DSS problem

[dark star]

Last night I took some what I thought were OK images of M51, using my 250mm f4.8 Orion Optics UK reflector, Canon 20d DSLR, guiding with finder guider and PHD. i used an Atronomik CLS light pollution filter.

I thought my polar alignment was OK. The individual images look OK, but when I try to stack them in DSS the result is horrendous, the stars turn in to lines! Is there any way to choose the best images to stack, there is a way to calculate the offset in DSS but I don't know how to use this to choose which images to stack?

Looking at the images does not help as to me they look OK! The best thing would be if I could check the images when taking them. APT has a FHWM tool which I use, could I use this, in which case what is the minimum value which is OK?

David

[Freddie]

Can you put up a single image and the output from DSS for us to look at?

In DSS you can select the % of frames to stack though if the single frames are OK there is obviously something odd happening in DSS at the moment.

[Keiran]

When you begin the stacking process you can select a percentage of images to stack. I think as default it is set to 95%. This means it will discard the worst 5%. You may also want to check the stack parameters. I think there is a stacking method in there, you can choose how it stacks the images, you may have the wrong one checked.

HTH

[dark star]

I will post some images soon, my brain is too frazzled at the moment, as I have spent the whole day messing around with DSS!

i now tried stacking just 4 images of M51, making sure that M51 is in the same position in the camera image each time, as I lost it and went back to it a few times over the night, managed to lose power once etc.

I now get a double image of M51 and double stars. Does anyone have any ideas what causes this? I definitely do not have a double M51 in the original images!

David

[davew]

From your description so far I've no idea what's wrong other than you may have a duff setting or your idea of an ok sub doesn't match DSS's idea of an ok sub

Did you know there's am FWHM tool in DSS ? Try registering all your subs then look on the far right of the results section ( Ignore the score ). There you will find the average FWHM.

Don't chuck out subs with the auto select without good reason !

You can set up DSS to live register as you go.

With regards to an acceptable FWHM figure then it depends on your gear. Take no notice of the figures you hear about from others when focusing. They are normally only a few seconds long. As a wild guess I'd say you would need somewhere between 3 and 5.

Dave.

[dark star]

Hi, my FHWM in DSS are between 3 and 4. I attach 2 images, both 60 seconds at ISO 800 and the third image is what happens when I stack the images, I didn't stack any with what I thought were really bad stars. Are my stars just not round enough? But the FWHM doesn't seem so bad. I was guidng, maybe the fact that it was windy caused the problems? Any advice gratefully received!

I am starting to wonder if I am getting some kind of double image in the originals caused by my Baader MPCC coma corrector and Astronomik CLS clip filter giving internal reflections. On the other hand I can't see any double stars myself and the MPCC is the new version with anti-reflection coatings.

David

https://dl.dropboxusercontent.com/u/82184859/M51twice.png