Narrow band help please

[mike geisel]

Hi, I took this image of Thors helmet last night Ha, O111, Hb all 3minutes , all 20 subs. I haven't finished processing yet and hope to improve the colour. There are two problems I don't know what to do about. First the background is streaky. I have heard this may be due to temperature variation of the sensor and as it was hot last night the cooler at max shifted from -14 deg to -18 over the 3 hours. I have also read it could be flexure in the guide scope. Secondly there are a number of 3 coloured streaks where something has moved over the 3 hours. Are these hot pixels? If so any advice on removing them. I have Pixinsight but am only just starting to learn it. Image taken Sky Watcher Esprit APO150 EQ8 mount ZWO ASI 1600mm-c. Guiding was via a 80mm 600mmFL refractor mounted on top with an Orion pro auto guider and PHD2. Sorry I don't frequent forums often and I couldn't work out how to attach a reasonable size copy of the image so I have pasted a drop box link to the image. Will that do?
https://www.dropbox.com/s/ym448vhe3x...20RGB.jpg?dl=0

[glend (Glen)]

Not too bad but, imho, you need much more data, I would be aiming for something like 100 subs for each filter. There is much more detail there to be had. I always use 5min NB subs, on faint objects, but it does depend on your mount and guiding setup.
The ASI1600 will be fine at temps below -10C, so don't stress too much about that. Pick a temp it can hold. Imaging on warm night's is always a challenge. Just make sure you shoot darks and bias at the same temperature.

[barx1963 (Malcolm)]

Originally Posted by mike geisel

Hi, I took this image of Thors helmet last night Ha, O111, Hb all 3minutes , all 20 subs. I haven't finished processing yet and hope to improve the colour. There are two problems I don't know what to do about. First the background is streaky. I have heard this may be due to temperature variation of the sensor and as it was hot last night the cooler at max shifted from -14 deg to -18 over the 3 hours. I have also read it could be flexure in the guide scope. Secondly there are a number of 3 coloured streaks where something has moved over the 3 hours. Are these hot pixels? If so any advice on removing them. I have Pixinsight but am only just starting to learn it. Image taken Sky Watcher Esprit APO150 EQ8 mount ZWO ASI 1600mm-c. Guiding was via a 80mm 600mmFL refractor mounted on top with an Orion pro auto guider and PHD2. Sorry I don't frequent forums often and I couldn't work out how to attach a reasonable size copy of the image so I have pasted a drop box link to the image. Will that do?
https://www.dropbox.com/s/ym448vhe3x...20RGB.jpg?dl=0

Hi Mike
The bright 3 coloured streaks will most likely be hot pixels. When you have run image registration in PI the hot pixel ends up in a different spot so you get a streak as each image is shifting slightly. I am guessing you have a polar alignment issue, how do you Polar Align and how confident are you that it is correct? The fainter streaks are caused by the same effect on pattern noise in the sensor. Best bet is most of that will be got rid of by getting a good set of darks and bias frames, you didn't mention that in your post so I assume you have not used?

Malcolm

[mike geisel]

Hi, thanks for the replies. Glen, I was interested in your comment that the 1600mm-c would be fine at anything below 10deg. I always try and get it as cold as possible, but I also read somewhere recently that it was more important to keep the temp constant. At the moment I have it set to -26deg but maybe in the hot weather I should be more realistic and set it to something like -14deg. Also one of my biggest problems with astrophotography is patience!. 3 hours total on this was all I could bear. Your suggestion of 3x100 subs at 5min would drive me insane!
Malcolm, thanks also. I have my gear permanently set up and aligned in an observatory and thought the polar alignment was pretty good, within a few arc minutes. But I think I will revisit this and see if I can get better. I use Sharp Cap pro for capture and it has an automatic apply darks feature, but I think the dark files need updating, so I will try that. I have been lazy and slack and not doing flats and biases and up until now have been getting away with it, because subs have been fairly short, usually 60 seconds. I plan to start using these calibration frames soon. Perhaps the lack of flats is the reason for the background streaks? I do alignment usually in Nebulosity 4 and at the moment only use pixinsight for modified hubble colour correction. But I hope to learn more about pixinsight over the next year, (New Years Resolution??) Thanks guys
Mike

[glend (Glen)]

Originally Posted by mike geisel

Hi, thanks for the replies. Glen, I was interested in your comment that the 1600mm-c would be fine at anything below 10deg. I always try and get it as cold as possible, but I also read somewhere recently that it was more important to keep the temp constant. At the moment I have it set to -26deg but maybe in the hot weather I should be more realistic and set it to something like -14deg. Also one of my biggest problems with astrophotography is patience!. 3 hours total on this was all I could bear. Your suggestion of 3x100 subs at 5min would drive me insane!
Malcolm, thanks also. I have my gear permanently set up and aligned in an observatory and thought the polar alignment was pretty good, within a few arc minutes. But I think I will revisit this and see if I can get better. I use Sharp Cap pro for capture and it has an automatic apply darks feature, but I think the dark files need updating, so I will try that. I have been lazy and slack and not doing flats and biases and up until now have been getting away with it, because subs have been fairly short, usually 60 seconds. I plan to start using these calibration frames soon. Perhaps the lack of flats is the reason for the background streaks? I do alignment usually in Nebulosity 4 and at the moment only use pixinsight for modified hubble colour correction. But I hope to learn more about pixinsight over the next year, (New Years Resolution??) Thanks guys
Mike

Mike I was referring to imaging at -10C with the 1600mm-c in summer. It is near impossible to get down to -20 or less on a warm summer night. A Delta T of 35C from ambient can be difficult for that small Cooler chip and fan. You will probably find the camera case starts to heat up, a sure sign it cannot shed heat. You could install a small booster fan to increase airflow over the case, I did that a few times in mid summer. As to building more data, consider returning to your target over multiple nights, as this can be the only way to build sufficient signal to improve your image. Of course, imaging over multiple nights means that you need the skills to return the scope to exactly the same spot in the sky with camera alignment to match. In an observatory, where your setup can remain untouched, this is pretty easy, even without Plate Solving. However if your tearing down each night it can be difficult, but Plate Solving can help alot. Even if your sub alignment is slightly out you can still align them with DSS if they are close, you may need to crop overlaps slightly but it can work. Obviously full plate solving as in SGP or other sequence automation software, makes it easy, but increases the complexity of your operation, at least until you have it all working well.

[barx1963 (Malcolm)]

The moment I saw the streaks, I knew they had to be hot pixels. I have had a similar issue from time to time but as I dither, they usually turn into clusters of multi coloured dots. he hot pixel is always in the same place on the image, but if each image is pointed at a slightly different point in the sky, when your software uses star to align all the images, the hot pixels end up in different spots. The fact that the coloured streaks and the faint streaks are all oriented the same way indicate that they originate from the same cause.

One thought that occurs to me, are you using "dithering"? PHD2 has an option that allows dithering in RA only. The streak appear to be oriented east west, so if you are dithering AND dark frames subtraction is not getting rid of your hot pixels, that may be a solution.

Not sure about the Sharpcap "auto dark" feature. Me, I wouldn't use it, you are effectively ceding control of calibration to software. Much better to calibrate properly in pre-processing. I have tried Nebulosity and several other processing programs but have ended up using Pixinsight almost exclusively. Pre-processing and calibration is very easy once you know what to do and gives great results if you have good data. I would strongly recommend Warren Kellers book (see https://www.bintel.com.au/product/in...v=6cc98ba2045f ) which is excellent for guiding through the pre-process "process".

Malcolm