What is the typical FWHM that I can get in the UK?

[SlimPaling]

Hi ...

I have just started to get into CCD mono imaging and I am concerned that my set-up needs some tweeking.

Last night, after using the focusing process in Maxim DL, I managed to get the FWHM only down to just over 4.0 before I started imaging. The seeing wasn't brilliant and there was a full moon.

Is this FWHM typical for our UK skies ? ... or should I be able to achieve a better measurement by doing something despite the fact that, after much effort, I think that I have got the back-focus distance between my CCD sensor and my flattener spot on???

Any help & suggestions would be much appreciated.

Mike

[Davey-T]

Are you focusing manually ?

Have you worked out the theoretical resolving limit of your setup ? Dawes limit.

I can usually get under  2 FWHM using 8300 sensor and WO Star 71.

Dave

[SlimPaling]

Hi Dave ....

I have an Optec Focus Lynx focusing system fitted to my 3.5" Featherlight  focuser and used the V-curve focusing system in Maxim DL ... which gave me the best fit focus setting before I started to take images.

My camera is a Moravian G2-8300 ... I don't know what is the "WO Star 71" that you refer to :-(

I live about 15 miles north of Nottingham and there is a fair amount of light pollution here caused by city lights and being under a flight path from USA to Europe :-(

Mike

[wxsatuser]

I can get under 2 with the Samyang.

[SlimPaling]

46 minutes ago, Davey-T said:

Are you focusing manually ?

Have you worked out the theoretical resolving limit of your setup ? Dawes limit.

I can usually get under  2 FWHM using 8300 sensor and WO Star 71.

Dave

Hi again Dave ...

After looking on the 'net I now know what a WO Star 71 is :-)

I am using a APM 130-780 Super ED Triplet

I still don't know what my Dawes limit is for my system .... or how to work it out.

Mike

[SlimPaling]

3 minutes ago, wxsatuser said:

I can get under 2 with the Samyang.

Do you suffer from much light pollution where you are???

Mike

[wxsatuser]

Just now, SlimPaling said:

Do you suffer from much light pollution where you are???

Mike

Yes.
Football floodlights across the road and all the other usual sources.

The Samyang is quite hard to get right but in Artemis I can get under 2 with careful prods
on the Skywatcher focus motor. I really need a smaller pulley on the motor but it's the smallest there is.

[Davey-T]

6 minutes ago, SlimPaling said:

Hi again Dave ...

After looking on the 'net I now know what a WO Star 71 is :-)

I am using a APM 130-780 Super ED Triplet

I still don't know what my Dawes limit is for my system .... or how to work it out.

Mike

I can't remember the maths of the top of my head, will have to look it up, one thing I found extremely useful is a motor focus, doesn't need to be computer controlled, makes it so much easier to make small adjustments.

Dave

[DaveS]

Dawes Limit for a 130mm 'scope is 0.89" (When I put my 'scope into Astronomy tools) Formally it's 4.56"/ D (Inches)

[Merlin66]

I wish imaging scopes would work to the Dawes limit.... they are limited by the seeing disk, much larger than Dawes.

 

[sloz1664]

Hi Mike

I live in south Nottingham and typically get 1.6 FWHM in Maxim with my QHY8 Pro AA80 Triplet combo.

Steve

[Stub Mandrel]

Doesn't it depend on the brightness of the star?

[sloz1664]

Always try to grab a mag 5 star or you tend to blow out higher mag stars which affects the fwhm anyway.

Steve

[Davey-T]

10 minutes ago, Stub Mandrel said:

Doesn't it depend on the brightness of the star?

Yes and no, works best on something about mag 5

Dave

[ollypenrice]

Whatever you do, the focus star must not be saturated or the top is chopped off the bell curve whose 'half maximum' height is key to the measurement. For my CCDs that's 65000 ADU, shown alongside the FWHM in my software. If I reach 65000 I must shorten the exposures or find a less bright star. I like to use longish exposures (3 secs) because they average out the effects of seeing.

I rather hope that someone comes up with a proper theoretical explanation of the FWHM values because I don't really understand them. From experience, the coarser the pixel scale of the system the lower the FWHM. At 3.5"PP in our Tak 106/Atik 11000 it is not unusual to get down to 0.85. The seeing-related range is also low, with a likely 'worst' value of about 1.2. However, at higher resolution (0.9"PP in the TEC 140/Atik 460) the FWHM is both higher and more variable with the seeing. On a glorious night we might see 1.2 but it can be far worse, as bad as 2.5 or more. In this case I would only shoot colour and I'd wait for better seeing for luminance.

When we were imaging at 0.6"PP with a 14 inch for a couple of years we found FWHM totally useless. The values jumped around all over the place and told us nothing whatever. We had to use a Bahtinov mask on that setup.

Olly

[pete_l]

1 hour ago, ollypenrice said:

I rather hope that someone comes up with a proper theoretical explanation of the FWHM values because I don't really understand them. From experience, the coarser the pixel scale of the system the lower the FWHM. At 3.5"PP in our Tak 106/Atik 11000 it is not unusual to get down to 0.85.

Olly makes a good point. I wouldn't get too obsessed by "chasing the numbers". Ultimately what matters is the quality of your final image. For example, I use a 50mm finder-guider. That has a theoretical Dawes limit of 2.3 arc-sec. Somehow PHD2, with a 5.2µm pixel QHY5Lii manages to tell me I'm getting sub arcsecond FWHM stars!

Just don't look too closely at the "rough" guiding graph (dragging cables).

[Merlin66]

Interesting topic.

In spectroscopy we are very focused (excuse the pun) on the FWHM of the target star. To get maximum efficiency and photon collection we try to set the slit gap to at or just below the FWHM. This and having the imaging camera pixel size matched to give a sampling of at least 2 (preferably 3) is one of the keys to success.

When I see a FWHM quoted as 1.7 or 0.9 or what ever, I automatically think that the star image is undersampled.

For the C11 at f10 and a good night of 2 arc sec seeing, the FWHM image size would be 27 micron. x2 binning with an Atik 314 would work OK.

The guide figures above are showing (??) the guide image FWHM NOT what the imaging camera sees.....

[ollypenrice]

7 hours ago, Merlin66 said:

 

When I see a FWHM quoted as 1.7 or 0.9 or what ever, I automatically think that the star image is undersampled.

 

This is certainly compatible with my experiences because it is in undersampled systems (like our Tak 106 pair with 9 micron pixels giving 3.5"PP) that I see the very low FWHM values. Binning also lowers the FWHM but obviously has no effect on the focus.

Olly

[coatesg]

FWHM is simply the measurement of the width of the star profile at half its maximum value - it follows that it's dependent on seeing, focal length, pixel size and aperture. Light pollution shouldn't affect this, though it will decrease signal to noise ratios.

If you're measuring in pixels, and if you're undersampled (eg KAF-8300 on Star 71 above - 3.2"/px) then FWHM values in px could be very low, and it's not necessarily a good indication of actually how good the seeing is. Software interpolates the star profile a bit here, and if virtually all the star light falls on one pixel, then it's going to give an inaccurate answer.

On long exposures, I get best figures around 1.3px FWMH as measured on the subs in PI - 7.4um pixels at 632mm FL gives this at about 3.1" but this includes any guiding errors as well as seeing (and is still undersampled!).

If you want to verify focus, a bahtinov mask is a cheap and effective way of doing so.

 

[Merlin66]

Graeme,

the FWHM is only dependent on the seeing.

measuring the FWHM can be influenced by undersampling.

[coatesg]

FWHM is a measure of how much the atmosphere *and* telescope have smeared out the point source onto the imaging surface - so it has to be dependent on scope aperture (this defines the angular size of the airy disc) - for large apertures and short fl, I agree the airy disc is << seeing. If you measure in pixels rather than arcsec, the pixel size and FL dictate how many px the star's profile covers. If you measure in arcsec, the combination of pixel size and FL don't matter as long as you're not undersampled.

Of course, we're assuming perfect optics as well (which they won't be)

[Merlin66]

In my ten year experience in spectroscopy the FWHM is seeing dependent.

The linear star image is directly related to the focal length. The aperture is a minor player. The pixel size just affects the sampling.

 

[coatesg]

Yes - I agree - for all intents, FWHM is most dependent on seeing (in spectroscopy, the aperture won't be tiny). 

But, if you're using a camera lens (small aperture) it can affect - wxsatuser might be (talks about a Samyang).1

 

[Stub Mandrel]

FWIW I get values of 4 - 4.5 on my guidescope, but that uses a 50mm coated achromat objective that cost about £6 from Astromedia.

[SlimPaling]

I see that there has been quite a few responses to my original question about the typical FWHM values that I should be getting in my system since I last wrote anything here.

I have been away since last Friday morning and I haven't been able to respond to all of the thoughts and suggestions that you have been sending in ... many thanks. You have given me a lot to think about here!

After reading all of the replies I suspect that I happened to select a couple of stars, for testing the FWHM values, that were probably too bright and so became over exposed. I thought that I had chosen my stars carefully as I knew that they didn't want to be too bright ... the ones that I did chose were certainly not the brightest on my test images.

On Thursday night, despite my worried about focusing and the FWHM values that I was getting, I decided to plough ahead taking sets of LRGB of M15 .... today I have just done my very first attempt at calibrating and stacking these images. As it is my first attempt at doing any if this I am quite happy with my efforts thus far.

I am attaching a copy of my first attempt at LRGB imaging .... not too sure about the correct colour balance yet. 

Any helpful comments would be very much welcome :-)     Mike

M 15