Astro Picts.

[tkerr]

There is a huge difference in a Celestron CG5 GT Mount and an AP Mach 1 GTO. Even if you were to push the load limits it would still track better than mine.


In that thread they briefly touched on the mathematical formula, I don't worry about the math, instead I prefer practical application and seeing with my own eyes.


Signal remains a constant from one exposure to the next, but the noise in each exposure is random, and when you stack multiple images the noise cancels itself out.


Increasing the SNR you're also increasing the Dynamic range which might make it appear, or make you think that you have increased exposure length, when instead you're only revealing those fainter details because there is less noise blocking them. Does that make any sense?


Anyways, I was one of those who thought multiple exposure added up to a greater exposure length, E.g 10 x 60 seconds would equal to 10 minutes exposure. I was wrong, through trial and error I have learned differently.


The best thing I can suggest doing is to experiment; on the same object if you can. You can even do it in your home with your camera and camera lens taking pictures of a gray card or something. For Example take 40 x 30 second exposures to stack and process, and also take 20 x 60 second exposures to stack and process, and then compare the differences.


I think were a lot of the confusion comes from is from SNR and people equate the Signal to an accumulated exposure length.


If you really want to get those faint details you will need longer exposures. I try to push my exposure to the maximum limited by the skyglow, not the noise level evident in each exposure due to the high ISO. That will clean itself up with enough exposures/light frames, and darks, flats and bias frames.

[Jon Ruyle]

There is a huge difference in a Celestron CG5 GT Mount and an AP Mach 1 GTO. Even if you were to push the load limits it would still track better than mine.

Sure- well, there had better be a difference, anyway. []

Increasing the SNR you're also increasing the Dynamic range which might make it appear, or make you think that you have increased exposure length, when instead you're only revealing those fainter details because there is less noise blocking them. Does that make any sense?

Sure it does. In some sense, SNR is all that matters. That and quantization noise.

Anyways, I was one of those who thought multiple exposure added up to a greater exposure length, E.g 10 x 60 seconds would equal to 10 minutes exposure. I was wrong, through trial and error I have learned differently.

If there is a difference, it is due to read noise, and maybe thermal noise. With exposures that short, I believe it. But if you compare a single 100 minute exposure to 10 10-minute exposures, I doubt you would see a difference. (Assuming you overexpose in the 100 minute exposure).

If you really want to get those faint details you will need longer exposures.

Even read noise can be overcome with large samples. There is no limit to what you can resolve with 1 minute exposures, provided you take enough of them.


I agree with you- noise level apparent in a single frame is not important.

[PaulM]

Hi Tim - thanks for the answer:

Generally, it's usually a matter of the Aperture of the objective lens.
A majority of astroimages are done with a telescope, but not all. I guess it depends on what you are trying to accomplish. A camera lens is usually smaller and will give you a wider field of view. Using the 100-400mm gives me more flexibility than using a fixed focal length telescope. But the telescope has a larger aperture and longer focal length which will have greater light grasp and resolving power.(Greater Detail), and increased image scale.


BTW, you don't need a telescope mount to take pictures of the night sky. You will if you want to do something like these two pictures, but there are other things you can do also.

I've been out all weekend and only now have the opportunity to reply. I was referring specifically about capturing magnified images of galaxies, nubulae etc. I remain a little confused about two things though.

Generally, it's usually a matter of the Aperture of the objective lens.



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However a Canon 600mm f/4 hasa bigger aperture than the telescope you used, which was 600mm f/7.5 if I read it right. I have no idea which is more expensive. The canon lens is not cheap.


Then you said:

BTW, you don't need a telescope mount to take pictures of the night sky. You will if you want to do something like these two pictures, but there are other things you can do also.

but that was my main point of surprise because the second shot was not from a telescope but instead a Canon 100-400mm f/5.6, yet it shows what I typically thought required a long focal length.


In my limited exposure to telescopes I've encountered the term "light bucket" which was explained to me as "not a particularly long focal length but very large aperture to capture the light". My expectation would be a focal length not dissimilar to a telephoto camera lens but a monster aperture. Conversely if it were simply this description then I don't know why people wouldn't use these "light bucket" style telescopes more often for land based photography, yet I don't believe they do...


I understand your reference to the telescopes being prime lenses when compared to a camera's zoom lens. I'd expect these long+stacked exposures are sensitive to distotions that are lesser in prime lenses.


Great video. Nice alternate use of your obvious skill at time-lapsefor stacking!


Paul

[Jon Ruyle]

However a Canon 600mm f/4 hasa bigger aperture than the telescope you used, which was 600mm f/7.5 if I read it right. I have no idea which is more expensive. The canon lens is not cheap.

I'll bet the 600 f/4 would do far better (due to its larger aperture) than the 80mm Celestron ED, but it is much more expensive and much heavier (thus requiring a far more expensive mount). A more balanced comparison would be between a fast 6" refractor and the 600 f/4, or between a 5.5" refractor and the 800 f/5.6.

In my limited exposure to telescopes I've encountered the term "light bucket"

The term "light bucket" originally referred to a large aperture telescope of not-so-great optical quality- usually a large dobsonian. Today people use the term for any big dob (I've never heard anyone refer to anything but a dob as a light bucket). The term, as far as I know, has nothing to do with focal length. But today's big dobs are getting *fast*. f/3.3 in apertures of 20" and up is no longer unusual.

[tkerr]

However a Canon 600mm f/4 hasa bigger aperture than the telescope you used, which was 600mm f/7.5 if I read it right. I have no idea which is more expensive. The canon lens is not cheap.


Then you said:

BTW, you don't need a telescope mount to take pictures of the night sky. You will if you want to do something like these two pictures, but there are other things you can do also.

but that was my main point of surprise because the second shot was not from a telescope but instead a Canon 100-400mm f/5.6, yet it shows what I typically thought required a long focal length.
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Let me clarify that comment. I should have said you don't need a Telescope or a expensive Mount to take pictures of the night sky with a DSLR.


Those images are cropped down a little which is probably making it look more like an image from a longer focal length. They are about 80% of the original image.
Additionally, Pleiades, and Andromeda are pretty large Deep Sky Objects, you don't need a lot of focal length for those.





The Canon 100-400mm lens is three times the cost of my C80ED. I started using it out of curiosity to see what it could do, and it gives me the versatility that a fixed focal length refractor doesn't.
A Canon 600mm f/4 would most likely cost more than all my telescope and current camera equipment together.
The things I could do with such a lens. [^o)]

[Baker]

How did you stack you star trails image, Photoshop Or Startrails?

Actually I used ImageStacker for that shot

[tkerr]

How did you stack you star trails image, Photoshop Or Startrails?
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Actually I used ImageStacker for that shot
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I think I've got that hiding somewhere, on my USB Hard Drive. I remember downloading it but never got around to trying it out.