March 12, 2023 102mm double and triple stack comparison with bonus full disk

[Oak]

Hi All,

It's been months since I had any time to image, but I finally got a chance yesterday. I tested for the first time a triple stack configuration of quark+two Lunt40 etalons vs. quark with only a single 40mm etalon in the telecentric output.

A prior attempt showed that there were serious reflections between the two Lunt etalons. This time, I added a circular polarizer between the Lunt etalons and the reflections were vanquished. Unfortunately, transmission was cut way down so I essentially couldn't use lucky imaging to break through the seeing (from 6ms exposure with 249 gain to 13ms with 350 gain, basically getting into grainy images). I had to eyeball the exposures as the histogram wouldn't load for some reason.

There was some increase in contrast for the triple stack but too great a loss in transmission to be worth it to me in this configuration. Visual impression was similar.

Below are the results:

Double stack (6ms 263 gain (65%) 158fps)



Triple stack (12.9ms, 347 gain (86%) 77fps)




Double stack (6ms 263 gain (65%) 158fps)




Triple stack (12.9ms, 347 gain (86%) 77fps)




The filter stack. Very important to note here that both Lunt 40mm etalons are in the telecentric output of the quark, which is being used as specified by the manufacturer. The whole imaging train is sun→AT102ED→Baader 35nm H-alpha→ Optolong KG3 UVIR →Quark Chromosphere (with built in 4.2x telecentric --> Lunt etalon 1→Hoya circular polarizer HRT 52mm→ Lunt etalon 2 → tilt adapter →>AS174MM. Lunt etalons are housed in the Raf camera adapters.



Bonus full disk - Lunt 40mm double stack with 174MM, very under sampled


I will keep fiddling with the setup to see if I can improve results.

Best,
Derek

[marktownley]

Ingteresting results Derek, I think i'd probably stick with double stack in this instance.

Try and see which of the lunt etalons gives the best result (singly) with the Quark, one will be better than the other.

[Dennis]

Very nice comparison, thx. So doublestack seems to be the sweetspot.

[ffellah]

Excellent ! The last one of the closeups in my opinion looks best. Pretty amazing contrast and clarity: maybe it is also the processing.

Franco

[Averton]

Very interesting experiment Derek.
We would agree that over 300 gain for the 174MM is bad territory. Even with our DS similar arrangement we are scraping on occasion to get enough photons to the sensor. It becomes a compromise of either too much gain or too long an exposure, neither giving us a good outcome.

[Oak]

Thanks all,
Yes double stack seems to be the sweet spot for now. I want to try a focal reducer and perhaps a different orientation of the polarizer (although I'm pretty sure I got it right this time given the polarized output of the quark)

[marktownley]

I'm not sure there will be much to gain even if you could get the exposure shorter (through focal reducer or changing orientation of polariser) - there's little difference in the bandwidth tightening between double and triple stacking in this instance, it's just etalon synergy. I would try clocking the 2 lunt etalons relative to each other first to see if that improves things in this case.

[Carbon60]

Nice experiment, Derek. I’ve triple stacked my Lunt 60, but it’s really tricky getting all three etalons to work together to effectively reduce the bandpass. Given there’s no discernible difference between your DS and TS images other than noise, I would suggest the tuning is out of step between the three units. TS is really only working as a DS in other words.

I guess the question is, does the potential gain outweigh the loss resulting from image noise and slower exposures? For FDs I’ve found a significant improvement with triple stacking my particular etalons, but on a Lunt 60 with a 2.5x PM for FD mosaics the increased exposure time makes no significant difference to the resulting image.

Stu.

[Oak]

Good points. I will work on tuning and rotation first. I did some rotation in my previous attempt and didn't see a difference.

[arnedanielsen]

Interesting experiment and very nice results!

Best regards,
Arne