New photometric slit for LISA

[Terry Bohlsen]

Dear All
I have received my new photometric slit for my LISA and used it last night.
It has created some interesting dilemmas.
The first was the taking of flats. Previously I have used 45 sec exposures with the internal tungsten lamp and processed them in Isis using the create flat process.
Because of the wide section of the slit, this part of the flat was widely over exposed so I had to drastically reduce the exposure time. This means that the part of the flat frame that the narrow slit occupies is somewhat underexposed. I’m not sure how to get around this apart from taking 2 different exposures and deliberately over exposing the wide part. When I did this, the create flat process gave an incorrect result so I used the under exposed version last night. I find that if I don’t touch my setup, I can reuse flats over many nights as they never change unless I remove the slit of reposition the camera- both of which I rarely do as I leave the LISA permanently on my scope.
The second dilemma was the positioning of the star on the slit. I positioned the star near closer to the wide end of the slit. The result was that the sky subtraction zone on the reticule crosses over to the wide slit part of the image. This will then subtract an incorrect sky response from the spectra. I tried omitting one side of the reticule but Lisa comes up with an error if you do this. I decided to make the reticule just 1 pixel wide on the side of the wide slit as a compromise but will position the star further down the slit next time.
I haven’t tried to take an image through the wide slit yet as cloud stopped my imaging.
That will be the next challenge.

[Terry Bohlsen]

Another thought about the photometric slit.
How do you calibrate a spectra taken through the photometric part of the slit?
The neon or "filly" lines are very wide and would not be suitable.

Terry

[Robin Leadbeater]

Hi Terry,

As I understand it, the wide slit is just used to provide the information needed to produce an accurate absolute flux calibration which is applied to a second high resolution accurately wavelength calibrated spectrum taken with the narrow slit.
See method 1 here
http://www.astrosurf.com/buil/calibrati ... ion_en.htm

Cheers
Robin

[Terry Bohlsen]

Hi Terry,

As I understand it, the wide slit is just used to provide the information needed to produce an accurate absolute flux calibration which is applied to a second high resolution accurately wavelength calibrated spectrum taken with the narrow slit.
See method 1 here
http://www.astrosurf.com/buil/calibrati ... ion_en.htm

Cheers
Robin

Yes I understand that but it still needs to be calibrated. I tried last night and it did calibrate with the very wide neon lines so it is OK. The R was about 200 but that is OK.
There were other problems but I will post them separately.
Cheers
Terry