Optics or focus problem?

[deckardbr]

Hi everyone,

During my last observing session I noticed something weird while using stars to do the alignment.

Most stars I look at, once in focus (or so I believe) appear as a couple of bright red and blue dots, very close to each other.

I know for a fact that I'm not looking at double stars so I believe it is an artifact of my scope or optics.

Have you ever experienced something like that? Am I doing something wrong? Is that the way they are supposed to look via SCT?

I was using my 2" SWAN WO EP when I noticed it.

Any help diagnosing this would be appreciated.

Cheers

Andrea

[michael.h.f.wilkinson]

Were these stars close to the horizon? If so, you might be seeing atmospheric refraction (the atmosphere acting as a prism). An SCT should certainly not be showing that kind of error.

[John]

I've seen stars looking like that too - when I had a low cost broadband filter on the end of the diagonal and forgot it was there.

[deckardbr]

Michael,

No, not close to the horizon. I noticed it also when I was looking at M45 which I believe is ~50+ degrees up from the horizon.

John,

Well, I actually was using a light pollution filter. I was told I should use it all the time... It should be narrow band and not cheap (£170).

I'll try and remove that and see if it is defective.

Thanks

Andrea

[michael.h.f.wilkinson]

What star diagonal are you using? The standard (prism) type? That, in combination with an LP filter might cause what you saw.

[John]

Michael,

No, not close to the horizon. I noticed it also when I was looking at M45 which I believe is ~50+ degrees up from the horizon.

John,

Well, I actually was using a light pollution filter. I was told I should use it all the time... It should be narrow band and not cheap (£170).

I'll try and remove that and see if it is defective.

Thanks

Andrea

That might be your problem. You really don't need a narrowband filter in place all the time. They really only work on nebulae.

[deckardbr]

John,

Ah, that makes me feel better. Tonight it should be clear skies and will give it a shot...

Michael,

No, the 2" WO one + 40mm EP SWAN + Narrowband light pollution filter.

Thanks both

[michael.h.f.wilkinson]

I have on occasion seen this effect (with the same WO diagonal, and UHC filter) if my glasses were not at right angles to the optical axis of the EP (quite far away from it actually). They show a prismatic error under those conditions.

[John]

Just out of interest, what sort of filter is it ? - I mean is there a brand / type printed on it ?

£170 is a lot for a filter, even a 2" one.

[deckardbr]

John - Astronomik UHC Filter 2" - http://www.widescreen-centre.co.uk/Products/Astronomik_UHC_Filter_2_00.html

[alan potts]

John,

I used to think 170 pounds was a lot for an eyepiece, wrong. I think the 0 III filter by Astronomik is now about 164 pounds, I was shocked after talking to you I thought, I will get one of them . Then I thought maybe I will wait.

Alan.

[deckardbr]

I was shocked as well. But in the end, I figured I'll need it sooner or later in London... and went all in.