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Doublestack H-Alpha, next try. Questions about polarising filters.

Posted: Mon Sep 04, 2017 7:11 pm
by bart1805
Today part 3 of the doublestack experience. Base is a Bresser 127, two PST etalons and a Powermate 2,5x
First a cropped image of saturday:

Image020917_13.26_Br127_DS_PM2,5x_AR2674_CropPS by bart moors, on Flickr

And today.

First one shows severe banding / reflections, did not try to remove them in postprocessing.

Image040917_10.21_BR127_DS_PM2,5x_AR2673 by bart moors, on Flickr

Second one from the eastern limb shows the same. You can also see that the double limb is not there on the upper side, but the more you go down the more pronounced it is. So something is still not right.

Image040917_10.50_BR127_DS_PM2,5x_Oost_Inverted by bart moors, on Flickr

I tried tilting the D-Erf, did not notice any difference. With or without the 35 NM filter: no difference. So more an more tilt of the rear etalon vs the front one. And in the end I managed to get this one. Full size of the sensor of the Grasshopper IMX174.

Image040917_10.45_BR127_DS_PM2,5x_AR2673-2674 by bart moors, on Flickr

So there is still a long way to go, but I won't give up, yet. Next try will be with a polarising filter. I could install it between the etalons or on the nosepiece of the Grasshopper. Any thoughts about that?
And what would be better? A linear or a circular one? My guess is a circular one (that is the one I don't have) , but I am not sure. And are there circular polarising filter with a high transmission? Already lost a lot of light with the extra etalon, so hope I don't have to really double the exposure time.
Thanks for watching and your advice!
Bart.

Re: Doublestack H-Alpha, next try. Questions about polarising filters.

Posted: Mon Sep 04, 2017 11:30 pm
by Merlin66
Bart,
You're breaking new ground...
I haven't heard about tilting between the etalons, are they "sagging" in the front adaptor? Was the tilt you applied in the direction of possible sag?

Re polarising filters - I've only ever used one of the elements of a CP set up and rotated that in the nose piece of an eyepiece to suppress a rogue reflection.....minimal light loss as you're not using crossed polaroids.....

Re: Doublestack H-Alpha, next try. Questions about polarising filters.

Posted: Mon Sep 04, 2017 11:44 pm
by bart1805
Sorry Ken, English is not my native language. What I meant was rotating the back etalon. So the front one is stuck with the 2inch nosepiece in the focusser of the scope. But I can still rotate the other etalon to try and steer the reflections out of the FOV. Not with much succes by the way. Bart.

Re: Doublestack H-Alpha, next try. Questions about polarising filters.

Posted: Mon Sep 04, 2017 11:51 pm
by Merlin66
Ahhh, that makes more sense......
Ik woonde al een paar jaar in Belgiƫ maar kwam altijd in de Franse "sector" voor winkelen!

Re: Doublestack H-Alpha, next try. Questions about polarising filters.

Posted: Mon Sep 04, 2017 11:58 pm
by bart1805
Hahaha!
In het Franse deel is het eten beter!

Re: Doublestack H-Alpha, next try. Questions about polarising filters.

Posted: Tue Sep 05, 2017 12:34 am
by Merlin66
Ja, de moule en frites waren geweldig!
They don't taste the same back here in Australia.....

Re: Doublestack H-Alpha, next try. Questions about polarising filters.

Posted: Tue Sep 05, 2017 7:01 am
by rbenton
If you are looking for a high transmission Polarizing filter, B+W makes a very good one that you can see here:
https://www.schneideroptics.com/ecommer ... &IID=10366

I used one on a double stacked Lunt LS100 to get rid of ghost images. Worked beautifully for that, with minimal light loss.

Randall Benton
www.randallbenton.com

Re: Doublestack H-Alpha, next try. Questions about polarising filters.

Posted: Tue Sep 05, 2017 9:03 am
by GUS
Spectacular captures Bart. I also use a CPL on my LS100, eliminates the ghosting completely. I've tried the Hoya CPL, and at the moment use a Marumi branded CPL, and visually can view at 200x without image breakdown in good seeing conditions.

Re: Doublestack H-Alpha, next try. Questions about polarising filters.

Posted: Tue Sep 05, 2017 11:55 am
by bart1805
Hi Gus and Randall,
Thanks! Do you place the CP between the etalons or at the back of the train?
CS, Bart.

Re: Doublestack H-Alpha, next try. Questions about polarising filters.

Posted: Tue Sep 05, 2017 12:48 pm
by GUS
Bart, the CP is placed between the two etalons, and is installed(in my setup) backwards from how you would set it up for a camera lens, as that gave the best ghost suppression.

Re: Doublestack H-Alpha, next try. Questions about polarising filters.

Posted: Tue Sep 05, 2017 6:49 pm
by bart1805
Thanks again Gus. I would have started the other way around with the CP. Bart.

Re: Doublestack H-Alpha, next try. Questions about polarising filters.

Posted: Tue Sep 05, 2017 9:37 pm
by GUS
If you haven't seen this already, these threads might be of interest.
[url]viewtopic.php?f=10&t=17982
[url]viewtopic.php?f=10&t=18020

Re: Doublestack H-Alpha, next try. Questions about polarising filters.

Posted: Tue Sep 05, 2017 9:58 pm
by bart1805
Gus, I found a second hand B+W High Transmission CP which I should be able to mount with a bit of tilt in the adapter between the two PST etalons. Really curious what will happen. I'll keep you posted.