Optimizing the Double Stack
Posted 17 October 2011 - 08:51 PM
Good to see your ideas fleshed out. I'm surprised by the markedly increased transmittance as evidenced by your integration times, I wouldn't have expected this dramatic and increase. I don't believe there should be a issue when using a single etalon visually however.
Posted 18 October 2011 - 01:24 AM
All these little tweaks definitely have a cumulative effect.
New, less flexible etalon hinge plates are also on my 'to do' list. Mine certainly can induce a bit of lateral twist due to their flexibility, which can lead to banding - can be seen on my last full disk image from the 16th. The bottom of the image has subtle banding - this is caused by wind buffeting as I exposed these panes, wasn't (as windy) when I did the flat frame so they weren't removed.
Posted 18 October 2011 - 05:01 AM
Posted 19 October 2011 - 09:43 AM
Sunny skies to all!
Posted 28 February 2012 - 04:57 PM
Do you think it would be worth doing this if one only had a single SM90 filter?
As it happens, I already have a 135mm D-ERF filter. May be a bit big to fit over the SM90.
Posted 29 February 2012 - 01:17 AM
Too big is better than too small ;-) You’ll just have to engineer the adaptor, and again be sure to tilt the ERF to avoid reflections. Your single stacked view may already be bright enough, and the Baader will provide even better transmission. So you likely might have to use additional ND filtering on the eyepiece at lower magnifications.
I’d still go through with it for one very important reason: The D-ERF will be substantially better at reducing IR transmission to the blocking filter. This can lead to thermal loading and cycling of the blocking filter components, and is one of the primary causes of premature failure of the ITF (Induced Transmission Filter) - and is why Baader adopted the D-ERF for use with the Solar Spectrum line of filters. I have had two ITF failures in use with my SM90 filters.
Here’s the transmission curve of the RG red glass ERF used by Coronado:
Here’s the transmission curve of the Baader D-ERF:
As can be seen the Baader is substantially better at blocking IR out to 1400 nm.
The ITF of the blocking filter further blocks the far infrared, making the filter system safe for visual use:
The ITF is composed of a complex layering of “soft coated” silver and dielectric materials, and is usually sealed with an optical epoxy around the perimeter. Thermal cycling can cause microscopic cracks in the sealant and allow atmospheric moisture to invade the layers and “rust” the ITF from the perimeter inwards. See picture below. Again, using the D-ERF will greatly mitigate this thermal loading and cycling.
Interestingly, Lunt appears to have now replaced the “traditional” ITF with a “piece of glass” with simple antireflection coatings (Lunt had some recent isolated AR coating failures on this “glass” which seems to have been easily fixed, and is using a D-ERF like coating on it’s internal optics ahead of the etalon and blocking filter). This piece of glass might be a version of Schott KG heat absorbing glass, which performs very similarly to an ITF in the 1400 - 2500 nm range without the need for complex multiple silver and dielectric coatings, and has very good transmission at the H alpha wavelength:
Combined with a D-ERF for blocking the UV, this glass would therefore be ideal for protecting one's eyesight and the trim filter of the BF. And given the excellent transmission at 395 nm, these types of heat absorbing glasses would appear ideal for use in CaK filters as well to reduce thermal loading and prevent premature filter coating failure for apertures above 100 mm (a future modification I plan on incorporating pending location of optically polished versions of KG glass).
Picture: premature failure of the ITF in a Coronado BF30: