Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

stargazine_ep28_banner.thumb.jpg.b94278254f44dd38f3f7ee896fe45525.jpg

PeterCPC

Deep Sky Images with an Alt/Az Mount and no wedge

Recommended Posts

Can I ask what camera you use?I have an 8se and only recently for the first time saw M42 which looked beautiful through the scope. I have a phillips 900 webcam and was wondering if this would be able to capture it at all?

Hi

I use a Canon 550d. I'm sure that you could get M42 with a webcam but the resolution will be much lower so you won't be able to get the detail that you would get with a DSLR.

Webcams are really for Solar System objects and have much smaller sensors that a DSLR but it's worth a go. I tried it with an Orion G3 colour camera and the resolution was not very good.

Have a go, if we ever get any clear nights that is.

Peter

Share this post


Link to post
Share on other sites

Hi Paul

That's a very kind offer which I'm afraid that I will have to decline. The StarSense certainly sounds good and easy to use. I'll have to have a think about that. I also use a Red Dot finder as well as the ordinary finder which makes finding stars quite a bit easier. 

Regards

Peter

Share this post


Link to post
Share on other sites

My friend has a Mead 10 inch LX90 without a wedge and an unmoded Canaon 1000D. He managed to get this image of M 51  with lots of short exposures and then used  de-rotating software in the processing.

post-2076-0-97882900-1392997464_thumb.jp

Edited by reddoss
  • Like 1

Share this post


Link to post
Share on other sites

I have an old cannon 350d I might get the t-ring and adapter for and give that a go.

Share this post


Link to post
Share on other sites

My friend has a Mead 10 inch LX90 without a wedge and an unmoded Canaon 1000D. He managed to get this image of M 51  with lots of short exposures and then used  de-rotating software in the processing.

attachicon.gifM51.jpg

Hi

Thanks for that. I was going to have a go at this when we get a break in the weather so I think I definitely will now. I think that with a bit more post processing that would be a great image.

Peter

Share this post


Link to post
Share on other sites

Can you explain how this is so please because I don't see how longer exposures will change the total number of skyglow or signal photons arriving on the CCD.

For example, if skyglow photons are arriving at 10 per minute on average then 30 second exposures will have an average of 5 photons each.  120 fields like this will contain 120*5 = 600 photons.

One 60 minute exposure will contain 10*60 = 600 photons.  Where's the difference?

The same thing applies for the signal of course.  The same total acquisition time will acquire the same amount of data.

Chris

Hi Chris.

Firstly, it's an empirical fact that under real conditions two images with the same total exposure time are not the same if one of those is made up of lots of shorter frames and the other is made up of fewer longer frames.  This is true from my personal experience but it's also clear from the many deep sky images posted on SGL.  Imagers take exposures as long as they can to go as 'deep' as possible and bring out the detail in the faintest parts of the object.  Also, if this were not true then imagers wouldn't go to the trouble and expense of autoguiding and having expensive mounts capable of accurate tracking over many minutes.

As rdfdesigner says, read noise is more of an issue with lots of short subs, but I am not so certain it is the whole story.  Readout noise is relatively modest in modern sensors - including DSLRs - and can be partially addressed by bias frames.  So, with bias calibration, it should be a small effect - maybe big enough to have an impact on lots of really short exposures, but would it really be big enough to make a difference between 40 x 5 minute exposures versus 10 x 20 minute subs?

This article on Cloudy Nights explains the impact of skyglow (skyfog) and short exposures on images, and it seems to make sense to me...

Share this post


Link to post
Share on other sites

No explanation at all.

Skyglow makes a difference to what you get but changing the number of exposures will makes no difference.

Read noise does make a difference and it is possible to determine what difference it makes. No amount of hand waving or opinion will change it.

Astronomy is supposed to be a science, let's see some.

Chris

Share this post


Link to post
Share on other sites

No explanation at all.

Skyglow makes a difference to what you get but changing the number of exposures will makes no difference.

Read noise does make a difference and it is possible to determine what difference it makes. No amount of hand waving or opinion will change it.

Astronomy is supposed to be a science, let's see some.

Chris

Fair enough.

Do an experiment and if you get as good results with 30 x 10 second exposures as you do with 1 x 300s, that'll be some science.

I don't think you'll be able to and I *think* that's because at very short exposures the signal is restricted to a very small dynamic range and taking the signal from sky glow out of the image (by moving the blackpoint) doesn't remove the shot noise associated with the sky glow.  I may have that wrong and please feel free to correct me; but what I am sure of is that my experience of imaging under suburban skies indicates that total exposure time is not the only issue at work.  If that's all down to read noise then that's also fair enough...

Edited by x6gas

Share this post


Link to post
Share on other sites

There is no magic that makes sky glow go away if you use longer exposures rather than short exposures. All that helps is longer total exposure, not longer sub exposures.

The square root of 1000 is exactly the same as the square root of (10 * 100) and the square root of (100 * 10)

Read noise will make a difference.  Taking an example of 10 electrons per read 1 read will give a variance of 3, 10 reads will give a variance of 10 while 100 reads will give a variance of 30.

But if the sky glow is significantly more than this then that will dominate the noise.

Chris

Share this post


Link to post
Share on other sites

There is no magic that makes sky glow go away if you use longer exposures rather than short exposures. All that helps is longer total exposure, not longer sub exposures.

The square root of 1000 is exactly the same as the square root of (10 * 100) and the square root of (100 * 10)

Read noise will make a difference.  Taking an example of 10 electrons per read 1 read will give a variance of 3, 10 reads will give a variance of 10 while 100 reads will give a variance of 30.

But if the sky glow is significantly more than this then that will dominate the noise.

Chris

I'm not saying that there is.  I am saying that there is a difference shooting with lots of skyglow and without so much.

What the article on Cloudy Nights says is that you need 15 times the integration time (that's total integration time not number of subs) at the urban site than at the dark sky site in the examples given to get the same signal to noise ratio.  That's a lot of short exposures and thus a lot of read noise.

Putting it another way, you need more subs under skyglow than under pristine skies to get the same SNR (because of the shot noise in the skyglow signal), and if you are shooting more subs you get more read noise so there is a relationship.

What's your explanation for autoguiding?  Is it just a myth and we should all be shooting 30s unguided subs?  Please explain?

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.