Batch Processing and Rotator Angle

[Luigi]

If I image a target on both sides of the meridian can the batch preprocessing script take advantage of the rotator angle when selecting which flat to use?

For example if I have my rotator set at PA60, I will have flats at that angle plus rotated 180 degrees (I'm using a GEM).

[Geoff]

If I have understood your question correctly you are not rotating the camera relative to the OTA. It's just the whole assembly rotating due to the meridian flip. In that case it's irrelevant which side of the meridian you are on. Flats only measure inconsistencies in the optical train and this is independant of where the telescope is pointing. Many people do them with a light box and the telescope can be pointed anywhere--even taken off the mount as long as you don't disturb the optical train.
Geoff

[Luigi]

I shoot a set of images on the east side of the meridian, then do a meridian flip and rotate the camera 180 degrees.

So I have flats with the camera in both positions.

My software (CCDAutopliot) apparently writes the rotator angle to the FITS.

I was wondering then, if the batch script would be 'smart' enough to calibrate the light frames with the flats taken at the same rotator angle.

[Geoff]

OK, I misunderstood what you were doing.  I would simply load all the flats into the BPP script and see how it sorts the flats. If it sorts by rotator angle as well as bin mode and filter, then you are good to go.  If it doesn't show a distinction between different rotator angles at the outset then my guess would be that it won't take it into account during processing.
Geoff

[Luigi]

I just read through the source code. It does not have support for rotator angle.

[Geoff]

Presumably this would not be too difficult to add.
Geoff

[Juan Conejero]

If your data set includes different rotation angles, then you have two possibilities:

(a) Calibrate your frames in separate batches, one per rotation angle. Note that your master bias and master dark frames will be generated in the first batch, if necessary, and you can reuse them in the following ones. Only the flat and light frames are rotation-dependent. Use the same registration reference image for all batches.

(b) Use pseudo-filters as follows:

1. Click the Add Custom button.

2. On the Add Custom Frames dialog, select your light frames for a particular rotation angle. Select: Image type = Light Frame, Filter name = 180DEG (for example), binning and exposure time as appropriate. Click the OK button.

3. Repeat step 2 for your flat frames with the same rotation.

4. Repeat steps 1,2,3 for as many different rotation angles as you have.

Now your light and flat frames will be grouped by the pseudo-filters specified in step 2, and the script will generate separate master flats, which will be applied to the corresponding lights.

Irrespective of the method you use, you should integrate all the registered frames *after* running the BPP script. Remember that the integrated results (the master lights) generated by BPP are only useful for fast previewing purposes; image integration must always be done manually with the ImageIntegration tool.

My software (CCDAutopliot) apparently writes the rotator angle to the FITS.

There are no standard FITS keywords for this purpose. The solutions given above work without problems if you have your files well organized. However, a future version of BPP will allow you to enter custom keywords to classify frames, which will simplify even more the process in these cases. We have other priorities right now so don't hold your breath.

[Juan Conejero]

If I image a target on both sides of the meridian

If the only reason to rotate your camera is a meridian flip, then you shouldn't do that if you preprocess your data with PixInsight. Our StarAlignment tool corrects for meridian flip automatically.