More homemade focal reducer testing

[kinetic (Steve)]

Hi,

My previous thread I put in Deep Sky for some reason,
http://www.iceinspace.com.au/forum/s...ad.php?t=39567

however, I tried a few combinations of spacing and lens
arrangement for the 30mm objective....not really anything
outstanding.
Coma was pretty bad, and it's quite disappointing, as this
was my trusty FR from webcam days for years on the 8"F7 Newt.

It just doesn't suit the GSO and it's short F.L.
In the best spot I get vignetting, so I made a 2" barrel on my lathe
to sit it in.I adjusted spacings and the 2nd shot below from Jan 04
is the best from a few nights testing.

Then I tried a 40mm doublet I have, that worked better. Pic 3 from Jan 08/09.
Still some slight coma, not a really good night to test it. Stars were
blurring on a 5 sec exposure with the DSI.

Compared to my Prime/Newt focus shot from Dec 02 2008, they don't
even compare.....Dec 02 was an outstanding night of seeing
(Pic 1)

Pics also of the 2" adaptor I made. Old VCR heads are a great source of 2"
Aluminium stock!


Steve

[pjphilli (Peter)]

Hi Steve
I too have experimented with home-made focal reducers. I currently have one fitted to my guiding camera to reduce the focal length of my guiding scope from 1000 to about 660mm (ie focal reduction of 0.66). I trust that you already know that:
Focal reduction ratio = 1 - distance of lens centre to the CCD focal plane divided by the focal length of the lens.
I had a dinky finderscope that came with a scope that I bought. However, it did have a nice glass objective lens. I found that the lens had a focal length of 85mm and buy popping out the lens in a cheap screwon Moon filter the little scope lens fitted exactly in the holder.
I then measured the length from the screwed on holder to the ccd as 29mm. Using the above equation 1-29/85 = focal reduction of 0.66.
I do not used this focal reducer for imaging but I do note that the stars showing in my guiding camera are perfectly round with no coma or "rooster
tails".
Cheers Peter