Here's one for discussion

[Martin Pugh]

Hi.
So the Celestron RASA optical responsiveness has been expanded to include 700-800nm. The Celestron FAQ on the RASA suggests that you would not need an IR cut filter.

Whereas most QHY CMOS colour cameras e.g 183C require you to add a UV/IR cut filter in the optical train otherwise your colour is impacted.

So I presume the right course of action would be to exclude the filter if you want to take advantage of this additional capability.

But of course, monochrome cameras are going to use some sort of filter which typically are UV/IR blocked, unless its a 'clear' filter. So there is not much you can do about that I suspect.

thoughts?
Martin

[Slawomir]

Hi Martin,

I’ve seen a few QE graphs for CMOS and CCD and there was still a non negligible amount of sensitivity past 800nm for I believe most if not all mono sensors. Therefore I suspect an IR filter is a must to minimise bloating. I have little experience with OSC (used only in early days of my astro adventures), but if a manufacturer recommends IR filter then I would use one for the same reason as for mono.

Having a telescope corrected for a wider range of wavelengths IMO should result in sharper DSO images, even in narrowband imaging, since no filter blocks 100% out-of-band light. If RASA brings a wider EM spectrum into a focus, this should be helpful for sure, but I would be using filters as normal, if chasing perfection.

[Atmos]

I am currently using an ASI094 with RH200 and no IR/UV filter. Although I’m not noticing any bloating which is a testament to its colour correction, I do notice some weird coloured stars. Most noticeable, a fair number of purple ones.
I have really thought about adding a UV/IR filter for nothing other than fixing some colour issues that the added signal puts in.

[Slawomir]

That’s awesome - another advantage of using mirrors vs glass. Would be interesting to measure FWHM with and without UV/IR filter.

[multiweb]

I dabbled a bit with IR imaging with a QHY9 mono. I removed the UV/IR cut filter from the imaging train. I used an FSQ106N and previously a 90mm F/6 APO. The FSQ was a lot faster. I got some good results.

[h0ughy]

Martin,
I bought the light pollution filter for the RASA 11 and the RASA 8. Not sure of the light curve but it did make a difference with my asi 071 and the 294

[gregbradley]

I have a clear luminance filter. I used it for a while. Bright stars were a bit larger. Its not always a bad effect.

I did not notice much gain though. Its subtle if its there.

Greg.

[The_bluester]

I found with my SCT that a lum filter was a big help in restraining reflections that were otherwise very intrusive, I keep seeing RASA images with halos that look like reflections of either the corrector plate or the mirror and wonder if a tightish lum filter would help with them.

The reflections were what tipped me in to buying a really nice APO 80mm rather than a RASA 8". I keep finding them as intrusive as I find the lens diffraction issue in the ASI1600 and similar cameras (Which has stopped me buying one of those too)

[gregbradley]

What lens diffraction issue is that? I ask as I may get one of those cameras at some point.

Greg.

[Atmos]

Orion Nebula (https://www.astrobin.com/full/321338/0/), Check out the square reflections around ALL of the brighter stars.

[gregbradley]

OK yeah a bit rough. The Sony A7ii could be made to have that effect with really bright backlit scenes As you say its a reflection off the Bayer Matrix.

I wonder if putting a luminance filter in front if that would help with antireflection coatings and UV/IR block.

Greg.

[Atmos]

There’s nothing you can do with it. It’s due to there not being any AR coatings on the sensor cover slip. It’s not off the bayer matrix, this was taken with a mono camera. It’s not reflections on the filters, it’s off the sensor itself.

[gregbradley]

It looks like the circuitry pattern around the pixel magnified somehow.

I'd be surprised if the sensor cover slip does not have AR coatings. I'd still try that out.

But a good quality luminance filter with good AR coatings in front of the sensor may stop that. What seemed to be happening with Gen 1 filters was reflections bouncing back from the corrector lenses of a lot of modern scopes back to the sensor forming a halo (not a square though).

All gen 2 filters were simply better AR coatings.

AR coatings have advanced a lot in the last several years per Roland Christen from AP.

Greg.

[The_bluester]

I have not so far been able to make out any rhyme or reason behind which images show severe issues and which show virtually none, not by way of what filters are in use anyway. I though the same thing as I had pretty severe reflection issues with my ASI294 until I fitted an UV-IR filter to the train, at which point they vanished.

There is a very long thread on Cloudy Nights about it and the common wisdom seems to be that it is an image of the microlenses.

[Atmos]

It’s a reflection pattern caused by the micro lens and it’s interaction with the non-AR coated cover slip .3mm from the sensor. It’s been very well modelled on CN.

My experience is that it’s a LOT worse on my 8” F/3 than my 130mm F/5 so aperture and faster focal ratios make it worse.
I’ve also used it with a Sigma Art 85mm and Zeiss 135mm APO Sonnar at F/2.8 and not noticed it at all.

Using it on an F/8 RC isn’t likely to show this micro lens issue but a F/4 newtonian will.

[Martin Pugh]

Hi Greg
You cant have a clear luminance filter. Its either clear (UV/IR allowed) or luminance (UV/IR blocked).

[gregbradley]

OK, so its a clear filter then. It has no UV/IR block.

Greg.