ripples in spectrum with CCD chips

[Merlin66]

http://www.aphesa.com/downloads/download2.php?id=1

The above paper (Section 3) may be of some interest.
If the same logic applied to the CMOS it may explain the issues I have with the ASI 174 on the SHG.
Looks like I'm screwed.

[lazjen]

I'll admit I have a very hazy understanding of the problem, but I assume the vertical lines in your image are the meaningful spectrum data, while the horizontal banding/ripples? are not.

Do you think this might be made usable by some form of software processing over the image to extract the data? Statistically process the vertical lines per column for the most common value?

[Merlin66]

Chris,
The thicker dark line across the image is the Ha absorption line. This is where I would extract the imaging data, any bandwidth from 0.2A upwards.
As each frame of the AVI (usually 2000 frames) forms part of the reconstructed image correction is almost impossible. No one as yet has come up with a way of applying flats to an AVI.

For more conventional AP imaging, this result also infers that the sensitivity of the chip will not only vary with the QE curve but also with the added ripple!
(The vertical lines are due to inconsistencies in the entrance slit gap and can be easily removed with synthetic flats applied to the final image)

[Shiraz]

Hi Ken. A few comments that may be of some use - or possibly not.

the ripple shown in the white paper looks ghastly, but is due solely to the use of a single angle of illumination. With real optics and a cone of illumination, there will still be some ripple, but it will be the average of that at many different angles and far less pronounced than that shown in the article. In any event, your optical geometry is fixed, so the effect can be flat-calibrated out in principle. I am not clear on how you process the data, but if nothing moves between subframes, you should be able to apply a flat to the final stack, rather than the individual AVI subframes.

However, I doubt that is what you are seeing. CMOS chips have a split conversion/amplification mechanism that operates both at pixel level and chip level. The pixel-level converters can all have slightly different bias levels, which gives the banding typical of older CMOS chips and similar to what you show. The chip makers are not forthcoming on details, but apparently some modern CMOS chips have a mechanism that adjusts the pixel level biases to remove banding - but that never settles to exactly the same place on each power up, so bias data is not fixed from session to session. Suggest that you try applying a bias subtraction to some data and see if that tidies up the banding. If the 174 is self-calibrating, you may find that the bias data will have to be taken on the same power-up as the lights (actually only a minor inconvenience).

Overall though, because CCD chips have only one amplifier and stable bias, they are probably a little easier to use than CMOS for spectrometry. However, the extremely low read noise of CMOS (which is due to the split amplification) must make it worth persevering with CMOS.

[Merlin66]

Ray,
If I could find a CCD which would give me an AVI output at up to 400 fps. I'd be very interested.

[OICURMT]

Question...

Are you implying that your flats change as the image is captured or do you need to apply a single / master flat?

If you have only one flat, then you could place it as a chroma on a second video track and combine them in a video editing program.

OIC!

[Merlin66]

Hmmm,
Sounds like a possible concept solution....
OK how do you process AVI files to divide one by the other?
I have and use VDub.

[Merlin66]

The issue of spectral resonance ripples has been the subject of lengthy discussions:
https://groups.yahoo.com/neo/groups/astronomical_spectroscopy/conversations/messages/12826

I attach two images extracted from the messages (Robin & Peter) showing some actual results from various cameras.....

[pluto]

I'd be happy to try one for you if you like as I use compositing software at work which could achieve this easily. If you'd like me to have a go I would need a capture video as well as either a flat video or flat image.

If you'd like to try it yourself there is a free compositing package called Fusion by Blackmagic Design, it's similar to the software I use and should be capable of subtracting (or dividing) one video/image from another (but keep in mind it's a big package and there will be a steep learning curve).

[Merlin66]

Hugh,
If I decide to progress this option further I'll contact you.