Home Made Focal Reducer.

[tilbrook@rbe.ne]

Hi,
For quite a few years I’ve been experimenting with home-made focal reducers, for my Sac-7 CCD.
The chip in this camera is small, 3.584mm x 2.688, which in my 200mm F/6 only gave me a FOV of 10.27 X 7.7 arc minutes. Frustrating to centre an object reduces sensitivity at this F ratio and shows up minute tracking errors.
The focal reducer I’ve built has taken a lot of time to make. It consists of three lenses housed in old screw type camera housing. These are available in second hand stores; they are cheap so you can afford to sacrifice them for the project. The lenses consist of two 30mm objectives, and one 40 mm from old binoculars and two small hand held telescopes.
The trick is to get the spacing right between the lenses, this was trial and error. I also needed to do some mods on the scope. I needed a low profile focuser and needed to change mirror positions slightly.
Today I succeeded in pushing the FOV out to approximately 29.34 x 22.00 arc minutes, making the scope nearly F/2 (See sun image) I’ll have to wait till tonight to see what the stars are like at the edge of the field.

Cheers,
Justin.

[bojan]

Very interesting!
Could you show us the optical layout (rough drawing of lenses, how they are oriented and spaced)?
Where is the focal point of your telescope?

[tilbrook@rbe.ne]

Hi,
Here’s some more images and basic layout of my focal reducer. Originally I started out using a camera lens reducer, fine for every day photography but no good no good for astro imaging! Way too much coma.
Then I tried various fast lenses, but to find a single one that didn’t produce coma proved difficult. I’m sure you could get one made at great expense. So I toyed with the idea of using slower lenses in line, this seemed to work. But spacing them was the bit that took time.
I eventually came up with an arrangement of two lenses, which gave me a reasonably coma free field of 22 arc minutes on the long axis. I used this setup for a long time, but wanted to see if I could get more with a third lens.
I did, 29 arc minutes, but the coma was quite bad. Then I replaced the third lens with a 0.5 focal reducer which gives approximately the same field, with less coma. But I do think I’ll need a coma corrector, or just back off on the field slightly.
Anyway, I’ll keep you informed of my progress.
Note, spacing depends on, focuser height - lens housing material thickness - where the CCD chip is placed in the CCD housing and the focal length of lenses used.
Cheers,
Justin

[tilbrook@rbe.ne]

Hi,

Just a quick image of NGC 253, with the wider field. You'll notice coma on brighter stars.

Cheers,

Justin.

[bojan]

I was thinking about focal reducer before (using doublets from old binos, I have a few in my drawers) and it's good to know someone tried and obtained acceptable results.
When I have some free time I'll certainly give it a go..