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Found 8 results

  1. I seem to be getting conflicting views on whether it is necessary to take both dark and bias frames to calibrate lights. Some web sites say yes, some say no - because the bias in included in the dark. In the past I've only ever taken lights, darks and flats What is the consensus here, and does it vary depending on the type of imaging device - DSLR, CCD, webcam etc thanks in advance Neil
  2. Hi! Yesterday I was reading about dark frames vs in camera long exposure noise reduction, and something caught my attention. As far as my (so far little but growing) knowledge goes, the best you can do is to take the calibration frames right after the imaging session. This can be a pain in the A, and as I read yesterday, many takes these frames separately, when there is nothing better to do, like on a cloudy afternoon. This is allright, it's a good idea, you can create different master darks and other master calibration frames on different temperatures (room temp, cold, hot etc), and use these when stacking images from your light sessions according to the temperatures the lights frames were capured at. But. As far as I know, my darks should have the exact same settings and focus that my lights have. If I know I use for an example a prime wide angle lens at F2.8 all the time, with ISO 1600 to capture the milky way, that's okay. But what if something changes? What if I use ISO 3200 for some reason? What about the focus (okay, inifinity, but not exactly the same all the time when manual focusing)? What if I use a zoom lens on different focal lenghts? What about the other calibration frames? It's definitely not impossible to be prepared for every scenario, but when you use lenses instead of telescopes, there are more variations. Extra info, if that matters: I'm using a Nikon D5500, which is "ISO invariant". I'm really curious about your replies, as this could greatly improve my image's quality, if It's possible to take calibration frames this way. Thanks in advance! Árpád
  3. Below is a comparison between single dark frames taken with the Nikon D7500 and D5300 with exposure durations varying from 1 sec to 240 sec ( my usual main light frame exposure ) all at ISO400. Firstly a graph of the standard deviation of the noise in the dark frames versus exposure time: The standard deviation of the noise is a fairly constant 2 ADU less for the D7500 compared to the D5300 ( pretty much the difference in the read noise between the two ) However, the difference is not just in absolute terms but also in the quality of the noise ... Below are the dark frames - ranging from 240 sec exposures at the top to 1 sec at the bottom: D7500 D5300 The D5300 dark frames clearly show the pattern in the read noise ( banding down the bottom ) and also have far more chrominance noise compared to the D7500. At 240 seconds ( the main exposure I have been using ) the difference is starkly different; the D7500 produces images with much lower noise that is significantly more even and random and hence more likely to be reduced during integration.
  4. I have completed a comparison of the level and pattern of read noise in bias frames between the Nikon D7500 and the D5300, please see the blog page ...
  5. Folloing on from a post I made in early November :- http://stargazerslounge.com/topic/198663-is-this-acceptable-or-only-30-mins-of-data/ I am finding myself frustrated by lack of time and the clouds.... (set up three times last week, clear skies, but by the time it was dark the good ole clouds had rolled in and put paid to another night.. So with the limited time I set myself a challenge to see what I could achieve with only 30 mins of subs, I have included three images of what I have so far. I would be interested to see what others could achieve with the same limitations, so are you up for a "30 Minute Challenge" ? Not too many rules :- Only 30 mins total of Subs (1 x 30m, 2 x 15m, 15 x 2m, 30 x 1m, you get the idea...),(Be Honest) Unlimited dark's, flats, bias etc. Include some capture information with any image submitted. Thats about it.. So heres my attempts :- all taken with a SW200P, HEQ5 Pro unguided, Canon 1000D at ISO800 M31 15 x 2m Lights 10 X 2m Darks 10 x Flats 10 x Bias M45 12 x 90s Lights 10 X 90s Darks 10 x Flats 10 x Bias M81 20 x 90s Lights 10 X 90s Darks 10 x Flats 10 x Bias Any One Else Up for It ????
  6. The Nikon D5300 has a well-earned reputation as one of the lowest noise DSLR cameras used for Astrophotography. Now that I have a new Nikon D7500, I was keen to see how it compared to the D5300 in terms of the level of read noise and the extent to which that read noise is non-random ( and thus needs to be removed using a Master Bias frame to prevent it summing up during image integration). So here goes ... ......... A single bias frame Nikon D5300: ISO400, 1/4000th second: This may look pretty bad but really the extreme stretch is bringing out the very small variations across the frame: Standard Deviation: 3.53 ADU ( note: in this context, 3.53 ADU means 3.53 "units" on a real number scale ranging from 0 to 16,383 ( ie. a real conversion of a 14 bit digital scale )) The master bias frame looks like this: Nikon D5300: ISO400, 1000 x 1/4000th sec frames Standard deviation: 0.48 ADU The bands at the bottom are each 1 ADU brighter than the one above. Now for the D7500 ... A single bias frame from the Nikon D7500: ISO400, 1/4000th second: It is immediately clear that the single bias frame is cleaner. The statistics confirm this: Standard Deviation: 1.37 ADU And the master bias ... Nikon D7500: ISO400, 1000 x 1/4000th sec frame Standard deviation: 0.07 ADU The band at the bottom is 1 ADU brighter than the background. The improvement is very obvious Std. Deviation single bias frame: 1.37 versus 3.53 Std. Deviation master bias ( 1000 frames ): 0.07 versus 0.48 In graphical form ... Conclusion: The read noise in a single frame from the D7500 is around 40% of that in one from the D5300. This should give me greater flexibility to reduce exposure times and still ensure that the read noise is only an insignificant component of the overall noise. I will need to consider further the impact of the very low level of pattern noise in the Master Bias; it is so low that I will think about whether or not I still need to calibrate my lights with a Master Bias ( particularly for long exposures when the noise is dominated by light pollution and thermal noise ).
  7. Now that may look depressing for a post title, but it isn't that bad. Since the weather is the way it is (= 1 shade of gray, and snow for the whole weekend), what else can you do but reprocess old data and play around with the software (in my case PixInsight) I found this script in PI that lets you calculate camera statistics such as gain, read noise, dark current, etc. While it is intended for CCD and not DSLR, some threads I read suggest that the numbers you get should at least be in the ballpark. So I thought to myself, why not, and took a few shots with my old (2009) Pentax. I took advantage of the weather (Yaay!) because conditions were low noise (0 deg C). First I was amazed at the results: readout noise < 2.7e and dark current < 0.035 e/sec at ISO 800. But after some more checking I got my explanation: DSLR's may use internal dark clipping. That made me think; if the camera alters the image data, then what is the use of taking a lot of bias and dark frames? Ok, dark frames do help with the amp glow of my camera, somewhat, and they do show I have so many hot pixels that even a dark frame looks like a clear photograph of the milky way. But still, amp glow is taken care of with a pair of scissors (read: cropping) and hot pixels can be removed by cosmetic correction. My question stands: what is the use of taking a lot of calibration frames if your camera does internal correction? BTW, the cause of the clipping was also revealed to me on a Pentax site: the ADC is 14 bit but the output is only 12 bit per channel. Two bits down the drain.
  8. Hi all, first off, very sorry for this incredibly basic question, I am very new to this DSO imaging game . So, I am waiting on a suitable mount to get into this properly, but I am keen to practise with my 80mm refractor on the Nexstar mount; so short exposures only at the moment. Last night I had a play with getting some images of M82, and I could very easily see the galaxy on the screen with a 5 second exposure and the gain set high (obviously not good for imagining). So, taking 10 second exposure is giving me some images which I am happy, with, so next step is to work out how to stack, but reading making every photo count, I need to take some Dark & Bias frames first (when I have got all three types I will try to work out how to use DSS). So tonight my imaging will be very basic, I will probably take 40 Luminance shots at 1x1 and 15 RGB shots at 2 x 2; all shots with just10 second exposures. From reading the book, I think I understand Dark frames and basically I have to take these at the same time as the proper photos, using the same gain / exposure / focus settings, but with the end cap screwed on the scope (will ten shots be enough?). The Bias frames do not need to be taken at the same time (?), but do the settings need to be similar in terms of Gain? The books states that the exposure time does not matter, but I am already on a short exposure, so how will this be different from the Dark shots. Any quick answers would be good as I am hoping to be out in a field in 3 hours time , Cheers, Dave
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