H-Alpha and IR Cut or 12nm secondary filters

[Starry Jack]

I'm experimenting between an IR CUT (Baader) filter on the nose of my camera and the 12nm I've used for months. Is the 12nm overkill?

I know it cuts my frame rates by half to get the same exposure, but I don't know enough about what I'm looking for to judge between the two.

I know both cut background glow off of the black sky around a prom, but I can't discern a difference on proms or chromosphere that I can appreciate.

Thoughts?

152mm + Aeries DERF + Quark + ASI174.

Thanks,
Jack

[marktownley]

Hi Jack,

Using either filter isn't going to make any difference in terms of the detail you will get with proms etc, they won't narrown the quarks bandpass. What they will do is keep thermal load off the Quarks blocking filter. I don't know any transmission data for the Airies DERF, but it is also letting blue through, so there will be some thermal load, using either or both will reduce this. Try and use them as high up the light cone as you can - put some 2" extension tubes between it and the diagonal to get them higher in the light cone, you will be using more of the filters surface area for energy rejection.

Mark

[Rusted]

Hi Jack,

Using either filter isn't going to make any difference in terms of the detail you will get with proms etc, they won't narrown the quarks bandpass. What they will do is keep thermal load off the Quarks blocking filter. I don't know any transmission data for the Airies DERF, but it is also letting blue through, so there will be some thermal load, using either or both will reduce this. Try and use them as high up the light cone as you can - put some 2" extension tubes between it and the diagonal to get them higher in the light cone, you will be using more of the filters surface area for energy rejection.

Mark

That is a very interesting suggestion.
I have a 150mm & '174 too but no Quark yet. [Quality issues!]
You are right about the considerable heat passing through the D-ERF and it needs serious attention.
I have my Baader 35nm and Beloptik KG3 stacked and screwed into the 2" Baader Clicklock at the backplate.
Only about 50mm before the PST etalon but still providing protection for the following elements.
These two could be moved further up the light cone using an extension inside the OTA.
Spreading the heat load across a greater surface area [without vignetting] should help.
It reduces the localised heat load on the filters themselves.
But demands that the filters are of a very high optical standard.
I'll have to check the light cone diameter before the backplate to avoid vignetting.

[marktownley]

When all the load is concentrated centrally on a 2" filter opposed to moving it higher in the light cone to use as much of the area of filter as possible the chances of wavefront error distortion is increased which then could translate as poorer image quality.

[marktownley]

I'll have to check the light cone diameter before the backplate to avoid vignetting.

It's a fairly simple calculation - free aperture of the filter (mm) x focal ratio = point ahead of focal point (mm) to position the filter.

You wouldn't want to use the 'absolute' free aperture, leave a bit of play to avoid vignetting like you say. 400mm ahead of the focal point in a f10 system is a good start.

[Rusted]

Thanks again.

I didn't mean to hijack the thread but wanted to emphasize the value of your suggestion. These ideas are easily lost in the flow.
Thanks to all the adapters I have collected over the years I was immediately able to achieve 100mm up the light cone from the etalon.
No obvious improvement in the image. No obvious deterioration either. The seeing is the main arbiter of any change in image quality.

P1440027 rsz 500.jpg (106.36 KiB) Viewed 312 times

[Starry Jack]

No harm or hijack to the thread! Fascinating.

My thinking was that I would trust the DERF then trust the Quark but then handle any stray light into the camera so I put on either the UV IR CUT or 12nm on to the nose of the camera.

I certainly get much better contrast than when not using either but I don't see improved contrast between the two at the cost of slower frame rates.

I had no considered putting one on the nose of the Quark.

Jack

[Rusted]

Moving a filter up the light cone pushes it well out of focus.
So dust goes unnoticed as well as spreading the thermal loads on the filter.

[Starry Jack]

Rusted,
Thanks for posting the results of your Test.

[marktownley]

I had no considered putting one on the nose of the Quark.

Don't put it on the Quark nose, at least put it on the nose of the 2" diagonal, preferably well ahead of that with 2" extension tubes.

[Starry Jack]

Thanks, Mark.
I don't have a 2" filter or adapters to use the diagnonal, but I did some tests today with filters on the nosepiece of the camera and on the nose of the Quark. Neither position appeared to make much of a difference.

However, placing the filters on the nose of the camera did increase contrast on the prom shots by reducing the sky haze around the proms become blacker significantly. Using the UV/IR Cut kept frame rates higher than the 12 nm.

Food for thought,
Jack