Jump to content

Recommended Posts

Trying to improve my focussing and tried the FWHM thingy in Artemis Capture. What sort of numbers should I expect , I got down from 3.something to

fluctuating between 1.09 and 2.03, is this OK ? Never achieved a stable number.

Subs didn't look any better but my seeing is rubbish, lots of LP.

Dave

Link to comment
Share on other sites

Make sure that you are using at least 3-4 s exposures as this helps to combat the seeing. Also you don't want to focus on a star that is fully saturated. I think you want one that is reading about 20,000 - I hope someone can clarify that. Because in Artemis along with the FWHM figure you have a Bri figure and you want to use that in conjunction to get the lowest FWHM figure you can, coupled with the less movement with the Bri figure.

I generally get my FWHM figure at below 1 unless there's haze, in which case I generally go back indoors!!!

Link to comment
Share on other sites

I generally get my FWHM figure at below 1 unless there's haze, in which case I generally go back indoors!!!

If I waited till it got below 1 I'd be permanently indoors :)

Just have to make the best of what I've got.

Dave

Link to comment
Share on other sites

The figure you get depends entirely upon the resolution of your setup.

I image at 0.74arc sec per pixel, so anything under 3 pixels FWHM is good for me (2 arc sec FWHM). If you are imaging at 2 arc sec per pixel then I'd want to be around 1 FWHM.

With a 10" Schmidt cass .. (assuming 2m focal length and ~5um pixels) then a FWHM of 4 is pretty good, on a ED80 I'd be wanting as near to 1 as I could get.

Derek

Link to comment
Share on other sites

Derek, can I ask how you arrived at your numbers for FWHM? What is the relationship you are using?

I looked into this before, but couldn't find a way of calculating FWHM based on set-up and site conditions.

Link to comment
Share on other sites

Ok... I hope I've got hold of the right end of the stick here.

All I'm doing is making the assumption that 2 arc sec is achievable (I can get this in short focussing shots >90% of the time, and on long shots when the tracking plays ball.. but then I'm surrounded by green trees and paddocks, no arable (warm soil), <1% tarmac/concrete etc. If you're at 3 or 4 arc second FWHM then your numbers will be worse.

My image scale is defined by my focal length (1500mm) and my pixel size (5.4um) and by the fact that I image in mono and not OSC, if you are using a DSLR or OSC-CCD then you will suffer a slight blurring when de-bayering.

Maths for image scale (excel formula):

=ASIN(pix/fl)*180/PI()*3600 (the 180/Pi is just conversion from radians to degrees)

pix = pixel size

fl = focal length

note: pixel size and focal length must be in the same units, be they meters, micro-meters, mega-parsecs etc. :grin:

Derek

Link to comment
Share on other sites

Ah.. ok now we're on the same page.

Ok, nyquist. On a 1D trace nyquist = 50% of sampling rate. On 2D this is also true along the vertical and horizontal axis, on the 45 degree diagonal you still need it to be true, which means you need smaller pixels, ~30% smaller or 70% of the size [ 1/sqrt(2) ] So you end up needing a FWHM of about 2.8 pixels vertical or horizontal to capture the full detail.

This is why I was very happy to mate my 12"f5 newt with a KAF8300 sensor, giving me optimal sampling for best resolution with 2 arc sec seeing.

I found this site, which has some nice example images

http://www.stanmooreastro.com/pixel_size.htm

Of course if you are more interested in going deep than resolving detail then you might want to err towards bigger pixels.

Derek

Link to comment
Share on other sites

I read that site before, but again I got lost about half way through. It comes to the conclusion that a FWHM = 3.33pixels is optimum. But it doesn't say what size the pixels are, it's doesn't describe the optics and it doesn't mention seeing or tracking conditions.

For example. If I take my set-up, I have a FL of 1200mm, apeture 153mm and camera pixel size of 5.2um. My site is "inner-city" so seeing would be nearing the 3 arcsec mark I'd say. How would I translate that into a FWHM measurement and what are the steps?

Link to comment
Share on other sites

well, lets plug your numbers into the equation (it's basic trig that's all)

=ASIN(pix/fl)*180/PI()*3600

pix = 5.2e-6 meters

fl = 1.2 meters

aperture has no bearing on this bit.

the 180/pi() is the radians to degrees conversion

the 3600 is the degrees to arc seconds conversion

The angle created by a triangle with an 'adjacent' side of 1.2m and the 'opposite' side of 5.2um is 0.9arc seconds.. so your image scale is 0.9arc sec / pixel

So on a good (2 arc sec FWHM) day you might get your FWHM down to 2.2 pixels, more typically you'll be around 3.3pixels

We can do the same with your aperture to work out your diffraction limit.

First we have to decide what wavelength we're talking about, if we assume it's 500nm (green) then we can work out the resolution,

we can assume a path difference of one wave across the face of your objective, so

=ASIN(wavelenght/aperture)*180/PI()*3600

and we get a rough estimate of 0.7 arc seconds diffration limit (it's really a 2d equation not 1d, but this is good enough for pretty pictures and our poor seeing)

So your seeing is limiting your view, not your scope.

Derek

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.