Re: Guiding and Magnification?

From: Chris L Peterson (clp_at_alumni.caltech.edu)
Date: 03/29/05 

Date: Tue, 29 Mar 2005 15:27:45 GMT

On Mon, 28 Mar 2005 14:57:56 -0500, Davoud <star@sky.net> wrote:

>Conventional wisdom once held that the guide 'scope should have a
>greater magnification than the imaging 'scope/lens by a factor of
>(something or other.)

True, but that conventional wisdom was based on visual guiding, not
autoguiding.

> With the advent of cameras that contain both
>imaging and guiding chips, or that image and guide using the same chip,
>this notion seems to have fallen by the wayside, and I haven't seen
>this "old rule" mentioned in some time.

It is, of course, a major disadvantage of self-guided and off-axis
guided cameras that the guidescope and imaging scope are the same focal
length. This results in a tiny FOV and frequent difficulties acquiring a
guide star. You would like the guidescope focal length to be much
shorter than the imaging scope focal length.

>This is not an issue if I am imaging with my little Tele Vue 76
>(particularly with the reducer/flatener is in place) while guiding with
>my Meade 8". But what about the opposite situation -- won't it be
>necessary to put a TV "Powermate" or a Barlow in front of the ST-4 when
>it is mounted on the TV 76 and I am imaging with the Meade?

I used an ST4 guider for many years (and still do occasionally on a
portable setup). I used an ST8i imager on my LX200 (image scale
0.8"/pixel) and the ST4 on a piggyback ST80 (image scale 7.5"/pixel).
Based on this, I've concluded that a 1:10 ratio between imager and
guider pixel scales is perfectly safe. (To be clear, it isn't the ratio
between scope focal lengths that matters, but between pixel scales. The
ST4 probably has large pixels compared with your imaging camera.)

So calculate your two pixel scales, and only use a barlow on the guider
if the ratio is greater that 1:10, or if you see obviously bad guiding.
In an observatory setting, I would not use the ST4 standalone. Programs
like Maxim produce better quality guiding from the ST4 that the ST4 does
using its internal code.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Relevant Pages

  • Re: Question about this setup
    ... Is a guiding scope ... >> diving right into imaging, but want something that has the potential ... > Whether you need guiding or not depends on the focal length and exposure ... That scope has all the required elements for long exposure astrophotography, ...
    (sci.astro.amateur)
  • Re: Guiding and Magnification?
    ... cross hairs easer to see. ... Guiding with a ccd one can centroid to better than one pixel in cases ... I usually try to find a guide star ... > imaging and guiding chips, or that image and guide using the same chip, ...
    (sci.astro.amateur)
  • Re: Question about this setup
    ... > diving right into imaging, but want something that has the potential ... What I like about these mounts is among other things the ability to ... Whether you need guiding or not depends on the focal length and exposure ... you need is a good motor like the MT-1/SD-1 shown, but you will not be able ...
    (sci.astro.amateur)