SteveNickolls

Hi Nige, Thanks for posting this. I very much agree with your observation that the longer the individual exposures are, then other things being equal, the resulting image (S:N ratio) will be superior and is of course why people use EQ gear to get longer individual frames unfettered as they are by the effects of field rotation. With our Alt-Az equipment we are limited by the length of individual exposures possible by field rotation and the ability of our mounts to track and their (lack of) stability. Within this envelope (perhaps 60-70 seconds, sometimes much less) we hope to collect as many photons out of the decent % of frames from both the brightest through to the dimmest parts of a DSO and hope the signal can be pulled out of the noise enough. Local light pollution will add to the task. By repeating the exposures many times we hope to detect as much as we can the faintest part of the DSO which statistically will show up in lower numbers and in some exposures not at all if the object is very dim or the pollution very bad (that's my poor way of explaining the different ways signal and noise are treated mathematically). Joseph Ashley in, "Astro-photography on the Go" mentioned a survey of images posted an the Internet he made and concluded that 120 minutes of exposure was the right amount to get a decent image, so with Alt-Az gear that can amount to hundreds of light frames especially as not all frames are good enough for DSS to use in stacking. In practice I think the most frames I have ever taken on one object was 160. Your images of NGC 6543 bear out the positive effect of taking more frames-reducing the noise more. As an aside I'm unsure right now about the pro's and con's of using dark frames at all. It's telling that you felt StarTools was happier processing the second batch of exposures with the greater number of exposures. As the object is a difficult (fainter) one you are adding to the difficulty of extracting a good image. There is also some gain we can make by getting more adept with our processing software. Yesterday for example I reprocessed a cropped image of Kemble's Cascade taken originally in August and through better use of ST's got a much improved colour and dark background. However NGC 1502 (Mag 6.9) is much easier to image than your NGC 6543 (Mag 9.8) and in there hangs a tale. Original- Most recent reworking- I'm really pleased you have posted this subject Nige for us to chew over :-) Cheers, Steve

With immediate light overspill like that Ken you are doing extremely well to take the images you do. I've just been watching one of the old Astro Imaging Channel sessions (on You Tube) about light pollution and the 'orange peel effect' noise we all see in images can be attributed to light pollution. I think you have started the subject of a ramble on this thread Ken-that is what our imaging locations look like at night! Will post up when I get chance to take a night shot. Can just make out the upturned baby and Elvis too in your image now you point it out. Cheers, Steve

He, he we know EQ mounts aren't beds of roses :-) Good luck sorting out your mount problem though. Best Wishes, Steve

Hi Ken, I've had a try with StarTools on your downloaded image. The COLOUR module did not like the data and would have washed the nebulosity firther away so I missed that step out. The background is becoming darker and some nebulosity is developing. If it had been a more regular shaped object using isolate/mask in the LIFE module can really bring out an object more. I hope you can get more data to get this to be a good success. Cheers, Steve

Yes, time is not our friend as it could place the nebulae too high to work with. Looking back on my records September tends to be a good month for clear skies, so fingers crossed. I'll have a look at your downloaded file now :-) Cheers, Steve

Well last night regardless of the Moon it turned out to be very clear here. I was surprised firstly that when I was imaging the Double Cluster in Perseus that the Live View on the camera showed a dark blue background, usually it's a various shade of 'light pollution orange-red'. This morning stacking the images the master image in DSS was a dark grey colour and not the usual mucky orange hue. I rather think last night the sky was largely free of moisture so the level of scattered light pollution was a lot less than normal. I took x60 fifty second images of the double cluster at ISO 800 and this morning took the flats and bias frames (both x50). Again I did not take any dark frames. I used the usual equipment, the Synscan Alt-Az mount, Startravel 102mm refractor and my Canon 600D DSLR with a lens at 300mm. StarTools was able to process the images very well and I had no trouble in the COLOUR module and got a nice dark background automatically. The image is reproduced below, in the bottom left corner is the open cluster NGC 957. I love the double cluster and will spend ages now looking at the patterns and colours of stars v Stellarium. Cheers, Steve

Hi Ken, Thanks for posting this. I'm going to download the FITs file to see if I can process in StarTools later today. Best Regards, Steve

Don't be too upset over this Ken, it's sometimes difficult to know how some DSO's will appear on the restricted images we can make with alt-az gear. I'm often uncertain over the different brightness values stated for DSO's on sites. Often you just have to try imaging to see what you find. What's more of a disappointment in the UK is we get so few clear nights that spending two or three on one object when you could be imaging three others is a hard choice to make. Here's hoping it will be clear tomorrow night as promised in the forecasts. Cheers, Steve

I'd put them down to your relatively short total imaging time there Ken. I've been re-reading Joseph Ashley's book, "Astro-photography on the Go" this afternoon and he did a review of the images people had placed on the Internet that they were pleased with. He came to the conclusion that 120 minutes total exposure was the point to aim for which practically might mean multiple sessions. TBH I've never achieved that level of exposures but if the night is dark then of course the more photons collected the better and the easier subsequent processing becomes. Don't be downhearted if the image of the two nebulae needs more work as you could add more images another night. Cheers, Steve

I hope you get to the cause of the green artifacts Nige. I haven't had anything like that all across the whole image but I have had green and red short tracks on a wide field image before where I hadn't employed any dark frames and did think that was the cause. Your mention of StarTools green specks, could they result from the use of the DECONVOLUTION module? I have noticed it sometimes leaves odd gaps of colour often only noticeable in a magnified view? Anyway good luck sorting it out and do let everyone know what it is/how it can be resolved. Cheers, Steve

I have just obtained a ball joint to attach to the top of my telescope to allow me to accurately align the camera when doing piggyback work (I was having problems with the camera mis-aligning in altitude). Now the finder, eyepiece and Camera all point to the same spot, quite helpful really;-) :-) Despite looking top heavy the ball joint has worked very well with no movement even with the big 75-300mm lens attached or the camera falling off at higher altitudes. The joint is rated for supporting 8kg. Here's a wide view below (at a 75mm lens setting) of the area around the Double Cluster in Perseus, the Muscleman Cluster (Stock 2) is in the image. The image was taken on the 7th September using x50 forty second exposures at ISO 800 plus x50 flat and x50 bias frames. No dark frames were used in the processing. On the night of imaging there was a lot of moisture in the air and the usual light pollution. I was using my Canon 600D DSLR piggybacked on the Startravel refractor and Synscan Alt-Az mount. The images were stacked in DSS and images processed using StarTools. I have tried reducing the worst effects of the bad light polution by removing the luminosity from the red channel in ST's. This doesn't affect using the COLOUR module however. Cheers, Steve

Hi Nige, Very well done with this sequence of M31 at different FL's, I do like them very much as a record and I'm not going to pick any one of them out as I feel each one gives a different perspective on the object. You have certainly got me itching to emulate what you have done :-) Here's for clear, dark skies to come. Cheers, Steve

Hi Ken, sorry to hear of your mishap but good to know you have resolved the problem. Look forward to seeing your images, particularly how much nebulosity your camera has picked up. Earlier in August I was trying out my 'new' 75-300mm lens on the Canon 600D and took some wise-ish angle shots including the area with the Heart and Soul Nebulae and Perseus Double cluster. The image below isn't great with much false colour but it was the sum of x80 45 second exposures at ISO 800, plus the usual dark, flat and bias frames (x50 each). There was a bright Moon up that night and moisture in the air but you can make out some shapes :-) On the right hand side you can see the Double cluster in Perseus. Having never seen the two nebulae before even this poor image felt an achievement. Good luck with your imaging. Cheers, Steve

No, there were clouds scudding around and today the rain has started. The outlook for the coming week is very mixed. We might be lucky and have one clear night. Take opportunity of any clear sky you get :-) Cheers, Steve

Thanks, must try that option also :-) Cheers, Steve

Yes, That's a lovely image of M31 Ken and one to try to emulate. I do like all the blue star forming areas around the galaxy edge. You must have quite dark skies you know :-) Nige, you are building up a lovely collection of images at the different FL's :-) Cheers, Steve

Hi, Thanks for your view on the image :-) All images had flat and bias frames. The image of M33 was definitely the poorer and this would be down to it being lower in altitude and intrinsically being fainter. It would be interesting to muse the dark frames added more noise than they were meant to take away. StarTools doesn't like poor images hence the red background hue. Cheers, Steve

Sorry to have taken so long to post these images. They are what I promised to do-restack the M31 and M33 images I posted before in DSS and reprocess using StarTools to see what visible differences might be seen when omitting dark frames but using kappa-sigma in DSS. I would class the M31 image as being easy to process in StarTools indicating the data quality was better (for a brighter object). First M31 with darks and 'median' setting in DSS- And now without dark frames but using sigma-kappa in DSS- And now M33, first with dark frames and 'median' setting in DSS- And next without dark frames and using kappa-sigma in DSS- Changes in cropping were down to me. Comments welcome. Cheers, Steve

So if I crop out the star at the edge...I get to image for longer? It's the beer talking. Cheers, Steve

Hi, Thanks for the kind comments on M31. Yes it is an easily accessible target, it can be seen in the finder scope so composing your image is much easier than say with M33 which required taking snapshots to check its position in Live View. Right now if you are using an Alt-Az mount and have to consider the implications of field rotation M31 is well placed in the sky both in terms of azimuth and altitude. If you had an equitorial mount I guess you'd wait until it was at its highest position. With my SkyWatcher Synscan Alt-Az mount I have been able to take individual exposures of M31 lasting 60 seconds and the mount overall has produced a high percentage of frames acceptable to DSS (76%). Definitely go for it! Cheers, Steve

I can't articulate this very well but the idea I have is if the light from a small star in a set period of time could be detected on two adjacent pixels, that would be the start of a trail. However if the light in twice that period of time remained in the same pixel (because the field of view was so much wider) then you wouldn't see any apparent motion. It is possible then that longer periods of time (when using shorter focal lengths) could also show no apparent movement. My head hurts. I'm off to have a curry and some beer. Cheers, Steve

Hi, Thanks for your kind comments. They would both have been subject to some cropping, the one taken with the Canon piggybacked would only have been cropped slightly a few pixels around the edge. The piggybacked image was taken at 200mm FL (on a APS-C camera like the Canon 600D I think this works out at 320mm) while the other was through my refractor (500mm FL). The image through the telescope was cropped more vertically to remove a lot of poorer background. hope that helps. Cheers, Steve

Jimbo747, A most interesting phenomenon you have imaged there for over an hour. Can I ask if you were imaging at 200mm with your DSLR? My thoughts would be that at that FL the percieved field rotation on the chip was insufficient to cause a streak to appear. When I process images in StarTools I often bin the original image by 50% or 65% as my 18M pixel camera image is mostly wasted (oversampled I think the term is). I know in his book, 'Using Short Exposures with Light Mounts..' Joe Ashley has a technique for overcoming field rotation based upon calculations involving 0.125 degree over 20 seconds which is close to Ian's value. You also need to remember that even if you can 'overcome' field rotation (using a tracked Alt-Az mount) by using a short FL lens your mount performance might limit your maximum duration of exposure from a mechanical viewpoint. Very interesting phenomenon though and well worth exploring more. Cheers, Steve

Thanks for this, will see what happens! Fingers crossed. Cheers, Steve

Ha, for that you need some typing skill, knowing where the keys are laid out. I often don't know where the hudl is ;-). I will put that down as an aspirational achievement, but thanks for the idea happy-kat.