL of 600 and 900mm respectively. I do remember reading the brochure which said up to an FL of 1500mm. If you throw on a 0.63 focal reducer and you will be right.

[04Stefan07 (Stefan)]

Originally Posted by SuperG

I have the zwo. But I use it with either two imaging scopes with an FL of 600 and 900mm respectively. I do remember reading the brochure which said up to an FL of 1500mm. If you throw on a 0.63 focal reducer and you will be right.

Prob the best solution for the weight. I will have the reducer in the thing the whole time while imaging.