Spots on stars? Any ideas?

[[1ponders]]

While going through some NGC104 shots I too the other week I notice for the first time some strange spots on the stars. This was using the ED80 and FR. Image is a stack of 240sec@ ISO400. The first one is a crop of the top left hand corner of the second shot, prior to curves. The second shot is the whole image but with very heavy curves.


Strange bit is, I took other 240 sec shots, but at ISO800, the same night of another area of the sky and there are not dark spots :shrug:
Any ideas?

[joe_smith]

I have seen same pages that explain this but I'm having trouble finding them. I dont even know whats it called to google it.

Here is one page I was on last night the talks a bit about it but not with your camera.

http://www.leadbeaterhome.fsnet.co.uk/1004xcam11.htm

[2020BC]

Hi, Paul.

The shape of the stars (left pic) looks like a classic case of field curvature (similar to "coma" in newtonians).

The Orion ED80 has very pronounced field curvature. I was surprised by how strong it was when I bought mine.

Using a focal reducer without some sort of field flattener will only make it worse.

The problem is sensitive to focus, so without buying a field-flattener the only thing you can do to minimise the effect is to spend more time on acquiring exact focus. This won't eliminate field curvature but it will help a bit.

When I buy a focal reducer for my 80ED it will be something like the Williams Optics x0.8 that has a field flattener built in so it yields (according to those who champion it) pinpoint stars out to the edge of field.

[[1ponders]]

Thanks Bill.

That is part of the reason I'm trying the Meade FR as it is "supposed" to flatten the field as well as reduce the FL, besides these spots are right across the field. You can see the curvature in the triangular stars near the corner, but much less so in the center. In the center of the original, when viewed at full size the stars are nice and round, and for quite a way out.

You're right about the focus. Checking back over the 120 sec and 180 sec of 47 tuc that night the focus was much sharper and there are no black dots. There was still the curvature out towards the edges and in the corners but the centers were fine. The focuser may have slipped marginally as the scope moved more and more towards the vertical during the night.

Thanks. Time to invest in the 10:1 focuser add-on I think

[2020BC]

Where do you get a 10:1 focuser upgrade for an 80ED? How much does it cost?:)

[[1ponders]]

There are a number of places Bill

http://www.clearvueoptics.com/focusers.htm

http://www.agenaastro.com/Pages/Products_AllOther/products_Crayford_Focusers.htm

http://www.focuser.com/storefront.php

http://www.william-optics.com/wowebs/prod_acc/foc_2s110_mfr/features0.htm

And my personal favourite 'cos it comes with a decent finder bracket
http://www.william-optics.com/wowebs/prod_acc/foc_syn_2in/features0.htm

[joe_smith]

I might be wrong but the shape of the stars could be caused by field curvature problems, but I think the dark shadows on the stars is a ccd problem. Dosent coma type erorrs change the shape of the star not put shadows on them?

[[1ponders]]

Not sure Joe, but this was the only imaging session where the dark shadows came out. :shrug:

Are you telling me I'm going to have to get my wife to by me a 5DH? :D

[joe_smith]

do you think it would work to help you get a new one ;)

I have seen simlar images with the same thing and they all say its something to do with the way the ccd works (if i only bookmarked the site :doh:) might of just of been a glitch in the matrix, never to return again. The images in the link above look very close to what you had.

[2020BC]

Ah, yes, the Williams Optics one looks just the thing...:).

[tornado33]

Are the spots present on the individual exposures, or only on the stack? If on the stack, it must be some sort of processing artifact.
Scott

[2020BC]

Paul, Is it possible that you accidentally included one or more lights as a dark frame ? I've done this with a similar result. Is this possible? It would explain a lot. Say you've got a star at x,y on a sub. If the reference light is offset by x+n,+y+n you'll get dark stars (i.e. a negative) offset from the lights. Here's your image magnified x2.

[[1ponders]]

Good point Scott. I've just been checking and there are none in the raw images but they are present in the Fits format. I'm doing a reconversion and see if it happens a second time

[[1ponders]]

You are spot on Bill

:ashamed: ummmmm.....:ashamed: I went back through the processing of the images an they were fine up until I calibrated them. I then checked my darks and ......ummmmm :ashamed: I had a light in with them :doh::doh::doh:

Thanks Joe, Bill and Scott

[2020BC]

No problem. Let me know how the Williams Optics Focal Reducer/Field Flattener goes when you get it....:lol:

[[1ponders]]

np :lol:

[RB]

:lol: Paul the pun-master.......:rofl: