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AR2680, doublestack pst mod: getting closer!

Posted: Fri Sep 15, 2017 9:25 pm
by bart1805
Almost two weeks ago I posted some pictures made with the doublestack version of my Bresser 127 mm. There were extreme reflections and I asked here what I could do to controll thes reflections.
Gus and Randall gave me the advice to use a high transmission circular polarizer. I installed the polarizer between the etalons the way Gus told me to. And waited for a moment to try it out. Today it was finally possible. The view on the screen was heaven compared to earlier tries!
D-erf - Bresser 127 mm - Baader 35 NM - etalon 1 - circular polariser - etalon 2 - Lunt BF1200 - Powermate 2,5x - 2 degrees wedge prism - PGR Grasshopper

The western limb

Image150917_15.11_Br127_DS_PM2,5x_westernlimb_PSV6 by bart moors, on Flickr

AR 2680 in two versions

Image150917_15.20_Br127_DS_PM2,5x_AR_PSV5 by bart moors, on Flickr

Image150917_15.20_Br127_DS_PM2,5x_AR_Coloured_V2 by bart moors, on Flickr

There is still room for improvement, but many thanks to Randal and Gus, the polarizer makes a huge difference! Because of the clouds, and the rain, it was a short session. Now I need a long one and a beautiful blue sky to really tune the etalons the best I can.

Thanks for watching!
Bart.

Re: AR2680, doublestack pst mod: getting closer!

Posted: Fri Sep 15, 2017 11:50 pm
by Ronnie_Clayton
Excellent results!!! Your pictures are amazing!

Re: AR2680, doublestack pst mod: getting closer!

Posted: Sat Sep 16, 2017 12:29 am
by MapleRidge
Hi Bart...

The addition of the polarizer certainly improved the performance of the scope...very nice set of images!!!

Brian

Re: AR2680, doublestack pst mod: getting closer!

Posted: Sat Sep 16, 2017 12:37 am
by Valery
Hi Bart,

Been here, done that. I have experimented with two PST etalons during last two years.

Yes, a circular polarizer is the way to go. The only drawback is the light loss. Without a polarizer my exposures were somewhere 6-8ms (center of the disk to the limb). With a polarizer in place the images were so dim that I were need to triple the exposures. In such a high resolution imaging becomes impossible. Too long exposures - no effective enough seeing freezing, too long movie cadence (to gazer enough frames) leads to a too much details smearing due to their proper motions.
So, I returned back to the pair of etalons where their tilts (to come to the CWL) are different enough and this allows to place the ghost reflections out of the FOV covered by camera chip.
Exposures remains short enough for high res imaging even with the 11" SCT. See the attached image.

Re: AR2680, doublestack pst mod: getting closer!

Posted: Sat Sep 16, 2017 7:32 am
by bart1805
Thanks Brian and Ronnie!
Yes, Valery the longer exposure time is exactly what I was afraid of. But.... The first picture needed 6.9 ms. The one of AR2680 6.4 ms. So I was very happy with that. Thanks to Gus and Randal I bought a high transmission circular polariser, it is the one in the link:
https://www.bhphotovideo.com/c/product/ ... n_htc.html
Because of the clouds that kept rolling in, I made 15 seconds AVI's. And stacked the best 3% for the AR2680 image, the best 5% for the western limb. Off course you are right, there are easier ways to create a more narrow bandpass. And I wish I could choose between etalons. But these are the two I have. Beautiful picture with the C11 by the way.

Re: AR2680, doublestack pst mod: getting closer!

Posted: Sat Sep 16, 2017 6:51 pm
by Montana
These are exceptionally beautiful images, well done :bow :hamster:

Alexandra

Re: AR2680, doublestack pst mod: getting closer!

Posted: Sun Sep 17, 2017 2:34 pm
by bart1805
Thanks Alexandra!

Re: AR2680, doublestack pst mod: getting closer!

Posted: Sun Sep 17, 2017 9:28 pm
by Derek Klepp
Bart that's a great results with the DS.

Re: AR2680, doublestack pst mod: getting closer!

Posted: Mon Sep 18, 2017 3:42 am
by Merlin66
Well done mate...
Looks like you're having some success.

Re: AR2680, doublestack pst mod: getting closer!

Posted: Mon Sep 18, 2017 7:12 am
by GUS
Fantastic images Bart.

Re: AR2680, doublestack pst mod: getting closer!

Posted: Mon Sep 18, 2017 8:44 pm
by bart1805
Thanks!
Gus: the real difference was made by using the circular polariser.
This week the weather seems to improve, so maybe I am able to really compare the SS and the DS. Fingers crossed. (-;

Re: AR2680, doublestack pst mod: getting closer!

Posted: Wed Sep 20, 2017 3:35 pm
by marktownley
Really impressive results! I like the scale and level of detail.