How to chose your tele for AP?

Started 6 months ago | Discussions thread
rnclark Senior Member • Posts: 2,653
Re: How to chose your tele for AP?

1llusive wrote:

rnclark wrote:

Astrophotographer 10 wrote:

rnclark wrote:

The problem is Greg that this long exposure idea dates back tom film and early CCD eras. The only reason for long exposures is because read noise in CCDs is quite high. Early CCDs were around 16 electrons read noise. Now with sub 2-electron read noise, exposures can be much shorter. With the uniformity of today's DSLRs and low read noise, 30 second and 1-minute exposures work very well.

See Technology advancements for low light long exposure imaging:

29 one-minutes exposure with no autoguiding, no flats, no darks, no bias on an astrotrac:

The equations have changed. One can do great imaging with much less today than even 10 years ago.

Read noise will continue to drop. In labs now for over a year are sensors with less than 1/3 electron read noise. That gets into photon counting and one can do video and assemble images in post processing like on planets with super resolution. This is not too many years away.


Yes I am aware of these advances and the short exposure revolution that is going on. I see the best results though with ASI mono cameras using the Olympus OMD sensor and filtered. Typically they use a certain gain and go for around 60 second exposures and stack. They also still do 10-15 minute subexposures for narrowband.

Sony A7s is below 1/3 electron read noise at higher ISO.

No it doesn't. If someone derived such a number it is probably because the raw data are filtered so masking the true read noise. Eric Fossum, the inventor of the CMOS sensor has even questioned the sub 1-electron read noise that some people have derived for sensors. I suspect raw data filtering may be the reason for some of the low numbers.

But even these ASI camera users are tending to longer and longer total exposures. Stacking 1000 images starts to become very intensive computer-wise. So no free lunches there.

Old ideas still infecting the strategy. Computers get faster every year. Processing 1000 images is not has hard as years ago and next year will be easier.

At the end of the day many of these objects are very dim and it takes a while to build up the signal no matter which camera or approach you are taking. Granted read noise has been a limiting factor but its got a lot of years left in it still.

With less than 2 electron read noise in current CMOS sensors, read noise for essentially most astrophotography exposures is a small component of noise sources, with the majority of noise coming from skyglow and dark current.

On the topic of sky glow, I gather that you probably don't use sky glow / light pollution filters. The idea is you could cut down some of that signal, and with the ideal ($) filter, avoid cutting out what you do what. However is it better to just fix it in post? I suspect yes, with the right processing. And, exposure times suffer when using LP/Sky glow filters. Am I on the right track with this? Or, would these filters benefit the final image?

Filters help reduce skyglow and the noise associated with the skyglow, and the noise can't be subtracted in post processing. But filters warp color balance and because some things are emitted light (e.g. nebulae), a simple change in color balance does not correct things equally. Light pollution filters are becoming less effective with the change to LED lights. So whether filters work for you depends on your desires for the final image and the situation.

I usually want natural color in my astrophotos, so I avoid using light pollution reduction filters.

If natural color is not that important to you, then filters can be a benefit.

Professionally almost all my work is narrow band (100 nm to 200,000+ nm).

Also: you posted images at 420mm f/4, ISO 3200. What ISO do you find the best dynamic range? I think I've seen you using 1600 as well. Is that giving you more DR than ISO 800? By your example, again I'm suspecting yes, as long as dark current and noise are under control...

I use both iso 1600 (typically 1-minute exposures) and iso 3200 (30 second exposures when windy). The dynamic range at 1 minute, iso 1600 will be slightly less than the dynamic range at 30 seconds, iso 3200. If there were no dark current, the dynamic range would be the same. Two minutes at iso 800 would also be the same if no dark current, but if dark current, then it would be slightly lower than 1 minute at iso 1600.


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