Rainbows in the image - OAG prism?

[physicus]

I'm getting these rainbow intrusions on my images and wondered if anyone would instantly recognise the problem !  They're a bit faint, but hope you can see them; they sure appear as full little spectrums when stretched.  The set up is C9.25, Feathertouch Crayford, 0.63FR, Celestron OAG and manual wheel, Atik460.  I thought at first I may be getting leakage through the wheel thumbwheel, but am wondering now if its the OAG prism - and have pulled it up a bit to try tonight.  (The pic btw is of course M57 with Ha substituted for Red - ongoing attempt at the second ring :-)).  Any ideas that might save further experimenting welcome.  Thanks.

Tim

[MarsG76]

Hi Mate,

I had the same effect on my 8" SCT when imaging M42 using a full spectrum modded DSLR and through a f6.3 reducer/OAG setup. The loops were visible on all shots taken thorough Ha, SII and OIII as well as RGB. I have never seen this when imaging on any other object so far.

There was a bright star off frame in the direction of from where the loops originate. A friend also imaged M42 using a unmodded DSLR and through a f6.3 focal reducer but no OAG and had the same loops in the same general location on his subs  so from this I suspect it might be caused by the focal reducer rather than the OAG. 

See if there is a bright star/object off frame from M57.

Later in the year I'm planning to image the Orion Nebula again but at F10, with and without a OAG to see if I can replicate the loops/reflections without the focal reducer, but as I suspect the FR I think that they will not be there. 

[physicus]

Hi Mate,

I had the same effect on my 8" SCT when imaging M42 using a full spectrum modded DSLR and through a f6.3 reducer/OAG setup. The loops were visible on all shots taken thorough Ha, SII and OIII as well as RGB. I have never seen this when imaging on any other object so far.

There was a bright star off frame in the direction of from where the loops originate. A friend also imaged M42 using a unmodded DSLR and through a f6.3 focal reducer but no OAG and had the same loops in the same general location on his subs  so from this I suspect it might be caused by the focal reducer rather than the OAG. 

See if there is a bright star/object off frame from M57.

Later in the year I'm planning to image the Orion Nebula again but at F10, with and without a OAG to see if I can replicate the loops/reflections without the focal reducer, but as I suspect the FR I think that they will not be there. 

Thanks for that.  I'm going to have a good look through recent subs and see if it's a consistent issue.  The only really bright object near M57 is Vega, but that's well out of frame.  Sky is clear tonight, so we'll see what turns up.  I've only just started using this arrangement with the Crayford between the OTA and the FR; hoping that's not it, because the set-up otherwise if delivering nicely.

Best

Tim

[ollypenrice]

I found that an OAG would create a shadow if too deep into the light path. I never found that it generated arcs or rainbows.

I've had these, though, and I think they come from internal reflections. Bright stars out of shot can be a source. But the truth is that some systems are just prone to them. They don't tell you this on the tin.

Olly

[physicus]

I found that an OAG would create a shadow if too deep into the light path. I never found that it generated arcs or rainbows.

I've had these, though, and I think they come from internal reflections. Bright stars out of shot can be a source. But the truth is that some systems are just prone to them. They don't tell you this on the tin.

Olly

It's intriguing; I almost hope it doesn't go away till I've sorted the mystery.....he lied.   I've just started using narrow band filters too: a whole new world of shiny surfaces looking for trouble.  Anyhow. I've lifted the prism a tad and turned it parallel to the long face of the chip, so hopefully one variable minimised.

Tim

[MarsG76]

I think its pointing to the internal reflections in the FR.

[ollypenrice]

I think its pointing to the internal reflections in the FR.

 A very common source of bother.

Olly