focal length of finder scope?

[1crem1]

Hi all
I have a Bintel branded right-angled finderscope (GSO?) converted to a guidescope. It's now straight through with about 40mm of spacers to achieve focus attached to a ZWO ASI120MC-S.

I've fed 185mm into PHD as a focal length. Is that about right? How can I tell? I'm trying to reduce the variables in the guiding equation as the results aren't ideal.

Andrew

[1crem1]

Forgot to mention it's 8x50mm

[Merlin66 (Ken)]

Andrew,
Probably around 220mm focal length.
You could take an image and plate solve to get an accurate result,
Something like ASPS -http://www.astrogb.com/astrogb/All_Sky_Plate_Solver.html


Or simply remove the camera and point the finder at the Sun, use a piece of card to find best focus image and measure the distance from the objective to the card.

[1crem1]

Hi
Thanks
Andrew

[MortonH]

TS RACI model: 208mm (under Technical Data tab)

https://www.teleskop-express.de/shop...l-focuser.html


TS straight through model: 180mm (under Technical Data tab)

https://www.teleskop-express.de/shop...e-bracket.html

[Merlin66 (Ken)]

Morton,
You may be correct, but the TS finder takes 1.25" eyepieces and the GSO version has a fixed eyepiece..........

[MortonH]

Originally Posted by Merlin66

Morton,
You may be correct, but the TS finder takes 1.25" eyepieces and the GSO version has a fixed eyepiece..........

Yeah, I really just posted those links to show that your figure of 220mm would be correct or at least in the ballpark.

[1crem1]

Thanks for your help.

Andrew

[1crem1]

Originally Posted by Merlin66

Andrew,
Probably around 220mm focal length.
You could take an image and plate solve to get an accurate result,
Something like ASPS -http://www.astrogb.com/astrogb/All_Sky_Plate_Solver.html


Or simply remove the camera and point the finder at the Sun, use a piece of card to find best focus image and measure the distance from the objective to the card.

I tried that and it was ~180mm; fed it into phd2 and it gave me calibration steps of ~2000ms. A previous problem was that calibration failed as star didn't move enough; will see how this goes when next we get a clear night...
Andrew