DayStar's comment that wouldn't you rather have a reflective 2" filter inside than a 5" heat-absorbing piece of glass in front. I think yes.
Hi George - yes indeed!
I found another plot of the distribution of solar energy that makes me actually more concerned for the need to block IR ahead of the filter system (if not at the front of the OTA).
The statement that "the sun doesn't have a ton of watts of transmission in the IR. The solar transmission watts is very heavy in the visual spectrum," appears inaccurate.
52% of the solar radiation at sea level is IR, 43% is in the visual spectrum, and only 5% is in the UV (what the GG and RG glasses are meant to block by absorption).
"We moved from Red to yellow in or ERF glass so the clients didn't need to buy new glass for a Sodium or Helium filter. The impact is perhaps 5% more heat, but 5% of a load that is borderline insignificant to begin with anyway." Again this statement does not recognize what is happening near the focus of the instrument where the filter is located. Using a "yellow glass" long-pass filter instead of a RG630 as an ERF is adding insult to injury - you are allowing all the IR and a significant portion of the visible spectral energy through to your filter system. I would estimate this visible and IR transmission amounts to about 75% of the total energy, not the 66% I had stated earlier.
It is difficult to see how either the yellow or red glass absorptive filters can do the job "to reduce the thermal load so that the telescope and accessories are not at risk of damage or overheating and poor performance," when it appears what they would absorb is only about a 25% of the total energy. On the other hand, if the filters and their mechanical components are designed purposefully to take the concentrated amounts of thermal energy where they are located, then all is fine. But again this may result in localized heating effects for the components and the air lying next to them, and give rise to undesirable thermals within the OTA. My opinion is that if I can increase the filter component's longevity by spending a little more on a true blocking filter or system of filters (which as noted previously costs way less than $20K), then this makes sense to me.
Regarding the statement "you saw my talk on how much energy makes it into your telescope" and the rationale that there is a minimal amount of energy: As I pointed out earlier, the 1.05 mW per square millimeter entering the objective is not what matters, it is the energy flux per square millimeter of area near the focus for the DayStar and Solar Spectrum filters. I think we all know this.
Only 5.3 W is spread out over a 3.1 inch objective; but concentrated at the focus this is enough to ignite paper or burn wood:
Therefore I would agree that a front mounted ERF consisting of an absorptive colored glass filter that passes all the IR and a significant portion of the visual spectrum is indeed a "dumb" idea. Spending over $1000 for one that is only 0.25 lambda, and then "heats, swells and deforms" is even dumber, and likely is not as homogenous or striae free as optical glass, especially when for about $100 more you can get a true UV/IR blocking DERF with dielectric coatings that prevent heating, swelling, and deformation, applied on optical grade BK7 polished to 0.10 lambda - and then maintains this specification (it does require a cell however).
In summary, if you are using an absorptive colored glass ERF, at least add a good IR or UV/IR blocking filter to the nose of the filter or telecentric lens system itself. Indeed, this seems to be a must if you are dealing with an expensive SE or PE grade filter, let alone a entry level Quark. The BelOptik UV/IR KG3 looks to be ideal as it will block IR out to at least 2500 nm. Add a nighttime H alpha filter in front for even better protection from the visible spectrum components which may contribute to heating as well.
Doing so could delay or even save you from needing an $875 blocker repair/replacement...
Edited by BYoesle, 20 February 2019 - 10:27 PM.