Re: [APML] Guiding (was M101)

[Steve Lindsey <stevelindsey@earthlink.net>]

Thanks for framing and asking the question Greg.  I'd like to hear the answer.  I make matters worse I think the ST4 pixel size is about twice what you're using below for calculations so I doubt the answer will favor that low of a fl guidescope.  Would love such a light weight set up but can't justify the expense of an STV.  Maybe it would work with the ST4 and a doubler on the BORG?

Steve

-----Original Message-----

Hi, Steve,

I've been imaging with my 152 mm f/7 refractor so the focal length is 1064 
mm. I think the focal length with the Mini Borg is adequate for most imaging 
applications with the STV but I'm not so sure about the ST-4. Then again, 
with the 60 mm and a 300 mm focal length (I think it's f/5, too), that might 
work with the ST-4. I used to know how well the ST-4 performed its 
centroiding computation but I lost those figures when I my old computer 
died. (D'oh.) Let's think a second... At 300 mm focal length, the image 
scale is (to lowest order) 0.0033 radians per millimeter. Let's assume a 
pixel size of 7.4 microns = 0.0074 mm. This says that one pixel corresponds 
to to an angle of 2.47 radians = 0.001413 degrees = 5 arcseconds at the 
focal plane. If the centroiding is performed to 1/5th of a pixel (a 
reasonable and, I think, slightly pessimistic guess), then you'll get 
something like 1 arcsecond guiding accuracy. Hmm. Might be marginal. 
Everyone I've noticed going the compact guidescope route these days has been 
using an STV. Has anyone here tried guiding with an ST-4 at something like 
300 mm focal length?

Regards,

Greg

----- Original Message ----- 
From: "Steve Lindsey" <stevelindsey@earthlink.net>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Wednesday, May 04, 2005 6:26 PM
Subject: Re: [APML] Guiding (was M101)


> Greg,
>
> I thought of getting a 60mm BORG for guiding but thought the focal length 
> with an ST4 was getting low for imaging at 980mm with my TEC.  What fl do 
> you image at guiding with the BORG/STV combo?
>
> Steve 


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