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What's this in my images?


AstroJOE
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i've been out imaging twice over the past week or so and each time I've ended up with a strange curved line across the light frames I've taken. I'm not sure what it is, a light reflection, something on the DSLR sensor or something in or on the telescope (CPC1100)

I've attached a few pictures to show what I mean.

Comet Lovejoy

post-21020-0-60758400-1423174365_thumb.j

Orion Neb1

post-21020-0-60758400-1423174365_thumb.j

Orion Neb2

post-21020-0-60758400-1423174365_thumb.jpost-21020-0-60758400-1423174365_thumb.j

When I take long and short exposure pictures with the DSLR on it's own I don't get the same problem. Could this be dust or a hair inside the OTA? Or something else?

Any help or advice would be welcome!

James

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Inspect everything in the optical chain carefully in good light and you will find the culprit. Clean everything really good. Might be a hair? And don't forget to inspect the camera's sensor as well.

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Thank you both. I found the culprit !!

I locked the mirror in the DSLR and on closer inspection I managed to spot a small 1.5" hair trapped inside. I've removed it with the help of a pair of tweezers....

Happy days!

James

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Personally I'm not entirely conviced that the two features are from the same source. The hair fits the first one perfectly. That is doubtless sorted. If you return to the Trapezium, though, and take the same shot I wonder if you'll still get the arc? And that's the source of my doubt; it's an arc where the first one wasn't. It just might still be an internal reflection of some kind. They are quite common. I may well be entirely wrong about this, though.

Olly

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Personally I'm not entirely conviced that the two features are from the same source. The hair fits the first one perfectly. That is doubtless sorted. If you return to the Trapezium, though, and take the same shot I wonder if you'll still get the arc? And that's the source of my doubt; it's an arc where the first one wasn't. It just might still be an internal reflection of some kind. They are quite common. I may well be entirely wrong about this, though.

Olly

Thanks for the input Oli. I was hoping to check it last night but the conditions were poor. I'll have another check at the first opportunity. What might be the cause of the internal reflection ( if that's what it is) and how could I overcome this? Never had this problem before.

Thanks

James

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Thanks for the input Oli. I was hoping to check it last night but the conditions were poor. I'll have another check at the first opportunity. What might be the cause of the internal reflection ( if that's what it is) and how could I overcome this? Never had this problem before.

Thanks

James

Unfortunately they are not easy to overcome. You need to check for anything badly baffled or incorrectly blackened in your light path. Some matt black finishes are not matt black in the infra-red, which they reflect quite strongly. Paints and finishes with pigments rather than dyes are needed. Barbecue paints are suitable because only pigments are heat resisitant. What about your dewshield?

The more I look at the images with the arcs the less I think it was the hair that made them. One potential clue is that the arc has moved between Trapezium images and you have clearly refocused between them as well because the second one is far better. While I can identify the clue I cannot, I'm afraid interpret it. I don't know whether it points to the focuser or not. There is a bright star out of shot about a frame's depth below your image. That might just be the cause - or I could be wrong from the start.

Olly

Edited by ollypenrice
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As Olly says, it's very easy to assume because you've found a reason for a problem it'll solve everything. Coincidences happen and two unrelated visual artifacts appearing together can happen. It's happened with my camera but in that case one problem caused the other.

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Thanks guys. The strange thing is I haven't introduced any new items to the light path at all. The most recent thing would be the 0.63 focal reducer although but I've take images with this in place before and didn't get the same problem. So I can probably rule that out as we'll. The dew shield I use is the same one I've always used (heated astro-zap) and the diagonal is removed altogether when I attach the DSLR so it's not that either. So I'd be at a loss to explain it if it where a sudden internal reflection issue. I'll have another go at the Trapezium ASAP to see if the problem is still there. Thinking out loud I wonder whether frost on the inside of the dew shield might cause a reflection? It's been mighty cold out there recently... May be I'll try it with out without the dew shield just to rule this out.

James

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