Photographing the Stars

[Alex Paul]

Here is one of my scopes that is wonderful for imaging.....Celestron Schmidt Cassegrain C8.... I just picked up a C9.25 which I will be doing my newest series of astro imaging with....I am including one of my last M42 shots from last year which includes The Running Man Nebula......Alex

[Don Hoey]

Well I cannot cope with that lot Alex.

In case you are wondering I am still here, but trying to get my head round a few things.

As previously posted I downloaded RegiStax v4 but seem to be having a few probs. Starting simple I tried a few exposures of the moon but program only seems to recognise the last image loaded. Downloaded v3 but still the same. Also a bit of a problem with image size dimensions as the D2X exceeding program limits. Had a play with wavelets on a single moon exposure and that is something I will definately go back to next time the moon appears.

I have downloaded DeepSkyStacker and that does work. Long processing times though, 20 mins to stack 10 frames and subtract the darkframe and that is on top of image registration time.

Next problem is exposure. I did a stack of 30 frames with a 30 sec f4 exposure taken the other night. Result looks underexposed in terms of the number of stars captured. Luckily last night we had a lull in the high winds and by midnight clear skies as well so I had another go. Starting with 6mins at f4. Totally overcooked. 3mins at f4 the same. The length of those exposure did show up the limits of my manual tracking system. A bit of a rethink and I decided on the 50mm at f1.4 at a dramatically reduced exposure of 4 secs at f1.4. A test stack of 10 frames showed we were now in the ballpark but still suffering from tracking errors. I then took a series at 2 sec f1.4 and have just downloaded to pc and things are looking promising. Just got to process now. I have not worked out a link yet between exposure and each frame added to the stack so it will be a bit of trial and error.

I think something else I will need to look at is sensor sensitivity at low temperatures. Comparing the image I posted the other day taken with a warmed up D100 that 30 sec exposure, although delibrately containing more noise has also recorded the planets better than the 30 stack from a cooled down D2X ( approx 3 deg C ) . Last night it was 8 deg C and I did not allow a cooling down time and was achieving better results at same ISO. Previous exposures f4 - 30sec, last night f1.4 - 2 sec. 3 stops difference.

Guess you do not have the same temperature problems in the Bahamas luck devil.

Don

[Don Hoey]

Originally Posted by Alex Paul

I just picked up a C9.25 which I will be doing my newest series of astro imaging with.

Stevie has just had a droole.

Certainly looks a big beast with high magnification. I noticed the TWO finders on top. You should get some impressive results from that.

Well I just lashed out as well. Managed to replace my recyled coke bottle converter with a Nikon TC201 mint for £69. Just got to do some tests with it various lenses. Hoping for better results with the moon.

Don

[Don Hoey]

Here is my pic from last night.

Taken with 50mm f1.4 lens at ISO 800 exposure time 2 secs.

35 frames + 1 darkframe were taken and stacked using DeepSkyStacker set to select the best 80%. During that stack I noted the best 10 frames ( the program gives points to each image ), copied each 4 times and stacked that lot. The stack was then copied 16 times and re-stacked. The result from that stack, was copied a further 16 times and stacked again. Each stacking operation took the computer 1 1/2 hours.

I have not tried taking the single best frame, multi copying that and stacking to see if it works.

From these pics I see I really need the exposure down to 1 sec as there is still some star trailing. On a wide angle ie 18mm you can probably get away with 15 secs exposure without an equitorial mount. For my 200 f2.8 to get in closer, I may have a problem as the X does not like ISO higher than 800. Hopefully the darkframe is a method of noise reduction, so I may be OK, as no NR was used on these images.

Pic 1 is the full frame shot.
Pic 2 a crop of pic 1
Pic 3 a 100% crop from a 4 sec exposure stack of 10 frames to show star trail over that time.

Don

[Alex Paul]

Don: You get great results from that 50mm.. One thing to be said for the Canon DSLR's is they really have excellent noise reduction and shooting 3200 is not too bad.... I generally try to use 800.... The dark frame does remove a substantial amount of noise... Hot pixels are another issue that happens with to many timed exposures one after the other as the CCD or Cmos Sensors do heat up.... We need to get you more magnification... The images you get are fantastic and with more mag I think what you will be producing with stacking will be mind bending.... I would consider getting Stevie the Orion ED80 with the CG5 mount.... It has motorized tracking and so it will do a wonderful job for both of you.... As long as you can decide how to split up the use.... I was thinking about your birding scope but it would be very difficult to focus with the aperture limit and you really need to have a mechanized tracking solution to get the 30 second and above timed exposures... Now none of this holds true for widefiled shooting as in the Milky Way, but to get the deep space Nebula you really do need it to get consistent images for stacking to build it up to its potential.... I will go through Registax work flow and write it down so that I can post it.... It has been a while since I have been actively shooting Astro stuff so I went through a number of previously shot M42 frames today and stacked them with Registax.To get back into the groove... Once you get the flow and logic of the layout it is a nice simple program to use... I will go through it again in the morning and write up the procedure and post it tomorrow... I would give the 200 a go with 15 second exposures with a cool down in between every 2nd shot. With the results you got with the 50mm on Orion, It would be interesting to see just how much more image size the 200 will give you..... Don: You make me wish I had a Nikon,,,,, When I see your images.... Noise reduction goes to Canon but oh how nice the rich images of the Nikon are.....Take care Don.....Stevie..... Get after your Dad.... We need to get the both of you that scope......Alex

[Don Hoey]

Alex,

Having seen how stacking brings out detail, I have to understand exposure, then relate that to size of stack required to produce the final image.

Equally I need to do some tests to establish the effects of sensor warm up on long exposures.

I will look around at motorised mounts but will stick with photographic lenses for now. The ED82 is a great observing scope and any advance on that should logically be in the direction of 102 - 105.

A workflow for Registax would be very handy in view of my initial experiences. I would also very interested to see the results from a 200mm on a tracking mount as that is the next logical step up for me.

Don

[Dave Smith]

Don

I have been watching this thread with interest, thanks for getting it going. I am a little puzzled about your stacking technique. As I understand it when you stack say 10 images, then they are averaged but as the noise is random it gets averaged out giving a better signal to noise ratio. If you stack identical images then there should be no improvement as 10 lots of identical noise added together and divided by 10 gets you back to where you have started. Have I misunderstood what you have done or the process?

I am (rather impationately) waiting for a clear night to have a crack at M42 with a 200mm lens or even 400mm. I am fortunate that I do have a drive.

Dave

[Alex Paul]

Dave: If I might..... This is where the darkframe is used by the software... It is shot during the same session with the lens capped off for a total darkframe... When this is used with the stacking program it takes all the dark background with included noise and uses the dark frame for cancelling the noise and turning the background black. I hope this made sense....It dramatically improves the shots by replacing the actual background of the stacked shots with the dark frame.At least that is how I understand it to work.... I can tell you it does work ....Alex

[Alex Paul]

Don: Though the extent of money that can be spent on quality scopes is outrageous and aperture fever is a common affliction, the reason I love the ED80 is massive bang for the buck.... Though I have and use my other larger scopes, so far none of them have beaten the image quality of my cheap but very good little grab and go Orion.....Anyway there are many ways to enjoy this particular hobby ..... Good tracking is the key to good stacking.....Alex

[Dave Smith]

Originally Posted by Alex Paul

Dave: If I might..... This is where the darkframe is used by the software... It is shot during the same session with the lens capped off for a total darkframe... When this is used with the stacking program it takes all the dark background with included noise and uses the dark frame for cancelling the noise and turning the background black. I hope this made sense....It dramatically improves the shots by replacing the actual background of the stacked shots with the dark frame.At least that is how I understand it to work.... I can tell you it does work ....Alex

Hi Alex

You are correct about the use of dark frames but as I understand it the stacking is a separate issue. I have been using ImagesPlus to process my images and the dark frame subtraction is done to each image before the stacking process. Using a dark frame removes the fixed element of noise from the image but not the random thermal noise, if I understand it correctly. Images will be slightly different e.g. due to fluctuations in the atmoshere, so the stacking process will average these fluctuations out and give a cleaner image. Stacking many identical images will do nothing to improve the quality.

Does that make sense?

Dave