Star trails

[Itsakpr]

Hi,
can anyone explain why my Minolta 7D gets a red haze in the upper left corner of the image when I take any photo that is longer than 30 seconds(bulb)? I have taken hundreds of star trails with my Minolta film cameras. But, have yet to get a descent one out of a digital camera. C*n*n and N*k*n digitals also.

I am looking forward to a new Sony camera soon(my hands are 2 big for the A200 to be feasible). I'm hoping this is not inherent in all digital cameras.

[David Kilpatrick]

It is a well-known thing with this Sony sensor. Magnus Fjørtoft has done some astro photo experiments, and night shot experiments, with stacking exposures - the articles include references to the purple haze corner. They are included in the back issue pdfs you can download from the main website.

David

[Itsakpr]

Thanx David for getting back to me so quick. I searched PCA, Google and others but could not find article you were referring to.

[David Kilpatrick]

It's in the final issue of Photoworld before Sony - the one you can download from the 'About' page of this website.

http://photoclubalpha.com/about/

It is a substantial download, a 9.5mb PDF, with Magnar's excellent article included.

David

[Greg Beetham]

Hi,
can anyone explain why my Minolta 7D gets a red haze in the upper left corner of the image when I take any photo that is longer than 30 seconds(bulb)? I have taken hundreds of star trails with my Minolta film cameras. But, have yet to get a descent one out of a digital camera. C*n*n and N*k*n digitals also.

I am looking forward to a new Sony camera soon(my hands are 2 big for the A200 to be feasible). I'm hoping this is not inherent in all digital cameras.

Hi,
Like you, I got to wondering about Digital star trails, after someone on another forum asked the same thing.
I'm not totally convinced of the suitability of digital for star trails.
The main problem as I see it, is that the individual stars in the field being exposed, stay too long in the "one place" over the corresponding sensor photosites thus potentially causing spot overexposures for every bright star in the field, especially if using a fast wide angle lens and high ISO.
What I was going to experiment with, was slowing things down to say 100 ISO and maybe even stopping the lens down to an f/stop (yet to be determined) that smears the stars out without any overload, also in combination with de-focussing slightly.
Defocussing could be useful with trying to record the spectrum of the individual stars, as this is one of the main reasons for taking star trails and one of the most difficult too capture properly, even on film, I've had a few tries at it and have never been totally satisfied with the results.
If your interested in the colour of the stars it might pay to limit the length of the trails anyway, so that the identity of each can be ascertained, from a star atlas later.
The following is some spectral info if it's of any interest.
Main spectral classes. O, B, A, F, G, K, M. (Oh Be A Fine Girl Kiss Me)
Spectral class O and B (blue white supergiants) Only a handfull of these in the sky anyway and would look like a white streak no matter what you do with exposure, and the same applies to class A (white giants).
The next ones have a potential to show up differently in the exposure,(depending on their distance and therefore absolute brightness)
Class F (yellow-white)
Class G (yellow), our star the Sun is a G,
Class K (orange)
Class M (orange-red)

Anyway David K. might be able to offer some insight into the pixel overload question, (if that is what is happening in long exposures) sensor behavour is not something I have much knowledge in.
Lots of luck.
Greg

[David Kilpatrick]

Magnar Fjørtoft is the man to answer this. He can be found under Njordsternen on the dPreview forums and may drop in here occasionally too.

The correct setting is normally f8 at ISO 100 or 200 for star trails, and if there is no night sky light or troublesome local stuff around, you can expose for as long as your system will allow. The old Canon 300D is fairly good at this - 20 minutes no hassle. The Sigma SD10 is hopeless - the shutter always closes after 30 seconds no matter what you do!

David