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alan4908

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Everything posted by alan4908

  1. I've recently implemented dithering and I've started at 1.5 pixels in both the RA and DEC axes. I've noticed that I get an improved image quality but on the down side I also have to wait about 120s for the guider to settle. The settle time is always much longer on the DEC axis which I understand is consistent with mount backlash. To reduce the guider settling time, I was thinking about reducing the amount of dithering on both the RA and DEC axis and maybe even turning off DEC dithering altogether. Is this is good idea ? Given that the actual settle time is variable, I was also thinking of swi
  2. Martin Thanks for the response. You've made an excellent point regarding the my settling time (120s) eg doing "nothing time" versus imaging time (300s) eg a wasted time of 40% ! but given that I've found dithering good for improving the quality of imaging - I think I need to increase my subframe duration and/or maybe I shouldn't dither in the DEC direction.....Given that I have a OSC camera - what sort of subframe time do you think I should be aiming for ? I'll also take your advice and reduce the RA Aggressiveness down to 7. Alan
  3. Freddie and Gav - thanks for the comments. Freddie - Not sure which peak you are referring to.... if its the one between 2340s and 2520s - this is the result of a dither and during that time I'd not be imaging since the guider is still settling. Are saying that if the guide error in arc seconds is less than the main camera pixel in arc seconds then you have good guiding ? Alan
  4. In an attempt to improve my guiding and hopefully the quality of my imaging, I recently spend 4 hours adjusting various MAXIM DL parameters from the default values and finally ended up with the guider tracking graph shown below. Please note that I haven't yet implemented PEC. With respect to the graph, the vertical axis is in guide camera pixels where 1 pixel = 4.42 arc seconds, the horizontal axis is in seconds. My main camera has 2.39 arc seconds/pixel. I was also dithering at the time by 1.5 guide camera pixels and the subframe length was 300s. I also found that I needed a guider settl
  5. Hi Huw That's an interesting question. Being new to imaging, I asked myself a slightly different one "what's the optimum exposure time for a subframe". I found a good discussion of this topic in the excellent new book by Chris Woodhouse (The Astrophotography Manual) - he suggests going for a minimum subframe of around 5 mins unless you are interested in imaging star fields where you might get clipping, here the recommendation is to reduce the exposure to yield peak pixel values of between 30,000 and 60,000. There's also video from Craig Stark ( ) who discusses the topic at length. With the e
  6. Hi All if you want to find out more about LLRGB processing, which includes how to split a one shot colour into Luminesence and RGB... then I'd suggest you have a look at this video tutorial from Scott Rosen. Be warned it is over 1 hour long..... http://www.astronomersdoitinthedark.com/dslr_llrgb_tutorial.php Alan
  7. Hi Alex Thanks for your comments. On your suggestion, that's interesting. I'm currently following a LLRGB workflow which takes a One Shot Colour image and very early on splits the image into Luminescence (gray scale) and RGB components I then do all my sharpening/noise reduction on the Luminescence layer and all my colour boosting/blurring on the RGB layer (which I also convert into LAB mode for part of the process). The Luminosity is brought back in two steps (hence the two L's) to create the final image. I haven't tried your suggestion of changing the blending mode to colour - I will try i
  8. Hi Erik I'd say the image looks very good. On the star bloat issue, I also found that this was a problem when post processing NGC7000. I ended up using the Photoshop plug StarShrink by Russ Croman which, when you combine with layer masks, is very effective at shrinking lots of stars simultaneously. If you want to do the star sharping one star at a time then I also experimented with two Photoshop filters which you might want to try: First select the star with a round selection (Elliptical Marquee tool whilst pressing the shift key) then 1. Filter->Blur-> Radial Blur (for making the sta
  9. I've an issue associated with the movement of my NEQ6 but I'm not sure if the root cause is associated with the mount or my software. To explain, I've noticed two "niggles" that repeatably occur but only a certain proportion of the time: 1. Niggle 1: when I'm attempting to calibrate my guide scope (using MAXIM DL in conjunction with EQMOD pulse guiding) - the telescope is firstly moved in the X direction and then in the Y direction and you are supposed to end up with an L shaped image on your selected guide star. For reasons unknown, whilst the telescope always moves in the X direction it some
  10. Thanks for all your repiles. I find it amazing the amount of detail that you can extract during post processing.... if you know what you are doing. So, given that my post processing abilities are a bit hit and miss at the moment, my conclusion is that I need more practice Alan
  11. Since it's been rather wet and cloudy recently....a reprocessed attempt using the data from my second attempt at M31..... I was trying to bring out the M31 dust lane colours/details which I found challenging to say the least. If nothing else I've learnt a lot about the LAB colour space in Photoshop. I've also come to the conclusion that post processing an image is probably the most difficult part of imaging. LIGHTS: 42 x 300s at -20C. DARKS: 30; FLATS: 40; BIAS: 50. Alan
  12. After a bit of searching on the internet - I discovered that Photoshop does provide the means to measure the SNR in an image. Since SNR = mean signal/standard deviation, you can derive the information from the histogram which gives the mean and standard deviation for the image. Furthermore, if you select a particular area, the SNR relates to the area selected. So, in my latest image of M31, I decided to compare the IMAGE->MODE Greyscale to the Lab Mode Lightness suggested by Steve. I measured the SNR at 5 difference places across the image covering the core, dust lanes and background. Th
  13. Thanks Steve I will have a go ! I was also thinking about using the channel mixer in Photoshop and setting the output to greyscale. I presume this would allow me to choose which colours I wanted to mix based on the input image. The problem is that I don't know how to measure the resultant signal to noise ratio in Photoshop - the only way I can think of doing this with my software is to create the grey scale image in Photoshop and move it back to MAXIM DL and then to measure the SNR via the information window - unfortunately this would be a bit laborious - what I really want is for someone to m
  14. Firstly, a bit of background: I've recently adopted an imaging post processing workflow for my one shot colour camera that uses the so called LLRGB technique. Basically, the idea is that you take your one shot (RGB) image and at an early stage in the processing workflow you split it into a Luminance layer (on which you perform all the sharpening to reveal the detail) and the RGB layer (on which you perform all the colour enhancement and colour noise reduction). Towards the end of the workflow you combine the image back together in two Luminosity steps, hence the term LLRGB. Apparently, all thi
  15. Thanks for your help Adrian, that all makes sense. I shall give it a go. Alan
  16. Thanks Adrian and Francis. Yes, I think I understand (almost) all of this. Adrian: just to check my understanding of your dithering point : after I finish changing the various parameters and find the optimum minimum move time (with dithering turned off). Are you then suggesting turning dithering on and then keeping all the MAXIM DL parameters constant apart from the guider settling time ? in other words, the minimum move parameter should be set independently of any dithering parameters. On the point made by Francis: I've currently set my guider settle time to 25s, since it seems to take that
  17. Hi Alistair This looks very good to me ! I'm also a beginner in Photoshop but I've found the free video tutorial given by Scott Rosen (http://www.astronomersdoitinthedark.com/dslr_llrgb_tutorial.php) an excellent way to improve my Photoshop abilities - be warned the video tutorial is over 1 hour long. Alan
  18. Thanks for all of your replies. After a bit more investigation: 1. EL Panel: I found that the scope was not exactly perpendicular to the EL panel. So, after checking with my spirit level, I made sure this was perpendicular and redid that flats at the 35cm spacing between end of dew shield to the EL panel. The results are: Centre: 100%; Top right: 76%; Top Left: 73%; Bottom Left: 75%, Bottom Right:79%. So, slightly better (1% to 2%) around the edges but not significant. 2. T-Shirt: getting out my best white T-shirt out ...I took flats without the EL panel. This gave: Centre: 100%; Top right: 7
  19. Adrian Thanks for your response - that all makes sense. I'll read the article and also see what the calculator suggests for my system. On the basis of not chasing the seeing, I believe I'll need to increase the value from the current (default) value of 10ms to something that is above 40mS (given that my EQMOD minimum guide pulse is set at 40mS). Anyway, given that this seems too complex to analyse analytically (particular when you throw in the fact that I'm also dithering by 1.5pxls after every sub frame and I have unknown PEC) I think you suggestion of watching the guiding graph, taking the
  20. Hi Olly The distance from the end of the dew shield to EL panel about 35cm, the dew shield is about 10cm, so in total the objective is about 45cm from the panel. Please note that my camera has quite a large array (23.4mm x 15.6mm), so I was thinking that perhaps the larger the camera array, the worse the vignetting would be ? Given that I haven't actually measured my vignetting before, I think what I need to do is redo the flat measurement without the EL panel and see if I get a similar result. Alan
  21. Thanks for the replies. Yes, I do like this image, I was a bit surprised about how much detail came out given that my automated focusing scheme gave me a FWHM of about 4 arc seconds on my selected focus star, so prior to Photoshop processing I decided to apply some mild LR deconvolution which reduced the FWHM of the stars within the image a little. On the colour balance, yes - this is something I find really difficult to gauge at the moment. I did think about a bit more blue but I quite liked the resultant colour, so I didn't pursue this - as you say, all a matter of personal taste. I do like
  22. Thanks for all your replies..... The EL panel is now screwed to the observatory wall and I took some flats. I selected an exposure time of 0.2s which gave me (according to MAXIM DL) about 1/3 of the maximum brightness for my CCD as measured at the centre of the image. I've stacked 40 of these and measured the intensity at the centre and the four corners. The result is below. I've normalised everything so that 100% illumination is at the centre of the image. Is this normal for my set up ? I wasn't expecting a 23% to 29% light reduction at the corners of my CCD ! Alan
  23. My first attempt at M31 from my new location last night and my second since I started imaging. LIGHTS: 42 x 300s at -20C; 30 DARKS, 30 BIAS; 40 FLATS. After experimenting with various Photoshop techniques, I've now decided to use the workflow created by Scott Rosen which is a LLRGB processing methodology. Alan
  24. Hi Gareth Yes, that all makes sense - I shall give it a go. Alan
  25. In an effort to produce more consistent FLATS I've bought an A3 EL panel, a piece of plastic diffuser and framed these in an old picture frame. Given that I want to head in the direction of imaging automation, I was thinking of "hanging" this panel on my observatory wall in a position where my scope could be about 35cm distant from the panel and centered in the middle of the frame. After the end of an imaging run, I'd move the scope into a predefined position, activate the EL panel and take the flats. Do you see any issues with this arrangement given that my scope only has a 80mm aperture ?
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