Problems with focal reducer

[jnp]

Hi,

I took a punt on fleabay, this time for a "Rini 1.25" to T 0.6X telescope reducer", but I don't think this one has paid off. Has anyone else used one of these?

The problem is that I am unable to retract the focuser sufficiently. The listing implies that the new focal plane would be 25 milimeters inwards from the normal focal length, but I have found this to be more like 25 centimeters when experimenting. I have tested using a 90mm F11 and a 70mm F8.5 refractors with a Canon 1000d DSLR.

I have found this useful link, Focal Reducer Calculator I am not sure what the exact parameters are for this 0.6x reducer. My guesses are: diameter = 31mm, the focal length = 90mm and the distance from the reducer to the sensor on my Canon 1000d is about 70mm.

Running the calculation on the web link yields a focus position shifted inwards by 245mm (which seems consistent with my observations).

Does anyone know of any workarounds. My scope is DIY, but accomodating an extra 25 cm inward travel will definitely be a challenge. As a last resort, the vendor will accept a refund, but I woud have to pay return postage.

I had previously thought that a focal reducer was just like a barlow in reverse, but it's really not that simple!

cheers

John

[Merlin66]

John,

These 1.25" reducers don't work on DSLR's.

I have a Atik version, the focal length is 80mm, so to achieve the x0.5 it should be 40mm from the CCD - this is impossible - the body is 55mm deep!

They are designed for use in the nosepiece of a webcam or smaller CCD's where the chip is close to the front surface.

[jnp]

Ahh, you confirmed what I was deading.

I assumed the reducer was suited for DSLR because it is fitted with a t-adapter.

It's a shame. Playing with the Focal Reducer Calculator shows just a small reduction in the distance to senser will make a huge difference, but as you say, its not practical with a DSLR.