Photometric Color Calibration: CLS-CCD Filter and Saturatiom

[monkeybird747]

In reading release notes from Juan I noticed this statement:

“We cannot expect any robust color representation when using narrowband filters, or filters located in the UV or IR wavelength ranges. ”

Would this apply to using a clip in CLS-CCD light pollution filter? I believe it has a uv filter component.

Someone else asked if adjusting saturation evenly across all channels effectively negates the PCC process, but that thread is a year old and unanswered.

Finally, is SCNR necessary if you use PCC? I’m currently applying it prior to PCC.

Thanks,

MB747

[pfile]

i think by that sentence juan meant IR and UV *pass* filters - CLS-CCD has an IR cut component... not sure about UV though.

rob

[monkeybird747]

I’m not sure where I got the UV part. It’s not mentioned specifically on the Astronomik website. For some reason I thought this was part of the filter, and needed for a fully modified DSLR. What you say about the statement referring to pass filters makes sense though.

[pfile]

right - full DSLR mods involve removing the IR filter completely, so you need an IR cut filter somewhere (which doesn't block Ha)... not sure how sensitive DSLR sensors are to UV but obviously some sensors are since people use UV cut filters to try to prevent 'blue bloat'.

rob

[monkeybird747]

Thanks Rob. Incidentally, in reading the stats on this filter it also boasts "near perfect color balance" for modified DSLR. For me there is still a significant blue-green color cast using this filter. Easily fixed during PCC, but still I'm not sure what their idea of near-perfect balance is, or if its my understanding of what they mean that is flawed.

Here is a link to my latest M31 image using PCC. There were no color tweaks other than increased saturation on total image. I was hoping to get close to what Juan refers to as "documentary" color.

https://astrob.in/366999/0/

[pfile]

i think the blue-green cast is inevitable as the filter cuts a lot of red wavelengths. i used to use one of these filters (when i used a DSLR) and concluded they cut out too much light to get proper images of broad-spectrum objects like galaxies. the CLS filter works great on emission nebulae though - in the end it's bandpass is something like the combination of a very wide Ha filter and a very wide OIII filter.

so i wonder if it's even possible to color balance a CLS image because of all the missing light.

one thing i used to do to make sure that my flats were well exposed in all 3 channels was to do t-shirt flats with a pinkish T-shirt. it's not really important to make your flats grey, but because of the blue/green channels saturating so fast, the red channel can be weak and thus have lower SNR.

i've heard that the IDAS filter is a little more forgiving on galaxies.

at any rate your m31 looks good.

rob

[monkeybird747]

Thanks Rob. On some other images I get the comment that my stars are lacking color. I notice this on the raw subs as well. I wonder if this could be a side effect of the LP filter. It’s been a while since I imaged without it, so maybe a test is in order. I’m nearing the end of my DSLR career and begun researching mono astrocams.