ISO and image brightness at f/15

[jambouk]

Most people say a scope with a long focal length like my 180 mak (fl 2700, f/15) is not so good for imaging smaller DSOs because the light is spread over over a wider area of the sensor so it takes much longer to collect useful data - well that's my current understanding.

So, if i had a DSLR (i don't want a webcam connected to a laptop) which had a much better signal to noise ratio and allowed me to go from ISO 800 to say 3200 or 6400, would this in part allow me to better image the smaller DSOs, more akin to scopes at f/8 say?

Is there a way to correlate how Increasing ISO can offset slower optics for a given target? I appreciate f/ values of telescope optics don't change in the same way as they do for camera lenses (f/4, f/5.6...).

Thanks.

James

[hobsey]

I can't answer this but have you not thought about a focal reducer which just screws onto the back of the scope?

[baggywrinkle]

Look at trying to source a focal reducer, this will certainly help.

The other thing while you are waiting for the focal reducer (if you go that way) is just to play with the ISO settings and exposures. The great thing about digital is you can ditch what you do not use and quickly learn by mistakes.

Try some single shot imaging to see what your base results are like. You already have an .AZ-EQ mount so set up in EQ

[jambouk]

Thanks both.

I've thought about a focal reducer; if i could say reduce to f/10 (made up number for arguments sake), would i get the field of view of an f/10 scope or would i get vingnetting and the FoV be more like the f/15 but just with the proportions and brightness (when imaging) of an f/10 scope.

I am quite interested in understanding the relationship between the ISO and f/ too though. I am saving up for a more sensitive DSLR anyway.

James

[jambouk]

I've tried various ISO settings. Image quality at ISO 1600 is noticeably poor. At ISO 800 noise is tolerable, but with such a slow scope, my unguided subs can only be 120 seconds or so at a push before star trailing becomes noticeable. So i'm keen to ascertain if i could image at f/15 at ISO 3200 for 120 seconds say, would this be similar to imaging at ISO 800 for 120 seconds at f/8 for example.

James

[baggywrinkle]

120 secs unguided is quite a push, try shorter times at ISO800.

[jambouk]

I'm trying to capture more data, not less

Why would i want to reduce exposure time if i can get 120 seconds without star trails? [confused face]

It doesn't look like imll be doing anything this weekend though, so much cloud here in Nottingham.

James

[Kropster]

I have read somewhere that increasing the ISO doesn't actually get you more data..it is more like turning up the contrast on your TV.....more contrast from more amplification, but more noise too.

The number of photons gathered is set by the exposure time and the pixel size.

You may be better off using a lower ISO and increasing the number of subs and hence the amount of data going into the final image.

A low noise camera would look to be more important than one which pushes the ISO.

[lensman57]

about

I have read somewhere that increasing the ISO doesn't actually get you more data..it is more like turning up the contrast on your TV.....more contrast from more amplification, but more noise too.

The number of photons gathered is set by the exposure time and the pixel size.

You may be better off using a lower ISO and increasing the number of subs and hence the amount of data going into the final image.

A low noise camera would look to be more important than one which pushes the ISO.

You are correct, the iso setting is like increasing the brightness of the image post sensor ( with the penalty of increase in thermal noise and loss of dynamic range ) , it has nothing to do with the sensor's sensitivity. DSLR sensor is no different to one from a cooled CCD . There is only one sensitivity for the sensor. Some people go on about the "parity gain" which from what I have learned is around ISO 1000 for most of the Canons. You can increase the recieved signal by either using a more sensitive sensor, a faster scope ( F8 is already slow, never mind F12 ) or take longer subs and for that one needs guiding. There are mounts that can track up to an hour unguided  ( 10 Microns and  the like ) but the starting point for these is around £15000.~20000 for the basic gear. There is no point banging ones head against a brick wall, long subs need to be guided.

Regards,

A.G

[jambouk]

That is interesting. Thank you both.

So, is there a objective measure of a sensors sensitivity which can be used to compare the sensors of various Canon dslr cameras?

James

[sharkmelley]

So, is there a objective measure of a sensors sensitivity which can be used to compare the sensors of various Canon dslr cameras?

Yes - it's called Quantum Efficiency (or QE for short) - it is the proportion of photons reaching the sensor that are actually captured and turned into a signal.  QE is wavelength dependent and dedicated astro-CCDs often plot QE against wavelength.

A table giving some QE figures for various DSLRs that have been deduced from analysing the data from tests can be found here:

http://www.sensorgen.info/

I think the quoted QE figures on that site are for green light. 

You also want to make sure you select a camera with low read noise.

Mark

[jambouk]

Ok, that's useful, thank you.

I thought most modern sensors had QE values in the 90% region for some reason, and that film was <5%. No idea what I got that idea from.

Interesting to see the camera I like (like and can't necessarily afford) has a QE of 50% which is amongst the highest of the Canons luckily:

http://www.sensorgen.info/CanonEOS_6D.html

More research to do!

Thanks.

James

[lensman57]

That is interesting. Thank you both.

So, is there a objective measure of a sensors sensitivity which can be used to compare the sensors of various Canon dslr cameras?

James

Hi James,

Here is a comparison graph, does not make for comfortable reading I am afraid , it also puts an end to the wild claims about modded Canons. One is the graph for a typical DSLR sensor and the other about various CCD sensors.

Hope that it helps anyway.

Regrads,

A.G

[jambouk]

Blimey. CCDs are slightly better than DSLRs aren't they!

I'm looking for a DSLR for widefield too, and for daytime fun. This page has really drawn me towards the 6d. If I did get a 6d, I'd have to see how my much slower f/4.5 10-24mm Tamron lens coped before splashing out £400 for the 24mm f/1.4 lens on this page:

http://www.extremeinstability.com/2013-5-4.htm

I also have zero interest in using the laptop outside at present. To be honest, I can't stand using it anyway for astronomy. If I had an observatory and a warm rood immediately adjacent to the scope it might be different, but at present being tied to a laptop drives me mad.

It's all about compromise.

But thanks again for the links and the info.

James

[lensman57]

Blimey. CCDs are slightly better than DSLRs aren't they!

I'm looking for a DSLR for widefield too, and for daytime fun. This page has really drawn me towards the 6d. If I did get a 6d, I'd have to see how my much slower f/4.5 10-24mm Tamron lens coped before splashing out £400 for the 24mm f/1.4 lens on this page:

http://www.extremeinstability.com/2013-5-4.htm

I also have zero interest in using the laptop outside at present. To be honest, I can't stand using it anyway for astronomy. If I had an observatory and a warm rood immediately adjacent to the scope it might be different, but at present being tied to a laptop drives me mad.

It's all about compromise.

But thanks again for the links and the info.

James

If you inspect  the graphs you will notice that there is very little difference between a modded camera and a standard one in the blue and green regions of the spectrum, a modded DSLR is way more sensitive in the Ha region though but only compared to a standard DSLR. The same graph also shows that in the blue region a DSLR is still way behind a cooled CCD but not as much as in the Ha region where the modern CCD sensors are just in a different world, that is why a typical DSLR image shows a reasonable blue response but really lacks in the Ha region. As the galaxies emit a full spectrum of light in general DSLRs do better here than with Nebulea with high emission in the Ha region.

A.G