Telescope Reviews: Lunt double stack

Note for newcomers: The free spectral range (FSR) is the distance between the primary etalon target wavelength peak (Ha @ 6562.8 Angstroms) and the side band (harmonic) peaks passed by the etalon.

For single stacking, I believe the original Coronado and current SolarScope filters have a FSR of 10 Angstroms, and the Lunt filters have and FSR of around 15 Angstroms. Thus using a Lunt blocking filter with a Coronado or SolarScope etalon would be inadvisable, due to the potential side band (continuum peaks) leakage of a wider bandpass filter of the BF with the narrower FSR etalon. The bandpass filter also has the potential of significant drifting with temperature changes, which could make the situation worse. This therefore could result in much decreased contrast and detail.

Using a Coronado or SolarScope blocking filter system with a single Lunt etalon would seem to be acceptable, as it likely has a narrower bandpass filter than the Lunt blocking filter, and thereby would be even less likely to pass the side band peaks.

For double stacking with say a Lunt and Coronado etalon with differing FSR’s, using either company's BF should work OK, as the bandpass filter portion of a blocking filter would seem to be rendered almost unnecessary. As Joe states the different etalon FSR’s should block, at least theoretically near the target wavelength, the offending side bands from either etalon.

Yes, but if the Coronado SMII etalon design changes have changed the FSR to that of or near the Lunt FSR, this would not apply. In any case it would be necessary to see if far off-band peaks line up.

Regardless of which blocking filter is used, the IR blocker (the Induced Transmission Filter) in the blocking filter would likely still be required for safety, as far off-band IR peaks for both etalons may line up and allow harmful IR radiation to possibly leak through.

Using a Lunt verses a Coronado blocking filter in place of a "rusted" Coronado BF: Unfortunately, the ITF seems to be the component of the blocking filter most likely to fail over time with the Coronado blocking filters. This is a relatively inexpensive component of the blocking filter*. If the bandpass filter of the blocking filter fails (the much more expensive part) it might be able to be dispensed with in a double stacked system with etalons of differing FSR’s - but you would still need the IR blocking component.

* I seem to recall reading that Coronado Meade is no longer replacing ITF IR blockers when they fail, and now require the replacement of the entire blocking filter assembly if outside the warranty period.

Single stack configurations:

Wide FSR etalon + wide bandpass blocking filter = OK

Wide FSR etalon + narrow bandpass blocking filter = OK

Narrow FSR etalon + wide bandpass blocking filter = Not OK

Double stack configurations:

Wide FSR etalon + wide FSR etalon + wide bandpass blocking filter = OK

Wide FSR etalon + wide FSR etalon + narrow bandpass blocking filter = OK

Wide FSR etalon + narrow FSR etalon + wide bandpass blocking filter = OK

Wide FSR etalon + narrow FSR etalon + narrow bandpass blocking filter = OK

Narrow FSR etalon + narrow FSR etalon + narrow bandpass blocking filter = OK

Narrow FSR etalon + narrow FSR etalon + wide bandpass blocking filter = Not OK