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White Light Filter & Ha filter at the same time?!

Started by StarTraffic, 05/28/2015 12:03PM
Posted 05/28/2015 12:03PM Opening Post
Ok, I have a DA question for the group.

My wife's work has purchased 2 8" Dobs (XT8's) for outreach & they are in the process of getting full aperature white light filters for them so they can be used for solar outreach as well. They plan on a PST next fiscal year.

I came up with a screwball idea of using a 1 1/4" Ha filter in the focuser AND the white light filter at the same time. What would this get in terms of a spectrum shift/filtering? Would this be Ha scope on the cheap? I'm trying to get them to the point of a true Ha scope but with an 8" reflector it isn't intuitive or easy. My thinking is that the WL filter will reduce the total light energy enough that it won't instantaneously blind someone, and the Ha filter would do a bandpass to see the Ha.


Clear Dark Skies
39.138274 -77.168898
Posted 05/28/2015 07:00PM #1
Image brightness would be extremely dim and most likely not narrow enough bandpass to allow prominence views. That requires bandpass of .75 A or less. The typical Ha filter is a hundred A more or less.

Posted 06/10/2015 05:44AM #2
Full ap filters on the larger scopes (even just for white light) suffer poor wavefront. Also, such filter will squeltch the H-alpha so you wouldn't see anything at all with the combo. H-alpha generally requires very slow feed at the filter itself - Daystar recommends f/30 or slower. This is why the red off-axis ERF filter up front is used on SCTs - it both evades the secondary obstruction AND slows the feed to f/30 or slower. The dedicated solar scopes with internal etalons like LUNT, even though the scope itself is ~f/7, uses telecentric optics downstream so the light passing thru the etalon is slower (much slower) than f/30. Front-mounter etalons are operating at object-space equivalent of f/120 (├ętendue of the Sun). Short answer though is to stick with the standard COMBINATION of filters for line spectra (H-alpha, sodium-d, CaK etc.) Otherwise it could risk damaging your eyes! For white light the dedicated "visual" Solar ND up front is safe as is the Herschel Wedge + polarizer (like Lunt makes) when coupled to recommended refractor f/5 feed or slower. I have found that the LUNT wedge has MUCH better wavefront than up-front ND. Always use the filter COMBINATION recommended else risk eye damage. Final thought: I found that amending a green GLASS (Wratten 56?) filter + ND "moon filter" to the Herschel Wedge gives the BEST imagery when using an APO (Genesis) refractor. I think that is because the naked CCD sees IR and the apo is designed for visual spectrum. The green glass both rejects the IR AND is quasimonochromatic so takes stress of the performance of the chromatic correction of the lens. The IR manifests SEVERE chromatic and spherochromatism if allowed to get to the CCD. Sorry for the rambilng! Tom Dey

29-inch Dob in a dome
36-inch upgrade soon
LUNT 80/80 solar scope
FLI 6803 cam
APM 100mm APO Binos
JMI RB-16 Night Vision Binos
Zeiss 20x60 IS binos