gfletcher Posted April 17, 2016 Share Posted April 17, 2016 Hi Guys I attempted imaging through my 1000D fitted with CLS Clip I Filter and Canon EF 28-135mm lens last night. I was using 28mm focal length at f4 and at ISO 800 in Manual Mode and was shooting in RAW. I took: 10 Lights each 120secs 5 Darks each 120 secs 10 Flats (In AV mode) Looking at the Lights in DDP they all had a blueish tinge (as to be expected due to he CLS filter), they looked pretty good to me. Stars seemed quite tight. I put it all through DSS and arrived at the resultant TIFF image. When I post processed in Photoshop CS6 the right hand 1/3 of the image developed a colour cast which was obviously different to the rest of the image. I could not find any way of getting rid of this . Any of you guys got any ideas as to the cause of this and a way of getting rid of it to produce an even image? Attached are sample of one of the lights (prior to putting through DSS), the resultant flat and dark (after processing in DSS). I have also attached Autosave 01 (after DSS processing) Graham This is the Flat The Resultant Autosave01 after DSS processing Link to comment Share on other sites More sharing options...
+Graham Posted April 17, 2016 Share Posted April 17, 2016 First question is why are you changing modes to take the flats. I also notice your flat is not an even consistent brightness how are you taking your flats. Try processing again but without the flats. Link to comment Share on other sites More sharing options...
gfletcher Posted April 17, 2016 Author Share Posted April 17, 2016 Hi Graham I took the flats using AV because i had read somewhere that it was the method to adopt. They were taken at the same ISO as the lights and darks I took the flats by placing my ipad over the camera lens . The ipad was displaying "white light" Is this method not correct? Your help would be very much appreciated cheers Graham Link to comment Share on other sites More sharing options...
+Graham Posted April 17, 2016 Share Posted April 17, 2016 (edited) Hi Graham. I always take flats in the same mode as all the rest. I use my laptop screen for the flats but you have to make sure they are showing only halfway up the histogram. Looking at yours I would say they are too bright. Also if you look at yours it is bright in the centre and dark at the edges it needs to be the same all over. Try taking some more but decrease the exposure time. What capture software are you using. Edited April 17, 2016 by Graham Link to comment Share on other sites More sharing options...
Werisit Posted April 17, 2016 Share Posted April 17, 2016 You do need to take the flats at the same aperture and focus settings as your picture frames. Therefore you will need to change the shutter speed to get the correct exposure which aperture priority auto should take care of. I agree, your flats do look a bit bright so you can use a bit of exposure compensation to adjust your histogram (or as Graham says, use manual). The purpose of a flat frame, as I understand it, is to compensate for vignetting (from imperfect optics) so I'm not surprised to see some darkening at the corners. You may still have light pollution showing in the corner. LP filters help but they don't remove it completely. Link to comment Share on other sites More sharing options...
Perkil8r Posted April 17, 2016 Share Posted April 17, 2016 The ISO is unimortant for the flats so you can bump it up to 100 ISO to get the shutter speed easier. As above, stay with the same image format though. That said I am not sure why DSS would get to that result with those flats, but then small things can make all the difference so perhaps it is the format. Link to comment Share on other sites More sharing options...
Bassinghamobservatory Posted April 18, 2016 Share Posted April 18, 2016 Hi Graham, The discussion re flats has been well discussed but the main question was about the gradients across the image darker on the left and very bright at the right. The usual cause of gradients is light pollution, but anyone would suffer gradients when the moon is out and it is the latter that is the cause here. It does show slightly on the unstacked image but is brought out more in the processing. I have found that my widefield shots always have a gradient from the horizon due to light pollution form nearby towns. I have never tried it but there is a programme called GradientXTerminator that is reputed to solve the problem, but it will cost you $50. This is the link to the website http://www.rc-astro.com/resources/GradientXTerminator/. Pixinsight also has a gradient removal process and possibly there are others. Regards, Graham Too many Grahams! Link to comment Share on other sites More sharing options...
glider Posted April 18, 2016 Share Posted April 18, 2016 IRIS has a decent gradient removal tool. Link to comment Share on other sites More sharing options...
gfletcher Posted April 18, 2016 Author Share Posted April 18, 2016 How does IRIS compare overall to DSS ? Link to comment Share on other sites More sharing options...
Guest Posted April 21, 2016 Share Posted April 21, 2016 I've been having issues with flats, so I've given up on them for now. I read somewhere (Sky at Night mag I think) that one should take at least 8 darks. I notice you took only 5 but I used to. Since taking at least 8 darks in the same session as the lights, my results have improved. This is without using flats or bias frames. You might be able to get rid of the blue tinges and gradients by using Levels to remove/reduce the blue component. It's probably "old hat" to many astrophotographers (or astroimagers for the experts!) but I think there's a lot that can be done with just a DSLR. Link to comment Share on other sites More sharing options...
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