[Aavso-photometry] Focal Reducer

[Walt Cooney]

Here is a link to Ted Agos' solution to getting the right distance for Meade and
Celestron .63 focal reducers.  It has some good info on focal reducer spacing.
Ted is a very nice guy.  I was traveling on business less than an hour's drive
from his house so I took the opportunity and visited him when I was looking at
buying his focal reducer adapter.

http://www.ziplink.net/~lester/FLR.html

Clearest skies,
Walt

-----Original Message-----
From: aavso-photometry-bounces at mira.aavso.org
[mailto:aavso-photometry-bounces at mira.aavso.org]On Behalf Of Walt Cooney
Sent: Sunday, January 23, 2005 1:09 PM
To: 'Jim Jones'; aavso-photometry at mira.aavso.org
Subject: RE: [Aavso-photometry] Focal Reducer


Jim,

I have also seen the effect of the guider chip when using a focal reducer on
SBIG cameras.  A friend has an SBIG camera and I believe he uses a .33 focal
reducer with it.  He gets a good bit of shadow on one side of his images from
the shadow of the mirror that reflects the light off to the guider chip.  I
believe it flats out OK.  Even if you flat it out though, you lose some S/N that
you can't get back by flatting.

The bright spot I was seeing wasn't a vignetting effect.  If may have been
reflections in the focal reducer itself, the Agos adjustable spacer tube I was
using, or something in my OTA itself.  I never found out for sure.  Technically,
vignetting causes the brightness of the edge of the field to fall away rather
than a brightening of the center.

Clearest skies,
Walt

-----Original Message-----
From: aavso-photometry-bounces at mira.aavso.org
[mailto:aavso-photometry-bounces at mira.aavso.org]On Behalf Of Jim Jones
Sent: Sunday, January 23, 2005 12:22 PM
To: aavso-photometry at mira.aavso.org
Subject: Re: [Aavso-photometry] Focal Reducer


Greg

These are definitely FWIW comments and are worth exactly what they are
costing you.

I use a Meade .63 focal reducer.  I have used it much more for
astrometry than photometry.  I'm pretty happy with it for astrometry.
  I'm still fooling with it to further reduce vignetting for photometry.

The 0.63 focal reducers are sensitive to the distance between the
focal reducer and the surface of the CCD chip.  If memory serves me
well, the optimum spacing for the Meade 0.63 is 89mm.  As you deviate
from that distance, your will get less reduction and will get a bright
spot near the center of your image (vignetting). To get a quality
image you really need to get close to the optimum distance.

I haven't used a 0.33 focal reducer but sent some time looking into
them.  By all accounts if the 0.63 is sensitive to the distance
between the focal reducer and CCD, the 0.33 is extremely sensitive.  I
don't know the exact distance but it is considerably shorter than the
the distance for the 0.63. You can see some of the results of one
observers testing at:

http://pws.prserv.net/testani/FR_Report.htm

I have also read one report that the the vertical mirror between the
guide chip and the main chip in dual chip SBIG cameras interferes with
the light cone with a 0.33 focal reducer.

Doc Greiner has an informative article on focal reducers at:

http://www.mailbag.com/users/ragreiner/opticlens.html#Top

Jim Jones


Greg Spear wrote:
> I am in the process of configuring a setup for Transit Search which
> consists of an Meade LX-200GPS and a SBIG ST-7XME. I'm looking for a
> good focal reducer suitable for high precision photometry (< .01 mag)
>
> So far, I've found:
>    Optec NextGEN MAXfield 0.33x
>    Meade Series 4000 Focal Reducer .33x
>
> Does anyone have any recommendations or other reducers?
> Does anyone have any experience with these or comparison information?
> I have found some information on the Internet but would like to hear
> something from those doing photometry.
>
> Thanks
>
> Greg Spear
> gspear at ucolick.org



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