Astrophotography – deep sky

Some Musings on Aperture and DSOs

Rob’s recent post on his new scope led me to reflect (!) on my own experiences. My main interest is deep-sky objects, or “faint fuzzies”. I have a rule of thumb that says that to see any appreciable difference in object  brightness, you have to go up 2 stops in focal ratio (f-numbers). This translates to a doubling in aperture (doubling the aperture increases the light grasp by 4X). So when I upgraded from my old 4”, I went to 8”. Doing the same again would suggest a 16” would be needed. For me, a 16” would be unmanageable, so I went to a 12”. This is only 1 stop advantage to the 8”, and visual results were a bit disappointing. This more-or-less coincided with getting the PD camera, and even using this “live” as an electronic eyepiece gave views so superior to the 12” that the 12” virtually never got used.

So I wondered why this was, so here is a bit of basic physics.

Firstly, the eyeball. This iris opens to about 7mm when fully adapted to the dark. Or maybe 5mm in an old fogey like me. Given the size of the eyeball this is around f/3. See http://www.faculty.virginia.edu/rwoclass/astr1230/human-eye.html

To get the brightest image from a scope, the magnification from the scope must produce an exit   pupil (the beam width leaving the eyepiece) pupil less than 5-7 mm. Otherwise some of the light leaving the scope doesn’t enter the eye and is wasted. This defines the minimum magnification you can use for a given aperture. For example with the 8” and a 5mm exit pupil, this minimum magnification is 200/5 = X40. (The SCT has a focal length of 2000mm, so this is an eyepiece of 50mm FL). A lower magnification than this is not harmful, it is just that you are then not making use of all the available aperture. 7X50 binoculars are known as “night glasses” for exactly this reason – their exit pupil is 7mm, making the best use of all night-time light. Therefore, going up in aperture does not necessarily make the view brighter, but rather allows you to us a higher magnification for the same brightness. This might be a huge advantage for small objects like planetary nebulae, but less so for extended objects. Bigger aperture also improves resolution, allowing you to split closer binaries, but this is usually not the critical issue for faint fuzzies. The other issues affecting brightness are the eye sensitivity (more of this later) and its “integration time”, or the time period over which the eye sums the image it sees. This is its “shutter speed” and there is some literature that suggests in the dark, this is about 0.2 seconds. Again. see http://www.faculty.virginia.edu/rwoclass/astr1230/human-eye.html

Now, from here, I have NEVER conclusively seen any galactic spiral arms visually, although sometimes I have persuaded myself I can. So when I first coupled up the PD camera to the 8”, and turned it on M51, this is what I saw, live, with no processing at all:

Bingo! Spiral arms!

So if we are now talking about imaging, rather than the eyeball, what comes into play for image brightness?

3 things.

  • Focal ratio (not aperture)
  • Integration time. The PD single image is 1/50 second. Its “senseup” parameter allows it to internally stack up to 1024 single images, giving an integration time of about 20 seconds, or about 100 times the eyeball.
  • If the CCD had the same sensitivity as the eye, the brightness of the CCD image would be the same as the eye if a senseup of 4 were used. An experiment is called for!. I attached a lens to the camera, set it to around f/3 (similar to the eye), then in a darkened room compared the image from it to that I could see with my eye for various senseups. Although this is a very crude experiment, I reckoned that about senseup=2 was about right-not miles away from the predicted value. So a senseup of 1024 suggests a sensitivity about 8 stops faster than the eyeball.

There is also another factor involved, and that is contrast. This is the brightness of the faint fuzzy compared to the sky “background”, and I have found that with our local skies, that is the controlling factor. You can improve this visually using filters (UHC or OIII for example). These tend to dim the whole view, but the right one can improve contrast. Light pollution filters used to be good in the days of low-pressure sodium street lighting but are not much use with LED lights. On the other hand, the camera has a “gamma” setting that allows you to “stretch” the contrast, live, or if you post-process, the sky is the limit, as they say! For example, stacked 11 of the basic frames of M51 (11264 frames in total), processed it with GIMP, and here is the result. It is flipped vertically to get the orientation right (See http://www.thornett.net/Rosliston/Astrophotography/DSO.pdf for the details). Stacking 11 frames with a senseup of 1024 gives another 3 ½ stops faster than the eyeball or 11 or so altogether.

Interesting as all this might be, let’s remember that this is a hobby – and you do whatever you enjoy!

As a final thought, Lord Rosse used a 72″ aperture reflector to first identify the spiral nature of M51!

M46 & M48 – wide field images

Again taking the opportunity of a half-hour imaging session, here is M46 in Puppis from the window-sill. Since it is a wide field view, the included planetary nebula, NGC 2438, (a line-of-sight effect, it isn’t in the cluster) appears pretty small, but after a bit of processing you can see it just above centre and slightly to the left. For those of you trying to see it visually, here is a quote from Stephen James O’Meara’s splendid book “The Messier Objects”:

“There is yet another illusion with M46. It appears to contain a tiny planetary nebula. NGC 2438 – – –  But the cluster and nebula are not physically associated because the cluster is 5.300 light years distant, whereas the nebula is 6,250 light years away. Positioned just a few arc minutes north of the cluster’s centre, this 11th magnitude planetary measures only about 1’ in diameter. I suspected it at 23X but 72X shows it clearly as a ghostly mote among the multitude”

(I see from my notes that I observed it visually and sketched it at 01:10 UT on 23/12/2001 with my 8″ SCT at X266)

Since there was no moon, I had another go at M48, showing a few more stars than the last one I posted.

For completeness with the Messiers in that region I have also included the recent wide-field image of M47

Wider angle view of M47

Managed a 10-minute observing session from the window-sill before the clouds rolled in. M47 is quite large so this time, in order to get a good context, I used a focal reducer. Using a reducer on an f/5 refractor is not optically very good – and it was rather hazy, so the image is not brilliant. You can compare it with the one without the reducer at http://roslistonastronomy.uk/m46-and-m47

Roger

M48 at full moon – – – 28/02/2018

Having posted an image of M46 and M47 recently, the other Messier object nearby that I hadn’t yet imaged was M48. I hadn’t managed to locate this object from the window-sill, probably due to its extended size. So, tonight, I installed the focal reducer on the ST80 and went on an M48 hunt. It is full moon (another “Blue Moon”), so it was hardly ideal for DSO hunting!

Still, this time I found it. So here it is after a bit of processing with GIMP.

 

Consolation Prize??

 

After finding out the other day that my ZWO 174 camera doesn’t seem to work anymore, I decided to hang my Canon EOD 450D onto the back of my 8″ Celestron SCT to grab some snaps of Orion while the weather was good (yes, that day really did happen!!)

 

I was quite pleased with this one although I can’t remember how to get a resizable image onto the blog

 

My camera’s gone back to 365 Astronomy and they are sending it back to ZWO for replacement (I hope)

 

Cheers

Horse head and flame

I’ve been reading a lot recently about camera noise and recommended ISO settings for each particular camera for astronomy. The camera I’ve been using was said to perform best at ISO 1600 which I think in astronomy terms is actually quite high for a Dslr. So I decided to test this out on the 8th Feb and managed 74x1min exposures at ISO 1600. I was very pleased with the low noise, although the light pollution south of me (Tamworth, Birmingham) was very strong since the horse head is relatively low on the horizon. I’m pleased with the result, although I think with a bit more processing experience I could possibly improve the image???  I was trying out Pixinsight for a second time so I hope to improve over the coming months/years. Canon 6d+Idas d1+Takahashi fsq.

 

 

M46 and M47

There are a couple of very interesting open clusters in Puppis, not far from Sirius, for different reasons. I found that I had not imaged M47 before, probably because I thought it was a relatively uninteresting plain cluster. Not so! Here is an image of it from the window-sill from yesterday:

I thought at first that the star colours were chromatic aberration, but, in fact they are there in images on the www.. (See https://en.wikipedia.org/wiki/Messier_47). Very colourful!

M46 includes a planetary nebula (NGC2438) in the same field that is very unusual. I didn’t manage to see this one yesterday, but here is an image from 3 years ago,