Hi SB!
There is e.g. a convenient EU source of 100mm Lunt ERF's at Teleskope Service (DE)?
https://www.teleskop-express.de/shop/pr ... ilter.html
There is a slight "flaring" of the ST120 lens cell to give a canonical 120mm.
But the internal diameter of the ST120 OTA is indeed about 112mm too!
However, this being my *first ever* Frankenscope, I decided to "stick with the rules" in the design. Since I was upgrading the Lunt 50 *BF600* version, I decided to go with a f=600mm Donor scope. You can calculate light cone width at the blocking filter, but I decided to stay within the quoted limits! That limit fixed, I followed the general advice to stick with the Lunt f/7 light cone. Solar scopes use lenses to paralellise then restore this before / after passing it (parallel) through the Etalon etc. This also fixes things to f/7 and an objective aperture to 86mm! Ergo my 90mm Baader ERF.
Whatever the *individual* experience, it is anecdotal that mounting the ERF internally may give rise to tube currents? But simply it is *easier* to mount the thing on the front of the scope! lol. You may sacrifice some (potential) aperture, but I also checked out the (Ronchi Tested!) *optical* performance of Bresser / Skywatcher Achromats. I looks like they work best in green light and may suffer some spherical aberration at H-Alpha wavelength?
I tried a few ad-hoc tests of my ST120 and concluded the performance of the objective WAS better using a (green) Solar Continuum than a (red) standard H-Alpha (nighttime) filter. It is (as ever) far easier to focus a white light scope using a GREEN filter! But stopping down a faster objective down a bit from full aperture would not hurt things? I decided to be content with an externally mounted ERF - An 86mm H-Alpha seemed a FAIR upgrade!
It is not a *Full Disk* Solar scope. I checked that out too... Vignetting, non-uniformity of tuning (sweet spot) limits it. BUT it does give some encouraging *Barlowed* images. Overall I am pleased and excited by the result of this. On my limited budget, it is all totally FINE by me! I have some vague notions of upgrades. A BF1200 + greater aperture? Use the ERF inside the tube? Like Dogs and CATS, an ERF is "for life"!