Hi!

Canon 300D un-modified.
30s x 30
Televue 101

There are some red and blue dots in the photo. Can anyone tell me what it is? How to avoid this problem?

Aw Yong

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Take similar exposures with the scope covered so no light can get in or the shutter closed if that's possible. Do you still get them? If so, it is noise generated by the camera's circuitry. If they vanish then you had some sort of internal reflection problem. I lean toward the former however. If they appear randomly, by stacking 6 or more shots using a sigma combine routine they should be eliminated. But they shouldn't be there so if still under warranty you'll want them to fix it.

Rick

This is an RGB CCD and hot pixels show up as one of these color. Based on the pattern I suspect that your 300D has some hot pixels and they stay in place when the pictures are aligned to the stars before they are stacked togethet. Many cameras can be set to subtract a dark frame automaticelly before saving it when the NR mode is selected (at the expense of a factor of two in time), but maybe not the 300D. In that case you have to create your own darkframes. The pattern looks consistent enough that one dark frame taken before and one after the entire sequence should be good enough. Hope this works for you.

hha

Hot pixels were my first thought but then I rejected it since exactly the same pattern is seen both above and below M42. Left red one is different from the right in the same way every time Hot pixels don't behave this way in my experience but then I don't use a DSLR. Without seeing individual frames its hard to tell. I've seen this as read noise a time or two.

Anyway, if on each frame they are the same pixels, irrespective of where M42 is on the frame, then they are hot pixels and dark frames definitely are the way to go. If they are different pixels then it is something else.

Rick

I don't see a discernable pattern across the entire photo and given my DSI will produce exactly the same effect without darks, I would have to go with hot pixels.

Kind regards
Matt

Thanks for you guys! I did not take the dark together with the light frames. So it seem my dark frames did not subtract away the hot pixel. Maybe this cause the problem. First time taking the Nebula images, I am still learning..

Aw Yong.

You don't say if they appear the same place on each frame. Hot pixels don't move, at least those that a dark can removed don't. So if they are the same place each frame darks will help. If they move as I suspect from the image they won't help as they expect the hot spots to stay put.

Rick

In my case, the tracking is imperfect so that over a series of numerous 1-2 min shots, the stationary hot pixel is imaged sequentially a few pixels in the opposite direction to the tracking error. The tracking error is not sufficient to be obvious in individual frames but the final stack shows this as a 'trail 'of hot pixels, the different colours being the same type of error through different filters. At least that is how I have interpereted this effect but I will happily be corrected.

Kind regards
Matt

Matt,

If you combine your frames using a sigma-combine algorithm, this will minimize hot pixels and other anomolies (e.g., cosmic ray hits) that are not in the same place in every frame.

Just a thought,

--Andy

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Observatorio de la Ballona

Thanks Andy. Between darks and the suprisingly good hot pixel fixer in Meade's envisage, I can clean up most of the errant pixels but I will have a crack at the sigma-combine.

Kind regards
Matt