Solar Hydrogen alpha detail is addictive, but the filters/telescopes are expensive. At $1000 you're usually at the "entry level." Unfortunately, even with the most expensive solar filter systems and telescopes, consumer-grade systems can have a great deal of variation in quality between manufacturers, and even within a particular manufacturers’ product line. I would always advise getting filters and telescopes with full money-back warranties, and if buying used get an ability to return the filter or telescope system if there is any question about quality or function.
Here's my tutorial for those just starting out into the world of solar H alpha. H alpha filter systems generally can be classified into three categories:
1. Front (objective mounted) H alpha etalons. These can be used with stand-alone telescopes, and are generally the most expensive filter systems in larger apertures. Front etalons generally offer the most uniform image contrast for low power full-disc views, and especially imaging. But some manufactures can have issues with quality in which this may not be the case any longer.
Double stacking two filters can be used to improve contrast (the second etalon usually requires some additional tilt to remove "ghost" reflections). These filters are made by Coronado (Meade), Lunt, and SolarScope. SolarScope also sells dedicated solar Ha telescopes with front etalon(s).
2. Internal etalons. Cost concerns resulted in placing a relatively smaller air-spaced etalon in collimating lens optical system within a larger aperture telescope, and these can perform almost as well as front mounted etalons as far as image contrast uniformity. Some models are more subject to circular “sweet spots” with good contrast, while the contrast falls off outside the sweet spot. Some models in the past which use tilting to tune the etalon could compound the sweet spot with “banding.” These issues become moot with air pressure tuning, and higher power views and narrow field imaging – as the whole view or image is generally within the sweet spot.
With the Lunt pressure tuning systems (which eliminates tilting for tuning), banding issues have been eliminated. Pressure tuning is the equivalent of changing the temperature in the DayStar and Solar Spectrum filters. Coronado introduced “rich view” tuning, which involves mechanical pressure to avoid the disadvantages of tilt tuning, but the implementation is generally not as uniform as air pressure tuning.
Coronado and Lunt produce these designs for their dedicated solar Ha telescopes.
3. Rear (focuser mounted) etalons. These etalons generally require a f30 optical system or greater (using an aperture stop Energy Rejection Filter) to perform optimally. The use of telecentric optics can be used in faster optical systems for reaching f30 +. However, low power sweet spot issues may occur depending on the particular telescope used – especially if using an ordinary Barlow lens for amplification. As with internal etalons, these are generally moot for close up views and imaging. Tuning of the etalon is usually achieved by heating (and with Solar Spectrum filters heating and cooling), and thus these filters generally do not suffer from banding. Note: If considering a Quark, DON'T BOTHER WITH THE PROMINENCE VERSION – you'll see prominences just as well with the chromosphere model, and disc contrast should be better. Due to the built in 4.2 x telecentric, you'll generally need a shorter focal length telescope and/or a longer focal length eyepiece for full disc views, and contrast uniformity may not be so good, but again this is entry level.
DayStar and Solar Spectrum make these filter systems. DayStar also markets complete telescopes designed around their filter systems.
Another issue that should be mentioned is the deterioration of the filter. The air-spaced etalons used in the Coronado, Lunt, and SolarScope systems generally have not had deterioration issues, although the Coronado’s have experienced blocking filter issues with premature failure of the Induced Transmission Filter, and the Lunt BG blocking filter element has had some cloudiness issues as well. Deterioration of the blocking filter’s trim filter has not (yet) become apparent for these systems.
The solid spacer etalons used by DayStar and Solar Spectrum are also subject to deterioration of the blocker and trimmer filters, and may require replacement and/or rebuilding after about 10-15 + years. However, Solar Spectrum has recently begun to use hard-coated blocking filters which promise greater longevity.
Filter bandpass can be difficult to address depending on filter location and implementation, and for the most part is meaningless. Generally a narrower bandpass is preferred over a wider bandpass for disc detail. However, most all single etalon systems have transmission profiles – no matter what the bandpass – that will allow some parasitic light from the photosphere to leak through, which decreases surface contrast. Double stacking two filters can eliminate most if not all of this parasitic continuum. Prominences generally will not be very affected by the narrower bandpass, but overall image brightness is decreased due to the overall reduced filter system transmission.
Lastly, daytime seeing is very much a limiting factor compared to nighttime observing. For visual use apertures over 150 mm generally will not increase resolution due to daytime atmospheric conditions (but will help image brightness for the rare moments of good image steadiness). For imaging, use of frame selection and stacking software can enable larger apertures to be of significantly greater benefit.
You may want to familiarize yourself with how etalons work with some references in the Best of Solar Forum posts.
Also see Lunt’s blog post on internal etalons and issues with tilting versus pressure tuning.
My personal recommendation is that for full disk views and imaging, a front mounted etalon system is the best for its superior contrast uniformity. For close up views and imaging, a large aperture scope with an internal or rear mounted etalon is best and more cost effective. Double stacking should be high on a list of priorities for both visual and imaging performance if it is possible. In order I prefer two front mounted etalons, followed by a front and internal, and lastly two internal filters. Rear mounted filters are monolithic systems and therefore rather difficult to double stack, but this has been done.
Hope you may find this of some help.
Edited by BYoesle, 27 October 2021 - 12:10 PM.