Slit width for low resolution spectroscope.

[Jaroslaw Grzegorzek]

Hi,
I'm interested in choosing the optimal slit width for a low resolution spectroscope (R ~ 100) and very faint objects (SNs, QSOs), depending on the camera pixel size and telescope focal length etc. I tried to search the web but didn't find much. Do you know any articles considering this issue?

Cheers
Jarek

[Jaroslaw Grzegorzek]

Hi,

I'd like to use the smallest slit (minimise the sky background) but NOT LOSE any light stream of the object.
SCT telescope: D = 250 mm and f/6.3

My "homely" conclusions so far are:
Calculating: 7.8um (1.03") Airy disk but assuming 3" seeing I've got 22.7um.

Conclusions:
23um slit for 13um pixel size should be good. Am I right or not?

[Olivier GARDE]

With an ALPY 600, the slit width is a 23µm and you can take very faint target with this spectrograph (around 17-18 with a 200-250mm telescope).

[Jaroslaw Grzegorzek]

With an ALPY 600, the slit width is a 23µm and you can take very faint target with this spectrograph (around 17-18 with a 200-250mm telescope).

Thank you Olivier. Yes, I know that Alpy 600 got 23um slit, but I have adjustable slit and I wonder what width is the best for my purpose.
By the way, 17-18 mag is a great achievement with Alpy 600 and 8-10" telescope, I suppose.

[Olivier GARDE]

By the way, 17-18 mag is a great achievement with Alpy 600 and 8-10" telescope, I suppose.

It's possible, look at this post :
http://www.spectro-aras.com/forum/viewt ... f=6&t=2552

[Jaroslaw Grzegorzek]

It's possible, look at this post :
http://www.spectro-aras.com/forum/viewt ... f=6&t=2552

Of course, I believe it's possible. It doesn't change the fact that it is extremely difficult. Great appreciation for Etienne Bertrand.

[Robin Leadbeater]

Hi,
I'm interested in choosing the optimal slit width for a low resolution spectroscope (R ~ 100) and very faint objects (SNs, QSOs), depending on the camera pixel size and telescope focal length etc. I tried to search the web but didn't find much. Do you know any articles considering this issue?

Cheers
Jarek

HiJarek,

I am interested in the same thing. It is a balance between the fraction of the light collected from the star and the noise added by including more pixels.(from the camera and the sky background). The exact calculation is possible using numbers for the fwhm of the star image, the camera noise and the light pollution, similar to the calculation here for photometry by Michael Richmond
http://spiff.rit.edu/richmond/signal.shtml

Christian Buil has done something similar for spectroscopy
http://www.astrosurf.com/buil/us/compute/compute.htm

Of course you must also consider the loss of resolution for a wider slit so the same resolution needs a higher dispersion which then adds more noise

Cheers
Robin

[Robin Leadbeater]

It is a balance between the fraction of the light collected from the star and the noise added by including more pixels.(from the camera and the sky background).

The number of pixels per slit width is also a factor which affects the SNR.

You also need to consider how wide the binning zone should be. There are algorithms professionals use to optimise this. ISIS uses one for example.

Cheers
Robin

[Robin Leadbeater]

You can also find a calculator to estimate the slit losses here on the CAOS group website
https://spectroscopy.wordpress.com/2009 ... alculator/
(Note sure what calculation Christian's spreadsheets use)

Robin

[Jaroslaw Grzegorzek]

Robin, thank you for these hints.

Jarek