[Aavso-photometry] Focal Reducer

[Jim Jones]

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