DSS still a problem, or is it me??

[Stridor]

Ok, so carrying on from my last post a couple of weeks back where I explained that I am completely new to imaging. About 3 months ago I did a pretty poor image of M31; even though it was poor it was definitely recognisable and I was quite pleased. I then did an image of M 37 (open Cluster) and this was slightly better.....all good, so full of enthusiasm I then tried to image M81 & m82 In Ursa Major. The individual subs,  I thought looked ok: by now though I found that Deep Sky Stacker 3.3.2 was not working properly and it was necessary to download DSS 3.3.4., so I put ( as I have already previously explained....sorry!) these through the new DSS, all I ended with though was a very blurry image with very wiggly star trails!!? I tried numerous times with varying numbers of subs but the result is always the same. As an analogy, the before and after are like having a sheet of A4 paper (my sub) and then putting it through a shredder (DSS!!).  Anyway, I had an opportunity to attempt The orion Nebula (m42) the other night. All the subs, about 20 of them, look pretty good (from my very inexperienced imaging point of view...... ((I enclose one)) so again I put them through DSS and end up with the same result as before; a very blurry blob and some wiggly star trails. What on earth is going on??  The star count was between 60 and 150 stars. Do I need to download a more serious bit of software to proceed; and also will my current computer cope; I gather the processor speed etc etc all need to be pretty top notch!?

[vlaiv]

Sub that you showed is to much out of focus - it might not seem so when you look at it zoomed out but if you look at it at 1:1 you will notice two things - stars are not "points" (they really never are, but they should be just a couple pixels across with well defined center), but rather circles. Also they are not perfect circles but a bit distorted - this happens when you are not guiding or when your guider setup is having trouble.

So what is happening, at least this is my guess is - DSS is not finding right stars - stars that you see and expect DSS to find - it probably finds what it expects to be more like star - hot pixels in this case, and you end up with smeared stack.

You can do two things: first make sure your focus is better (this does not help much with already taken data). Second thing you might try is to take all subs and before using DSS preprocess them yourself (calibrate, debayer, ....) and then bin them to much smaller size so that stars become as close to the size they are supposed to be (couple of pixels across instead of few dozen of pixels across)

[Stridor]

7 minutes ago, vlaiv said:

Sub that you showed is to much out of focus - it might not seem so when you look at it zoomed out but if you look at it at 1:1 you will notice two things - stars are not "points" (they really never are, but they should be just a couple pixels across with well defined center), but rather circles. Also they are not perfect circles but a bit distorted - this happens when you are not guiding or when your guider setup is having trouble.

So what is happening, at least this is my guess is - DSS is not finding right stars - stars that you see and expect DSS to find - it probably finds what it expects to be more like star - hot pixels in this case, and you end up with smeared stack.

You can do two things: first make sure your focus is better (this does not help much with already taken data). Second thing you might try is to take all subs and before using DSS preprocess them yourself (calibrate, debayer, ....) and then bin them to much smaller size so that stars become as close to the size they are supposed to be (couple of pixels across instead of few dozen of pixels across)

Thanks, I will try what you recommend, but when I did my very first image 2-3 months ago of M31, it was very out of focus (was using an inferior camera at that time) and DSS 3.3.2 coped with that ok!!

[PeterCPC]

I don't know how you are focusing but use a Bahtinov mask as it really nails it.

Peter

[vlaiv]

10 minutes ago, Stridor said:

Thanks, I will try what you recommend, but when I did my very first image 2-3 months ago of M31, it was very out of focus (was using an inferior camera at that time) and DSS 3.3.2 coped with that ok!!

It might be due to number of reasons:

- Resolution of camera might be smaller - and in spite of stars looking more out of focus they were taking up less pixels (more star like profile)

- If you used hot/cold pixel removal on previous image it might have removed hot pixels and hence they were not seen as stars so alignment was ok

- If you are using hot/cold pixel removal in DSS for this set of images as well - try to adjust filter to deal with more than single pixel - by inspecting this sub on large zoom settings I spotted hot pixel clustering - sometimes it happens that few adjacent pixels are all hot pixels so they form a cluster of hot pixels - something that hot pixel removal filter will not see (it expects a single pixel to be with wrong value not bunch of them) and it looks more like star - also when debayering OSC image hot pixel values tend to spread to adjacent pixels as well so you need your hot/cold pixel removal filter to have at least 2/3 pixels sensitivity instead of 1.

 

[dph1nm]

Try using superpixel mode in the DSS debayer options. This, in effect, works on an image 1/4 of the size (it just uses the red pixels to form the red image etc, there is no interpolation) and is much better at finding stars if they are bloated or trailed.

NigelM

[groberts]

36 minutes ago, PeterCPC said:

I don't know how you are focusing but use a Bahtinov mask as it really nails it.

Peter

Discovering the Bahtinov mask was one of my epiphanies and is so easy and quick to use.      

[almcl]

I think, as well as the focus issue, the mount has actually been disturbed (at least on the sub shown) as the brighter stars show a distinct figure-of-8 shape, all in a northeast/southwest direction.   

Now I am a champion at producing that sort of image, and have possibly thousands of subs that look like that or worse, so am probably the last person to take advice from, but not moving near the scope when imaging, using a remote shutter release, avoiding backlash and doing shortish subs at possibly higher ISO, may all help.

[Stub Mandrel]

> I found that Deep Sky Stacker 3.3.2 was not working properly and it was necessary to download DSS 3.3.4.

I'm curious what the problem was, I thought the only change from 3.3.2 to 3.3.4 is support for more DSLRs, nothing has changed that should affect a 600D user (I use 3.3.2 and it's only very rarely it fails to stack).

I've found that, when the tripod is on grass you can create funny shaped stars just by standing within a foot or so of a tripod leg - once the camera is clicking away, retreat to a safe distance and try and resist the temptation to watch every sub preview on the screen (very hard to do, I know).

[newbie alert]

Looks like a focus issue...make sure your ota is well cooled... what scope are you using a 8 inch can be over an hour ,and focus will change abit during its cool down

[Jannis]

On 19.1.2017 at 15:51, vlaiv said:

It might be due to number of reasons:

- Resolution of camera might be smaller - and in spite of stars looking more out of focus they were taking up less pixels (more star like profile)

- If you used hot/cold pixel removal on previous image it might have removed hot pixels and hence they were not seen as stars so alignment was ok

- If you are using hot/cold pixel removal in DSS for this set of images as well - try to adjust filter to deal with more than single pixel - by inspecting this sub on large zoom settings I spotted hot pixel clustering - sometimes it happens that few adjacent pixels are all hot pixels so they form a cluster of hot pixels - something that hot pixel removal filter will not see (it expects a single pixel to be with wrong value not bunch of them) and it looks more like star - also when debayering OSC image hot pixel values tend to spread to adjacent pixels as well so you need your hot/cold pixel removal filter to have at least 2/3 pixels sensitivity instead of 1.

 

This lower 5 dead pixel cluster actually looks like JPG artifacts created by the canon camera. I have the exact same issue on my 550D, not visible on the RAW. I actually had my first camera replaced because of them, thinking it was many dead pixels, but the new was identical so i compared to RAW and noticed it wasn't there at all. This leads me to think OP is also stacking JPG files from the 600D? If so, stack RAW only from now on...

As for stacking, it looks indeed liek the issue here is the stars are out of focus. A (very bad) solution is to resize the images by maybe 50% and then stack them. Make sure to verify first that DSS actually detect the stars and not noise (click the star icon in the preview vindow to confirm).