Re: [APML] Autoguiding Focal Length

[steve banbury <banburys@sonic.net>]

Hi Thom---I think there's a good bit of discussion available 
in the archives on this subject. My recollection is that 
guiding at f10 while imaging at f6 was appropriate for an 
ST4, but not necessary for an STV. I've had good results 
with the STV and a 200 mm lens to guide my TV101. Mirror 
flop plagued me on my C11 until I switched to an OAG. I used 
a focal reducer on the f10 pickoff of the OAG to get me back 
to f5 with the STV, and that has served me well. The f5 
pickoff on the camera side of the focal reducer in the OAG 
gave me sea gull shaped stars. Someone here on APML wrote me 
that careful shimming (of the pickoff mirror as I recollect) 
  could eliminate the sea gulls, but I didn't pursue it 
because the other approach was easier and worked for me.

Maybe some of the experts will chime in here too.

Steve

Thom Iwancio wrote:

> Hey all, I think this has been discussed before so I apologize for
> covering old ground.  I have been using a Lumicon OAG with my 8" SCT and
> the built in focal reducer.  This places the autoguider at f10 and film
> camera at roughly f6.  On of my partners says I should move the focal
> reducer and guide at the reduced fl as well. His reasoning is two fold,
> first it allows for more/brighter guide star choices and second it keeps
> the CCD from chasing the seeing and every little bobble of the mount.
> Guiding would essentially be at f6 and film at roughly f5. Is this the
> way to go?
> 
> I know people have been discussing the feasibility of using extremely
> short fl's like telephoto lens. It there a rule for guiding tolerances?
> I have read Covington's treatise and he speaks of x number of mm's being
> the resolution of the film and tolerances in mm's.  Unfortunately the
> CCD world uses pixels converted to arc seconds. Or is this a trial and
> error, see what works for me thing.
> 
> Thanks in advance,
> 
> Thom Iwancio
> Elgin, SC
> www.machunter.org
> 
> 
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