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philherbert

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About philherbert

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    Nebula

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  • Interests
    SkyWatcher ED100
    SkyWatcher HEQ5 Pro
    Canon 350D (unmodded)
  • Location
    Leicestershire
  1. I have a SkyWatcher ED100 and currently use a ZWO ASI1600MC-Pro so no filters to worry about. It sounds like a fairly slow process Olly - either taking subs or using some kind of live view while manually adjusting the focus to minimise the FWHM? Do you find this more accurate than a B-mask?
  2. Hi, I've been imaging for a while now, and only ever used a Bhatinov mask to get fairly good focus. Recently I have been wondering about getting an auto focusser. I have two questions: 1. Is an auto focusser worth it - the added complexity and things to go wrong? 2. How accurate is it? I use APT with the Bhatinov mask and focussing aid, but the reported 'accuracy' varies substantially. And yes, this is after waiting for the scope to cool down. I fear that an auto focusser could be badly confused by this (presumably caused by the seeing) and end up getting it wrong? Thanks, Phil
  3. I'm no expert, but could it be a very slight gap allowing light to leak into the OTA somewhere when in a particular orientation, perhaps in line with nearby lights? The fact that you can apparently see stars in the background is also odd - are they the right stars in the right location (can you compare to any previous subs of the same target)? Be worth checking that everything is tightly coupled.
  4. I just wanted to close this topic off, in case anyone is desperate enough to read to the end! I have spent nearly two months trying to sort the problem out, and was starting to question my sanity, but in the end the camera supplier (FLO) were good enough to provide me with a replacement camera to test. I'm pleased to report that the inverse vingetting and curvy lines that were visible in the 'background' of the stacked images have all gone, so I am concluding that there was some kind of problem with the camera (I have been careful to change nothing else in the imaging train or software settings, so I'm 99% sure that this is the case, although I don't know enough about the camera to work out what kind of fault could have caused it). To be more accurate there remains a little inverse vignetting, but ABE / DBE in PixInsight deals with it pretty well. In the end it was the curvy lines in the background that couldn't be dealt with through any ABE/DBE or alternatives that convinced me that the camera was to blame.
  5. Well I tried to test my first hypothesis (whether there is any stray light in the subs) but didn't end up showing anything useful. I have since been exploring ways of creating a synthetic flat which should at least remove the dust shadows. Still a work in progress. It shows promise (the dust shadows appear quite well) but it is a pain getting rid of the stars. I'm trying various techniques out but would be interested in others techniques for synthetic flats. Phil
  6. Well I've done the new darks being careful to block out any sources of stray light. I've also taken a new set of flats (the dull overcast weather is useful for something). However there is minimal difference when pre-processed, and I still end up with the reverse vignetting. I'm now wondering whether the problem might lie with the original light subs instead of the calibration subs? (Ok, there were some minor issues with the calibration subs, including the need for dark flats, that I believe I have now sorted out, but I still have the original problem.) My theory is that the flats are actually working correctly. My very basic test for this is whether they compensate for the dust shadows, and they do. If this theory is correct, and they are working properly, then they are also correcting for the vignetting. Which leads me to think that whatever remains after pre-processing is some stray light that was collected during the light subs. (When I say stray light, I mean not signal from the target, and not noise inherent in the sensor etc, so sky glow / street lights / house lights / internal reflections). Apart from re-taking the lights, and trying to minimise stray light, I can see two options: (1) Work out a way to test whether there is any stray light in the subs or (2) Create a synthetic flat. For (1) I haven't yet thought of a way to tell. There is some noticeable difference between the first sub and the last sub, so I was wondering about subtracting one from the other, once aligned, to see what remains as perhaps a difference in the stray light at the time? For (2) this article http://trappedphotons.com/blog/?p=756 is very interesting. Has anyone tried anything like this? Phil
  7. My scope is a refractor with a focal reducer. I have just checked and there was a very slight amount of slack in the connection between the focal reducer and the main scope, so conceivably this could have allowed a small amount of light in, potentially affecting the flats and possibly the original set of darks. It may be that my previous approach that worked fine with an old DSLR just isn't rigorous enough for such a sensitive CMOS sensor. I'll take another set of darks tonight (in the dark so that there is no stray light). I'll also see if I can minimise any stray light and take a new set of flats. And then I'll report back...
  8. Yes you make a good point about not being able to isolate the problem area from the amp glow, so that is not an option. The original darks you have analysed were taken with the scope lens cap on, but in daylight (on a cloudy day). This was not ideal of course. The second set of darks that I mentioned were taken in the evening with the lens cap on and a large dark towel over the telescope as well. I have looked at these darks and there is only the amp glow, no sign of the central signal. However after processing I am still left with the reverse vignetting from some kind of over correction. Here is one of the better darks: https://www.dropbox.com/s/uj6joog55nfzgtq/2018-01-21_17-49-53_D_180s_G139.fit?dl=0 (By the way, for anyone else with similar issues, the bright central area was not really visible in the original flats until it was debayered and then given a very strong stretch - it didn't show in the grey version of the dark. In reality it only showed in a few of the original dark subs.) So in summary I have a fairly good set of darks (unless someone can show me otherwise of course!), and using dark flats I get slightly better calibration than using bias frames, but I still get inverse vignetting. The effect is shown below: With PixInsight and the AutomaticBackgroundExtraction tool I get a result looking like: The left-hand image shows the removed signal; the right-hand image the result. As you can see it leaves a 'halo' around the centre of the image. It also leaves the snaking lines in the background which I think might be symptomatic of another problem. Any takers? I'm running out of idea! Thanks, Phil
  9. Hi Vlaiv, thanks for your analysis! I have checked the other darks, and there are certainly some variations between them. I have also compared them to another set of darks that I took under much better circumstances and there is much less variation between them - I used 'blink' to compare them. Unfortunately, processing again with this better set of darks still leaves the inverse vignetting. I was thinking - if the darks are over-correcting, can I 'reduce' the effect of them somehow (perhaps using Pixinsight's PixelMath) do you think?
  10. Hmm, I've created a set of dark flats / flat darks with same temperature, bias, offset and exposure duration as the flats, just without the light :-) . I've just run them through with Deep Sky Stacker and get no appreciable difference - the vignetting is still over correcting in the centre of the frame. Here's a link to a dark flat in case it is of any help: https://www.dropbox.com/s/tnwdg51aevtir94/2018-01-31_21-20-25_DF_1.55s_G139.fit?dl=0 I've stacked with lights, flats and dark flats (no difference), and also with lights, darks, flats and dark flats (still no difference). I've even thrown the bias in there as well as an experiment (no appreciable difference).
  11. Thanks to all for the suggestions. I'm just creating some flat darks (never used those before) matching the flats and will report back...
  12. Sorry, my bad. Here you go. Light, dark, flat, bias in that order. Phil https://www.dropbox.com/s/4erb24i7brxbsq9/2018-01-07_18-44-14_L_180s_G139.fit?dl=0 https://www.dropbox.com/s/5w1q7rmm6siyvdo/2018-01-14_10-38-21_D_180s_G139.fit?dl=0 https://www.dropbox.com/s/49hh0d4qoo8y3u7/2018-01-21_12-23-59_F_1.55s_G139.fit?dl=0 https://www.dropbox.com/s/yjp7prj6gcrbf4v/2017-12-30_13-12-09_B_0.001s_G139.fit?dl=0
  13. Sure. This are examples of the light (top left), dark (top right), flat (lower left) and bias (lower right). All are raw subs, just auto-stretched with the STF tool.
  14. Hi All, I have recently converted from an old Canon DSLR to a ZWO ASI1600MC-Pro. The individual subs look great with the new camera, but I am having seemingly insoluble problems processing, so I need the assembled wisdom here! The first image shows a single sub on the left (auto-stretched), and the result of stacking some 25 subs on the right, combined with flats. The flats have corrected quite nicely for the dust doughnuts, but given me inverted vignetting. (I should say that I have tried processing with darks and bias as well, but the basic problem of the reversed vignetting remains. Processing was done in Deep Sky Stacker, although I get the same results using PI and the BPP script). I initially thought this was down to using the wrong ADU values for the flats, so I've tried various from 5,000 up to 30,000 in 5,000 increments, but nothing improves - the higher the ADU the worse the effect seems to get. I've also tried different techniques for taking the flats - grey skies (easy at the moment) with t-shirt layers in front, or a tablet screen on white. Flats are seemingly ok - here is an individual sub on the left and master flat on the right. OK, they are rather brownish, but apart from that they look ok to my (DSLR-attuned) eye. Different ADU values seem to fail to correct for the dust doughnuts, as well as making the vignetting worse. I have one further thought / problem to add. The over-corrected centre of that processed image (top, right-hand side) shows some odd details. Here's a zoomed version. There are fairly broad swirly lines showing up in there. Actually the lines are across the whole image, but most visible in the centre where the vignetting shows it up. My only thoughts are that this could be condensation or similar? Might this account for the flats not correcting properly? As I said I'm using the ASI1600MC-Pro, and have it set to cool to -20C for all lights, darks and flats. I've not encountered a problem like this with flats failing to work so miserably, so I'm after any and all knowledge at this point. To a fair extent I can take out the vignetting with ABE/DBE, but that then leaves me the dust doughnuts. I really don't want to have to 'edit' my pictures to that extent, and feel that the flats should take care of the vignetting. Thanks for looking. Phil
  15. Eureka! I did the restore default settings, and then went through the recommended settings. The one that I didn't recognise says: You are using bias frames >Set the black point to 0 to improve calibration I wasn't sure whether I had set this before, so had a quick Google. That took me back to a four-year-old thread where "dph1nm" said: "Using this (with bias frames) kills my Canon 1000D reductions (flats no longer work), so I always have this switched off." And sure enough, switching this off has got my stacking almost back to normal. Now I think I have to take the flats in the right way since they aren't correcting 100%, but the bizarre colour correction is largely sorted out. Thanks for your help and suggestions! Phil
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