Focal Reducer?

Matty P (Matt) · #1 21-03-2008, 06:30 PM

Hi all,

I have been Googling around but can't seem to find and useful information on which Focal Reducer I should be using with a DSLR camera.

My main imaging scope will be my C8 with a FL of 2000mm. I am aware that I would be crazy to start off astrophotography using a 2000mm FL. So I need some help.

I have read that using a F3.3 reducer will create severe vignetting on an image. Is this true?

F6.3 reducer = 1260mm FL

F3.3 reducer = 660mm FL

Thanks in advance.

turbo_pascale (Rob) · #2 21-03-2008, 07:20 PM

My understanding is that the F3.3 reducer will be a real problem, and Chris Venter (author of DSLR Focus) categorically told me not to purchase the F3.3 reducer when I was looking to get one, and to get the F6.3 instead.

[1ponders] (Paul) · #3 21-03-2008, 08:10 PM

You will still get vignetting with the 6.3 and some out of shape stars at the edges and corners, but nowhere near as bad as with the 3.3.

Get yourself an ED80 and start with that and use the 8" as your guidescope.

RB (Andrew) · #4 21-03-2008, 08:13 PM

Matty the F3.3 is not suitable for DSLR use.
It will severely vignette the image.
The F3.3 reducer was designed to be used with the small size sensors, eg the Meade Pictor or similar size CCD's because it would only utilise the centre of the imaging circle thus not show the vignetting.

DSLR sensors are larger so they will show the outer parts of the imaging circle and any vignetting associated with these reducers.

You will still get vignetting with the F6.3 on a DSLR but not as bad.

EDIT: Doh, Mr Ponders beat me to it - yes listen to his advice Matt.

Matty P (Matt) · #5 21-03-2008, 08:22 PM

Thanks for your help guys,

I will start off with the F6.3 and go from there.

My plan is to get my feet wet and start off with my C8 for astrophotography. From there I plan to buy an ED80 and use my C8 as a guidescope.

[1ponders] (Paul) · #6 21-03-2008, 08:26 PM

RB had nailed it with the 3.3, in fact the 6.3 was not produced for DSLRs but for larger chipped CCDs like the DSI etc.

You might want to have a read of this to get a better idea of how FR work.
http://www.mailbag.com/users/ragreiner/opticlens.html