First light with my new Esprit 80: M13

[wxsatuser]

Thats miles better than my Borg and Canon 70-200mm.

Can't really help with your problem but just to let you know, your not alone.

[alan4908]

Hi Paul

I was wondering what might be causing this since, as you say for PA/guiding error appear to be too small to offer an explanation.

So, in an attempt to find another explanation... I looked at the web site  Astrophotography (http://www.astro-imaging.com/Tutorial/MatchingCCD.html) it tells me that for deep sky imaging the FHWM will be dominated by the seeing and to work out the FHWM you can get, you can use the formula star size = (seeing * focal length)/206.3.  If you assume a seeing of (say) 3 arc seconds and a focal length of 400mm for your scope you get a FHWM = 3 * 400/206.3 = 5.82microns. According to the Nyquest criterion, in order to adequately sample this signal you would need camera pixels of 5.82/3.3 = 1.76 microns.  If you are using the STF-8300M camera, this has pixels which are 5.4microns, which means that you be significantly undersampling the image by a factor of approx 300%. If the seeing was better than 3 arc seconds then the undersampling would be even greater. According to the web site this can seriously impact the star shapes eg it will give square blocky stars, or even stars that are missing completely.  So, do you think that this might be a possible cause  ?

Alan

[Pompey Monkey]

50 minutes ago, alan4908 said:

Hi Paul

I was wondering what might be causing this since, as you say for PA/guiding error appear to be too small to offer an explanation.

So, in an attempt to find another explanation... I looked at the web site  Astrophotography (http://www.astro-imaging.com/Tutorial/MatchingCCD.html) it tells me that for deep sky imaging the FHWM will be dominated by the seeing and to work out the FHWM you can get, you can use the formula star size = (seeing * focal length)/206.3.  If you assume a seeing of (say) 3 arc seconds and a focal length of 400mm for your scope you get a FHWM = 3 * 400/206.3 = 5.82microns. According to the Nyquest criterion, in order to adequately sample this signal you would need camera pixels of 5.82/3.3 = 1.76 microns.  If you are using the STF-8300M camera, this has pixels which are 5.4microns, which means that you be significantly undersampling the image by a factor of approx 300%. If the seeing was better than 3 arc seconds then the undersampling would be even greater. According to the web site this can seriously impact the star shapes eg it will give square blocky stars, or even stars that are missing completely.  So, do you think that this might be a possible cause  ?

Alan

Hi Alan,

If you look at the first image of M13 that I posted at full size, you will see that the stars are indeed blocky. Some are
barely more than a single pixel.

However, two factors immediately come to mind to discount this as a reason for the distorted stars:

1. The elongations are not uniformly distributed across the images;
2. The elongations are still very much evident where the brighter stars are several pixels in diameter.

I actually interpreted the obvious undersampling as an indicator of just how good the optics are: I suspect that it would get
close to the theoretical limit of 1.45" (about half my image scale) if I had small enough pixels!

I still very much welcome your input to this discussion

I am also currently in discussion with FLO and I am confident that a resolution will be found soon

[Pompey Monkey]

Huzzah! FLO has arranged for the Esprit to be inspected and re-aligned (if necessary) by Es Reid. It is in the box ready to be collected. At the very least we will find out if the problem lies in the scope or elsewhere in the chain.

Many thanks to Steve (both of them!) at FLO