Astro Picts.

[tkerr]

Thanks, Paul.


Tim-when yous say the dark points are clipped to much, do you mean I subtracted too much or not enough? On my monitor, space looks totally black.

I mean you adjusted it so that the dark points of you picture are too dark.


Here is a pretty good overview of using DSS and post processing.
[url="http://astrochat.co.uk/forum/viewtopic.php?t=13241]Astro Image Tweaking.[/url]


[quote]


As for darks and flats, I can't either to work right with DSS- maybe you can help.


When I dark frames in DSS (instead of in camera NR), I get little streaks all over the picture. I think it has to do with the fact that I'm getting a little drift (separate issue), but I don't know. I haven't heard of other people having the problem. Of course for the darks I use the same ISO and exposure time and make sure the temperature is close.


[/quote]


Make sure you are shooting your dark frames the same exposure as you shoot your light frames.
Dark frames don't correct for all the noise! Some of the random noise as well as hot or cold pixels, but there is also what some might refer to as read noise or fixed pattern noise (aka Bias or Offset) that is generated by the camera internal processor reading the data collected from the sensor. This is why you need to shoot bias frames. The ugly streaks you see all over are probably the bias error.


[quote]


Whenever I've tried flats the thing always overcompensates (gives me a bright edges and a dim center). Maybe I'm doing something wrong.


My refractor has almost no vignetting, so I never needed flats.
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[/quote]


Flats can be a bit tricky to shoot and get the right exposure. How are you setting the camera up to shoot them?


Flats should be brighter in the center and darken as you get closer to the outer edges. The transition will be more pronounced if you have serious vignetting. The trick is to get the exposure so the center where it is brightest is not blown out(over-exposed). You will probably have to take some test shots keeping a close eye on the histogram. You want to expose to the right, but not too far to the point that the histogram is jammed against the right side.


Maybe this will help give you a better idea. &gt;&gt;&gt; Flat Field Calibration


Additionally, Flats are not just used to correct for vignetting, but for other optical anomalies such as dust spots on your sensor, and artifacts caused by pixel to pixel variations.

[Jon Ruyle]

I just put a white t-shirt over the lens and shoot in av mode. I have a set for each iso setting. I try to expose them fairly bright, but make sure there are no overexposed points.


I think I understand the basic idea, but somehow it just doesn

[tkerr]

I thought you were saying space in the picture was not black enough. I agree that it would look better if I subtracted less background light, but until I fix the vignetting problem I have to get rid of all the skyglow (or I get a little glow in the middle and black on the edges- it looks terrible)



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That's just another reason it is so important to use flats.


You also selectively adjust isolated areas of your image using select by color range with layers and layer mask. That is if you are using Photoshop CS*. That way you can adjust around the areas you don't want your adjustments to affect and get it to blend real well..

[tkerr]

Here is a couple more from my collection.


Both of these were shot with my EOS Rebel XT/350D
ISO 800 through my Celestron C80 ED refractor


Monkey Head Nebula
30 x 120 seconds



The Rosette Nebula Complex
46 x 180 Seconds

[Jon Ruyle]

Yeah, I agree that the vignetting is a problem that must be solved.


Both of those are wonderful, Tim. Makes me wonder why bother with autouiding


I

[tkerr]

Yeah, I agree that the vignetting is a problem that must be solved.


Both of those are wonderful, Tim. Makes me wonder why bother with autouiding


I'm guessing you used all types of ancillary shots for these.


The 80ED has about the same aperture as your 100-400. Any thoughts on how they compare?


(I realize that the 80 ED is a bit longer, but not too much, and I would think the rebel has plenty of pixels to crop a bit)
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With my mount and the amount of weight I have on it Autoguiding for long exposures is critical. I am using a Celestron C6R-GT (CG5).


Although the 80ED is only a few mm larger than the 100-400 nothing compares to a prime lens, which is, in effect, what I am doing when attaching the camera to the telescope. And the focal length does make quite a bit of difference in the Image scale and field of view.

A lot depends on what kind of shot I am after, and the object or object within it, that will determine which lens or which telescope I use.

[Jon Ruyle]

You aren

[Jon Ruyle]

Okay, it looks like DSS does not allow the use of flats without dark frames or vice versa. I have no idea why that is.


That might explain why when I was using the refractor and used dark frames (but no flats because vignetting was not a problem), it did not work. By the time I got the 9.25" I had given up on dark frames (the in-camera solution works okay) and tried to use flats with no darks, and again it didn

[tkerr]

You aren't autoguiding those wide angle, 3 minute exposures are you?


I just meant- why bother with long exposures when stacking short exposures gives such good results.
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Yes I do use the autoguiding even for wide angle exposure that long. The CG5 GT mount isn't that accurate, especially when you overload it like I do. And no mater what camera and lens I use it does make a difference in the stars


Why bother with long exposures? Increased image data / SNR.
Stacking multiple exposure gives you a better SNR, but it doesn't necessarily increase your exposure / saturation. Stacking multiple longer exposure will result in a better image than stacking multiple shorter exposures or a single very long exposure.


An assumption that many people make is that if you stack 40 x 30 second exposures that it will equal a 20 minute exposure. When in fact what you will have is a cleaner looking 30 second exposure instead. Don't get me wrong, more exposure will gain more detail than a single longer exposure, but only because you are improving the signal to noise ratio.


This thread can explain it better than I can here.
SNR and Total Signal

[Jon Ruyle]

You sure get nice results with your overloaded mount


I use my mount nowhere near capacity- if I polar align carefully, I can do a 15 minute unguided exposure with an 800mm lens and the stars will look round when viewed 1-1.

This thread can explain it better than I can here.

I'm not sure what the guy in that thread means by "doing the math". There is no math. The only differences between the sum of 10 60-second exposures and a single 10 minute exposure are read noise and clipped highlights. As long as you take a long enough exposure so that read noise is small and a short enough exposure that you don't clip highlights, there is no real advantage either way.

An assumption that many people make is that if you stack 40 x 30 second exposures that it will equal a 20 minute exposure.

I'm one of those people, apparently. I believe that the only differences are the two I mentioned above. Then again maybe read noise would be a factor in a bunch of 30 second exposures.


I think there is a lot of confusion about this- I've heard people say you need a single long exposure to get dim details, or who knows what else. On the other hand, maybe I am the one who is confused