Observing the Sun with complex optical systems since 1966, and still haven't burned, melted or damaged anything. Not blind yet, either!
Quote: ...we can now see that Secchi’s needles (the “spicules”) are arrayed in long picket fences... Spicules lie at the borders of supergranule cells...
Zirker, J.B. (2002) Journey from the Center of the Sun (p. 142). New Jersey, Princeton University Press.
Quote:In a phone discussion with one of the GONG design staff earlier this year the subject came up, relating to why imaging disk and proms was difficult with a single stack system. The NSO staffer (who's been doing this since before most of us were born) said that in single etalon systems the sideband leakage would present exactly the situation observed...
Bob diagonally parked in a parallel universe...
Quote:Observing the Sun with complex optical systems since 1966, and still haven't burned, melted or damaged anything.Not blind yet, either!
David W. Knisely . . . . . . "If you aren't having fun in this hobby, you aren't doing it right." Hyde Memorial Observatory http://www.hydeobservatory.info Prairie Astronomy Club http://www.prairieastronomyclub.org
Quote:George, your plots are correct, if you're still wondering. I've been lazy in using Gaussians to show the effect of stacking filters. I had a Gaussian spreadsheet already built and didn't want to take the time to do it right. I spent some time today to make a Lorentzian version and it agrees with yours.
Quote:I made it so I could leave one ERF off, and that brightened the double stack.
Lunt LS80t ,LS60f, LS60t, LS35 + PST
ES 102 APO ,Lunt Herschel Wedge
Celestron cpc 800
CG-5,Vixen porta, Twilight II
PGR Chameleon mono
Quote:The thing is, when I incorrectly did the Gaussian the day before, I discovered that as you shift, it becomes dimmer but not wider.
Quote:I’m not sure I see much benefit to triple stacking compared to double stacking
Quote:Do you have any idea(s) as to why tilting a pair of DS etalons off-band would produce a dimmer image verses a brighter image? Going off-band would seem to allow more continuum in, and tilting an etalon should widen the bandpass (at least if it’s a Lorentzian), which would seem to be a double-whammy for giving you a brighter disk.
Quote:Banding and sweetspotting issues become an absolute pain and is tricky to get everything just right...
Quote:My assumption was that tilting the pair inadvertently shifted them with respect to each other. E.g., if you tilt the front one to avoid the ghost (and you optimized the tilt for the best view), then tilting the pair could either untilt the front one with respect to the sun or further tilt it depending on the direction.... but I would just experiment with single stack tilting to note the behavior, and then add the second element to the already tilted first element, optimizing brightness. In other words, I suspect you had to do some fiddling to get your on-band double stack optimized, so to be fair you need to fiddle to optimize the off-band double stack.George
Lunt LS60TS50DS/B1200/FT SolarMax 60mm filter Denk.Bino's with 21's/TV Genesis Lunt Herschel Wedge/ES 127ED Orion ED80T- CF/C4-R/CG-5 C6-R/Moonlite CF2 /Atlas GoTo 12" LB/Roundtable/Discovery 10"
Quote:With the recurring question of how to set filters (single, double, or any multiple) onto H alpha, I have a suggestion. Get a source of hydrogen emission that you have control of... With the spectral source there is no question where you are tuned...
Quote:Tilt tuned systems may have trouble finding a tilt value that doesn't give annoying reflections, pressure tuning is better for stacked systems for that reason. But even pressure tuned systems will show a wavelength variation across the field, such are the woes of filter systems.
Quote:(or evidently even three triple stacked front etalons as shown by Mark's SM40's)
SLAP Observer --- TMB130SS, SV102V(LOMO Lens), SV80ED Deluxe "The only thing necessary for the triumph of evil is for good men to do nothing." -- Edmund Burke. "People sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf." -- George Orwell "The measure of a man’s greatness is not determined by what he accomplishes for himself, but by what he accomplishes for others.” -- Some Bald Guy
Quote:Using two external (front) etalons seems to minimize the magnitude of both sweet spots and banding, as there are no instrument angles, and the field angles are just those of the sun itself ~ 0.25 degree. Of course it's the most expensive way to go, and the aperture of commercial etalons is currently limited to 100 mm. Tilting the etalons (front to remove ghosts, objective to tune), can produce some banding, which is a more subtle gradient than is seen with internal etalon based systems. However, when properly adjusted, double stacking two front etalons (or evidently even three triple stacked front etalons as shown by Mark's SM40's) can give good contrast uniformity with little evidence of banding or a sweet spot.
Quote:The DS internal pressure-tuned etalons also achieve large sweet spots (by large I mean good; i.e., bigger than the sun) and no banding.
Quote:Perhaps the Denkmeier OCS (more or less a Barlow that goes in front of the blocking filter so you can achieve focus) changed the light cone so that the blocking filter induced banding. Tuning the blocking filter did move the band up and down (the blocking filters used to be tilt tuned).
Quote:"The DS internal pressure-tuned etalons also achieve large sweet spots (by large I mean good; i.e., bigger than the sun) and no banding."
Is this a result of the etalons being 'oversized' to what you might expect?
Quote:I'm trying to figure out what is causing the banding while using the binoviewer. Nothing past the etalon will affect its bandpass. Is it actually a band or a central hot spot? If it a spot I suspect it's a relay imaging issue with the design of the binoviewer.
Quote:The [ASP-60] etalon is in the front element. The diagonal (PROM-15T) contains the blocking/prominence filter. This does not 'tune' the whole filter. The narrow element (etalon) has a temperature coefficient of passband shift of <1 Angstrom/2000 degrees C. However, the blocking filter is somewhat temperature sensitive and needs compensating, - by tilt, - for large differences in ambient temperature. This filter isolates the H-alpha passband in the etalon and eliminates all the others. If it moves with temperature, without being compensated, it would allow one of the other etalon orders to come through and, thereby, lower the contrast.
[Old Coronado blog - emphasis added]
Quote:I’m in no way an optics expert, but from what I have been able to learn here’s how etalon size relates to sweet spots:Typically the sweet spot with a collimated etalon is caused by a magnification of the field angles of an extended object. If the field angle exceeds the acceptance angle of the etalon (i.e the angle needed to meet the filter's band pass specification), that portion of the image (in this case the limb of the sun) will begin to fall off-band. A decease in contrast (a loss of chromosphere detail) or prominence visibility will be noted outside the sweet spot.For the sun’s limb, the field angle starts out being ~ 0.25 degree (i.e 1/2 the sun's angular diameter, or when optically centered the center-to-limb angle). For an etalon mounted on front of the objective, this is the field angle, and amounts to an f 108 optical cone convergence. Generally there is a very wide sweet spot at this angle and configuration.In a collimator based internal etalon system, the field angle magnification is the ratio of the objective to collimator lens focal lengths. Because of the optical geometry, this is also roughly proportional the ratio of objective diameter to the etalon (working) diameter. Therefore, the larger the internal etalon, the less the magnification of the field angles, and the better the on-band performance.For example, the Lunt LS80 uses an internal 50 mm etalon, and dividing 80 by 50 = 1.6. Hence the internal field angle of the suns limb can be generally determined to be 0.25 x 1.6 = 0.4 degree. For the LS100, which uses the same 50 mm etalon, the collimated field angle is 0.25 x (100/50) = 0.5 degree, and so on. So we can see that the smaller the etalon is compared to the objective (i.e. the collimator lens FL and geometry) the greater the field angle magnification, hence the smaller the sweet spot of good on-band performance.Of note for narrower bandwidth filters is that they also have a narrower acceptance angle to remain on-band. This perhaps can explain why a single stacked system with no evidence of a sweet spot may develop a sweet spot (and possibly banding from tilting the second etalon) when double stacked, due to the constraints of the system to have a narrower acceptance angle for proper on-band performance.Again I’m not an expert, so if I’ve erred, I hope someone will correct me.
Quote:Coronado’s first solar filters basically took a prominence filter, which was an ERF plus a PROM15T (prominence, 15mm, maybe T for tilting?) and inserted an etalon behind the ERF. The prominence filter by itself showed no solar disk details... Over time I guess they switched to a blocking filter designed for the etalon. I don’t know, but I suspect that the new blocking filters were wider: still narrow enough to avoid other etalon orders but wide enough to avoid having to tune it with temperature variation (and perhaps cheaper to produce).
Quote: I would definitely want to "Try Before I Buy" due to the cost of a DS. It would have to be a huge improvement for me to spend that kind of $$$$.
Quote:“once you’ve double stacked, you won’t go back.”
Others may disagree. Your millage may vary. Void where prohibited by law.