Offset slit calculation tool

[Peter Velez]

I have hesitated to offer this as I am not a programmer and the coding is probably ugly for true developers, but here goes....

I've written a couple of short Python programs that helps use plate solving to get a nominated target on a slit.

The slit in my UVEX is off centre (and I am not alone in this I am certain!) - in my case, the optimum position for my UVEX is to place a target 14 pixels to the left of centre and 16 centres above the centre of my guider image. My usual technique would be to slew to the target and centre it with guider images using one of the usual software-based tools eg Closed Loop Slew in TSX. When centred, I'd then jog my scope till the target sits on the slit, start guiding and then start imaging.

An easier approach is determine the RA and Dec required to get the target onto the slit directly. The Python script determines the offset based on your nominated pixel position. For a given target, it determines the RA and Dec of a point offset from centre in the opposite direction - in my case 14 pixels right and 16 pixels below the centre. I enter the calculated RA and Dec and use plate-solved slewing to centre on the nominated RA and Dec - depending on the accuracy of your plate solving, the target should be sitting comfortably on the slit.

The script takes a plate solved image, determines the position angle and separation of any nominated pixel position from the centre and displays the offset RA and Dec. You can adjust the offset as you see fit. The downside with this technique is that the generated position is only good for one side of the meridian so the program displays the offset RA and Dec for both East and West of the meridian.

There are 2 programs - the first allows the user to specify a target and the program will look up the RA and Dec from public sources. The other requires the user to input the target RA and Dec. I use the latter when looking at transients such as Gaia or AT alerts.

Its very unsophisticated but I'm very happy to share it. Just let me know (sadly, I don't have the technical skills to upload it anywhere for public access - if anyone is able to assist, then I'll do that too!).

Pete

[James Foster]

To: Peter,

Nice python app! I'm just learning the program for a seperate project, flare star monitoring and computing total flux energy when the
star flares up.

I dont have a UVEX (yet!) but when I use my Alpy600 or LISA-IR spectrascopes, I always try to set the continuum right in the center
of my calibration lines; see below:


Now, as we're painfully aware, where you place the star on the slit/guide camera fov may or may-not be where the middle of the calibration lines are.

When I use the Alpy600, my guide star needs to be just right of center of the guider's FOV (on slit) to have the continuum centered. On my LISA-IR it needs to be left of center on-slit in my guide camera fov (specifically at pixel X=115) to be nicely centered on the
Ne-Ar calibration lines.

I learned this lesson (painfully...i.e. wasted resolution) when using the LhiresIII. If my Ne-Ar lines were sharp, but I placed the conitumn on the "fuzzy-part" (non center) of the calibration lines, my resolution dropped from 14K to almost 9K!

James