bloom on images

[raymo]

Can anyone help? I have noticed that all my efforts at the Helix have a
sort of large cloud at the bottom right of the image. If I don't get the RGB balance right in DSS, the cloud takes on a tinge of the dominant
colour. It has happened with images from both sessions. I am looking
out over forest, away from town, so not localised light pollution. It's
the first time I've seen it, and most of my shots are taken in that general direction.
raymo

[traveller]

AMP glow? How long was the exposure? What's the ambient temp?
Bo

[mithrandir]

I'd agree with Bo. All uncooled DSLRs suffer from amp glow. You either have to stick to exposures short enough to avoid it, or fix in post-processing.

[raymo]

Thanks for the reply Bo. It happened both nights. Ambient 8 and 10 deg.
subs assortment of between 45 and 105 secs. As far as I am aware, amp
glow is usually at the edge rather than in the middle.
I have just noticed that I said that the cloud was at the bottom right of the image. I should have said the bottom right of the object. [the Helix]. Why would I be getting amp glow now in the colder part of the year? You can see it in the image I just posted. It doesn't look like any examples I have seen.
raymo

[traveller]

Yes, I can see it now. Odd.
Maybe an internal reflection off a shiny part along the optical path?
Any exposed shiny screws or bright objects near the optical path?
Did you use any filters and were they screwed in/clipped in properly?
A terrific image of the Helix by the way :thumbsup:
Bo

[raymo]

Thanks again Bo, No filters, but might have found the cause, which I will
mention in my reply to Cometcatcher, who has given me some advice in a
reply to my posted pic of the Helix. Kill two birds with one stone,as it were.

Sorry, Andrew, I neglected to thank you for your reply.
raymo

[MrB]

All?
I've never seen it in exposures from my 60Da, even with half-hour long subs.
I've also seen people post stretched darks from 1100D's that show no sign of amp glow.

[MrB]

Ray, it looks like vignetting to me, was this shot thru the newt? Is the secondary big enough for 100% illumination of the APS-c sensor?

[raymo]

Thanks everyone. As I have just said to Cometcatcher, I'm wondering whether it is stray moonlight entering the back of the camera because I forgot to fit the rubber viewfinder cover. Vignetting would be pronounced in the corners; what I am referring to is a large amorphous patch nudging
the bottom right of the Helix itself. My 80mm refr has vignetting, but I've not had that prob with the Newt.
raymo

[MrB]

Sorry, yes vignetting was the incorrect term to use.
I still think it may be an issue with your 100% illumination field width.

[MrB]

I've had a look around on the interweb for the specs of the Skywatcher, I could only find one reference to a 52mm minor axis secondary, not sure if this is correct.

I don't know the exact dimensions of the scope so I plugged some best case dimensions into 'Newt for the Web':
Assuming a 250mm diameter tube for a 200mm primary, a 52mm secondary and only 45mm max backfocus (essentially a zero-height focuser, that is, camera flange pressed against tube wall) the 100% illuminated circle is ~21mm.
The 1100D's chip diagonal is 26.626mm (22.2mmx14.7mm sensor)

Ofcourse the scope has a focuser so guesstimating some dimensions:
about 50mm or more high, plus maybe another 10mm in-focus, plus the 44mm backfocus needed for the camera... I get just shy of an 8mm 100% illuminated field, which would be about right looking at the Helix image.

Ofcourse this is for the 52mm secondary size I found on the net....
If your scope has a secondary around 70mm minor axis, you should have a 100% illuminated sensor area.

[raymo]

Hi Simon, you've been busy. Minor axis is 52mm. O.T.A. I.D. is 240mm
tube thickness 1.5mm. Focuser is 57mm high. 1100D is focused at prime
with focuser racked out approx. 11mm.
raymo

[MrB]

Hi Ray,

Using the dimensions you gave, 'Newt' reports a 100% illuminated circle of 8.17mm
I imported your image into AutoCAD so I could draw an 8.17mm circle over it, looks like a decent match, tho it looks as though the image has been cropped and I don't know by how much or where, so the AutoCAD overlay isn't scaled perfectly.

With the dimensions given, a new secondary that is 65mm or larger will give you 100% illumination over an 1100D's sensor.

[raymo]

Thank you very much Simon. I appreciate the effort you
have put in. I'll have to have a look and see what size
the secondary is in my 10" Dob. Maybe I could try it in my 8".
raymo

[MrB]

No worries, I actually enjoy doing this stuff :D (using Newt / problem solving / using CAD)
I've also never scaled an image in AutoCAD before, even tho I use it daily for work, so I learned something new :thumbsup:

[raymo]

We both had a win then.
raymo