Focal reducer - How to prevent reflections ?

[jarbi]

Hi Everyone,

I have started to use a cheap 1.25" 0.5x focal reducer with my SAC10 CCD, but the results are unusable... There are always reflection spots around the brighter stars on the image.

I guess my reducer has no coating to prevent this. Could you advise which are the good ones for a 10" Skywatcher Netwton ?

cheers,

Janos

[bern]

They might not be reflection spots, can you post an image?

bern

[jarbi]

They might not be reflection spots, can you post an image?

Bern,

Please find an M33 below captured by using the focal reducer in question. What do you think ?

(click to enlarge)

[bern]

The reducer is too far away from your chip which is giving rise to chronic vignetting and aberrations.

If you can reduce the seperation your problem should go away. If you can't see how to do this please post a description and image of your setup and we'll take it from there.

bern

[jarbi]

Thanks Bern,

I have an 1.25" T-thread attached to the CCD and the focal reducer is on the top of it. This evening I will measure the exact distances and I will also post a photo.

cheers,

Janos

[jarbi]

Bern,

I have measured now the distances: there is 4 cm between the focal reducer lens and the CCD chip. This is exactly what I am advised to have when using the focal reducer calculator. I have attached an image also, please find below.

I still strongly suspect that the problem is reflection. If you look at the example image, you can see the spider vanes of the diagonal and the shadow of the draw tube being in the light path round the brighter stars. When I look at the inner surface of the focal reducer, I don't see any trace of coating .

[bern]

I take your point but this type of vignetting and star aberration is EXACTLY what I see when I use a 1.25" reducer too far away from a chip which is the size of the one in your SAC10.

The 40mm distance you've been given is what's required to give a .5x reduction with this type of reducer, however this completely ignores the size of chip and F ratio of your imaging system. Your 10" Newtonian is around F4.7 so my experience tells me that a suitable chip to reducer distance is 25mm to 30mm for good image quality.

The easiest way to achieve this is to replace the nosepiece with one that is 10 to 15mm shorter than the one you're using presently. I stock a nosepiece expressly for this purpose which is about 14mm long from the flange, I'm hoping this is about right?

If so the good news is it's only a tenner and you can have your money back if I'm wrong

bern

[centroid]

I 'posted' this link for SteveL the other day:

http://timosastro.1g.fi/tools/focalreducer.html

Dave

[bern]

Dave, that's a useful link which I use myself for deriving reduction ratio vs distance but it unfortunately it gives no indication as to the quality of the image over a given chip size.

bern

[jarbi]

The easiest way to achieve this is to replace the nosepiece with one that is 10 to 15mm shorter than the one you're using presently. I stock a nosepiece expressly for this purpose which is about 14mm long from the flange, I'm hoping this is about right?

If so the good news is it's only a tenner and you can have your money back if I'm wrong

bern

Bern,

Thanks for your infos, I will PM you about the rest .

cheers,

Janos