Help with focal reducer needed

[Centus]

Hi,

I got a 0.5 focal reducer a few years ago, I'm not even sure what brand it is of it is any good.

The picture below is what I get with it when I use my Starshoot DSCI with my F/8 150mm Newtonian.

This is a processed/stacked picture, but the artifact is there in the single frames...

The field is bigger (as expected) by about 50% but I get this strange artifact near the stars which gets worse with distance from the middle.

Is this a crappy focal reducer? or something else?

Just to be clear, scope is well collimated, this is not seen without the focal reducer when I image with my Canon EOS DSLR on the same scope.

Thanks!

[michael.h.f.wilkinson]

Some form of coma or astigmatism in the focal reducer, I would guess.

[Grotemobile]

It could be the reducer.Perhaps if

you got a good 6.3 one

http://www.firstlightoptics.com/reducersflatteners/celestron-f63-focal-reducer.html

That is still not a bad picture.

Have a look on Astro Buy & Sell.

You sometimes get them come up

on there a lot cheaper.

Perhaps someone else can help.

Steve

[Centus]

So it is definitely the cheapness of the focal reducer?

Thanks for the quick replies!

[michael.h.f.wilkinson]

Your own experiments rule out the scope, so the focal reducer seems to be at fault. Even an expensive focal reducer can cause problems if it is attached to a scope of a type the reducer was not designed for. I use my SCT reducer (0.63x) on my SCT, not on the refractor.

[Centus]

Thanks, how do I know if the 0.63 sct reducer fits my newt? is there one that is known to work well with an F8 6" Newtonian?

[Guest]

It is not a problem with the focal reducer, this is normal when using them with certain scopes, it is all about getting the right distance between camera sensor and focal recucer lens, if you are reducing too much (distance to big from chip to reducer) then you will get odd shaped stars nearer the edges, so i sugest making the distance shorter, for a 6.3fr and my DSLR the optimum spacing from sensor to centre of reducer lens should be 95mm, with this set up i get very little coma or odd shaped stars.

Looking at your picture it looks like your collimation is way off, you need to spend some time on that first

MM

[Centus]

Great news MagnaMan! I like things I can solve.

Any pointers on where to start studying this subject so I can find the optimal setup for my Orion Starshoot DSCI camera?

Thanks a bunch!

[Guest]

you need to sort out the collomation issue first, and then experiment with different focal reducer spacings, it's all about eliminating one thing at a time and sorting problems out, it's a steep learning curve and we have all been there

MM

[Centus]

Thanks, I'll hit the internet then...