First light with a Star Analyzer 200 - Lots of filters

[robin_astro]

2 hours ago, Ags said:

I found what seems to be a better and more complete source of spectra via the pyckles Python library. 

Here they are in the database that comes with ISIS

[Ags]

I redid the the filters with better graphics and what I have learned so far about using the SA200. There are some glitches in ther calculation of the relative transmission of the filters, I will have another look in the morning.

Delta Cass with no filters, 8 seconds with my FMA135:

Using my Astronomik L3 filter:

My alternative IR block, the ZWO IR cut filter:

L3 vs ZIR:

Wratten 47 with ZWO IR block:

Wratten 47 without IR blocking:

Wratten 8:

IR 850 pass filter:

[robin_astro]

You will need to be a bit careful when measuring the response of the 850nm IR pass filter as there will be some contamination from the 2nd order spectrum in the unfiltered spectrum. (It is quite low with the blazed  SA200 but with a blue light source like Delta Cas some of the light at 425nm will also appear at 850nm)

To avoid this when measuring spectra in the IR, an order filter can be used (for example a filter only passing wavelengths above 500nm will allow measurements from 500-1000nm)

Cheers

Robin

[Ags]

1 hour ago, robin_astro said:

You will need to be a bit careful when measuring the response of the 850nm IR pass filter as there will be some contamination from the 2nd order spectrum in the unfiltered spectrum. (It is quite low with the blazed  SA200 but with a blue light source like Delta Cas some of the light at 425nm will also appear at 850nm)

To avoid this when measuring spectra in the IR, an order filter can be used (for example a filter only passing wavelengths above 500nm will allow measurements from 500-1000nm)

Cheers

Robin

Yes, but the picture tells me enough… the ASI120MM is picking up very little IR past 850 nm, I should get a 720 nm filter. And a more sensitive camera. Although I should try again with a redder star.

I don’t think 2nd order blue near UV is making much difference though. If you look at the combination of the Wratten 47 and a ZWO IR block, the violet light of the filter doesn’t show up lower down the spectrum.

Edited September 14 by Ags

[robin_astro]

1 hour ago, Ags said:

I don’t think 2nd order blue near UV is making much difference though. If you look at the combination of the Wratten 47 and a ZWO IR block, the violet light of the filter doesn’t show up lower down the spectrum.

Yes it is weak but it is there though and the signal in the IR is so low that it becomes a significant proportion of the signal (perhaps ~20% or so roughly from the plot?) when dividing the with filter by the without filter spectra to produce the filter passband curve

[robin_astro]

Leaking of filters in the IR though is an interesting problem. There are  examples of claimed discoveries of Planetary Nebulae in images which turn out to be  cool  bright stars  imaged using narrow band [OIII] filters, surrounded by out of focus leaked IR. Similarly with photometric filters affected by the same problem giving wrong values on cool stars.

Cheers

Robin

[robin_astro]

2 hours ago, Ags said:

the ASI120MM is picking up very little IR past 850 nm

It is not just the camera though. The grating response (and the lack of IR in the target star) is equally significant. The grating is blazed to give maximum efficiency at ~500nm and tails off either side of this.  Gratings designed for use in the IR have a higher blaze angle

Edited September 14 by robin_astro
clarification

[Ags]

Yes I must repeat the experiment with a redder star for the Wratten 47 and IR850 filters. Sticking to Cassiopeia, I think Schedar might be a reasonably bright candidate being K0-IIIa.

I couldn't figure out why the relative intensity I recorded for each filter didn't align with each other. I debugged my code to death and rewrote several sections and the data was still not correct. I only finally realized yesterday that when I crop the FITS files in GIMP, it automatically stretches the data when the FITS file is loaded. Fortunately I  created a FITS viewer that can convert the FITS files to another format that FITS doesn't auto stretch, so I finally managed to get this right (maybe). 

Unfiltered (8 seconds of Delta Cass:

Astronomik L3:

ZWO IR block:

ZWO IR block (grey) vs L3 (red):

Wratten 8:

Wratten 47:

Wratten 47 with IR block:

ZWO IR 850 pass filter: