The Solar limb on the morning of Feb 25th, taken from Sydney around 9:00am

Skynyx 2-2 Coronado 90 H-alpha filter.

Review: Coronado Double-Stack 90mm H-Alpha Filter.

I have  been a user of the original (pre-Meade) Coronado filters for some time, and frankly was very happy with the 90mm system.  That all changed when I went to the PATS Astronomy exhibition in late 2010, and happen upon Andy Lunt (yes, of Lunt solar) with one of his new 65mm 0.5A pressure tuned systems.

The solar view it provided was very impressive to say the least. Filaments that were hinted at in 0.7A systems, had significantly more contrast when seen through a  0.5A filter. The thought of a 100mm Lunt system had crossed my mind, but at $A10,000 or more, seemed a little extravagant.

Time to upgrade my 90mm!

This basically meant purchasing a second 90mm etalon to place ahead of my existing filter. Once upon a time this meant the system had to be returned to matched with a second etalon at the factory, but the Coronado II Meade now produce has a frequency shifter hence, no returns required.  I also had several Coronado adapters (to couple the filter to 155mm, 130mm and 106mm aperture telescopes ) which if re-made would collective add up to quite a few hundred $, hence staying with Coronado made even more sense.

I placed the order in October 2010. The etalon finally arrived in February 2011. Frankly not bad, as I have known H-alpha filters orders to take up to a year to arrive.  The filter arrived double boxed, with an impact (often fatal to etalons) absorbing foam cell nursing the instrument case of the filter.  For around $US2500 you get a case, etalon, metal caps, instructions , boxes and foam beads.

The instructions were, well, useless.  I suspect they were originally written in Spanglish. In any event, didn't describe how to set up a dual etalon system at all. I asked myself: how hard can this be?

As the threads of the old etalon could only be fitted to the front of the new one, the assembly order was pretty obvious. You also need to make sure the tilt stages of both etalons are fitted to both tune in the system and shift unwanted internal reflections. It helps to have either a short focus refractor, or arms as long as a Gorilla's when using a longer focal length scope, as you really need to be able to look through the telescope while adjusting these filters.

The sequence seems to be:
1) fit etalon 1 to the objective end of the telescope
2) use the tilt stage to get it "on band"
3) fit etalon 2 (while not twisting etalon 1) use its tilt stage to space secondary/internal reflections away from the primary image. 
4) use the doppler tuning ring to enhance the solar features you are interested in. 

The view?  Simply excellent!

The solar disk is very evenly illuminated with no hot or sweet spots.  Even a featurless-sun has features! Contrast is superb, and the system can be easily tuned to zero in on doppler shifted filaments, prominences and the like.

With 90mm of aperture you can magnify the image quite bit with no significant loss of light. Also at 90mm you are pushing the daytime resolution "seeing" limit of the atmosphere more often than not. Hence while going for more aperture can be seductive, it is more about light than resolution at this stage. The off axis reflection images of the sun  are somewhat annoying, but I suspect the physics of etalons is such that this is the nature of the beast.  Also the finish is not quite as pretty as the original (polished plus anodized) Coronado, but still to a high standard. Without doubt the Meade/Coronado 90mm double-stack sets a gold standard in H-alpha solar filters.

Copyright Peter J Ward 2011