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Also in long exposures, flexure can reduce the resolution (The average of lamp lines at start and end shows some smearing) For example in a 2 hour exposure last night there was ~0.5 pixel movement which reduced the resolution by ~5% This movement can be much greater if everything is not tight (As Benji suggested) A good check is to take lamp spectra with the telescope pointing in different directions and see how much movement you get. (My biggest movement (several pixels) was caused by slackness in one of my grating mounts, solved by increasing the spring tension)

Cheers
Robin

Robin,

Where is the "PSF" in ISIS. I don't see anything like that.

Al Stiewing

Albert Stiewing wrote:Robin,

Where is the "PSF" in ISIS. I don't see anything like that.

Hi Al,

Check out the screen shot I posted. Load the image through the "IMAGE" tab. Draw a box (in red) round the line you want to measure (keep the height small so any slant/smile does not affect the result) and click "line PSF" A graph of the line profile appears together with the calculated FWHM in pixels. (It is actually a plot by column of all the pixel values in the selected box)

Cheers
Robin

Hi all.

To Eric. Using FWHM of Ne Lines and PSF function of ISIS is a quick and easy way to focus the Camera on the doublet lens. I use this method almost each night (because I frequently change the grating), and most of the time, I obtain resolution around 14000 in H-alpha region with 2400 grating.

In LHIRES3 user manual, it is said that the optimal FWHM is between 3 and 4 pixels. I assume that such FWHM is obtained with a KAF-0400 sensor (pixel size=9um). Using a "rule of three", I obtain an optimal FWHM between 6.5 and 7 pixels. This is an approximation, but It allows to go quickly toward the focus, and to know if you are moving the lens in the right direction or not. With your camera, and using this rule, your optimal FWHM should be near 3.28 pixels (=3.29um*2*4/9um). As I said before, it should allow you to know if you are moving the focus in the right direction or not.


To Robin (and others). I assume from your previous post that the theoretical FWHM of the Ne Line you have chosen is near 32um. Am I true? Does the theoretical FWHM of Ne Lines change with wavelength? In other words, are the theoretical FWHM of the lines in Halpha region significantly differs from those in Hbeta region? I have looked for documentation on this question, but find nothing.

Best regards,
Ibrahima.

Ibrahima wrote:
In LHIRES3 user manual, it is said that the optimal FWHM is between 3 and 4 pixels. .

I think that just refers to the optimum pixel size when choosing a camera. For example for a 35um slit the optimum pixel size is 8.7-11.7 um. (larger than that and you risk undersampling, smaller than that and you are decreasing signal/noise due to the higher read noise) I think it is good not to bin when tuning for best focus though.

The actual width of the line will depend mainly on the slit width but there will be some diffraction effects too so there could be some change with wavelength. I think it will be small though so the line width in pixels at best focus should be almost constant with wavelength (You certainly need to refocus for the particular wavelength though because of the achromatic doublet)

Robin Leadbeater wrote:
The actual width of the line will depend mainly on the slit width but there will be some diffraction effects too so there could be some change with wavelength. I think it will be small though so the line width in pixels at best focus should be almost constant with wavelength

I just checked some of my lamp spectra and the FWHM in pixels or um of an in focus line with a 35um slit is very similar at both 4000A and 6600A

Cheers
Robin

Hi all,

I wasn't able to answer the messages. I now notice that there are a lot of tips and things I have to check, for which I sincerely thank you all !!

@Franck, the motorization of the focus lens in the LHiResIII looks nice

@Robin: I'm going to check the distance of the camera again, but I suspect it's correct. At the time I had an adaptor ring made with the correct distance so that the camera could be screwed directly onto it with a positioning ring. But I'm definitely gonna check it back

Here an image of my calibrationlamp with 2400 l/mm grating: I'm also going to try ISIS as a tool to achieve the best focus. I will try this out next weekend.

best regards,
Erik

Hi Erik,

There is a dedicated FWHM measurement tool for focusing process available in Audela software [1].

1. First you draw a rectangle (left click mouse button) around line's zone you want to do FWHM measurement
2. Click right mouse button and select FWHM: 3. Check radio button at window's bottom in order to get measurement refreshed when a new image is displayed during focusing precess: More over, Franck Houpert created a specific tool in Audela but I forget the url where he described it.

Benji

[1] : http://spcaudace.free.fr/installation.php ; then select AudeLA-3.0.0b4.exe or audela-3.0.0b4-amd64.deb