SolarChat!

I think there's a few people know who've done a variant with different scopes on what you're wanting to do so there will be plenty of info on here. You should be able to 3D print a reducer for your focuser, just go for a dense fill. Where abouts in the west midlands are you?

Should be a pretty straight forward mod. Let me know if you find anywhere local to mill the ali.

Mark

Here's mine, works very well: viewtopic.php?f=9&t=19238
Most of my solar images since then have been captured with this setup.

I have added a few practical details re. the making of my Lunt50 / SW ST120
86mm Frankenscope now. I acknowledge the ideas & encouragement of others!

viewtopic.php?f=9&t=23187

No skilled machinist *immediately* on hand here either. But sometimes easier
to make a "prototype" to get the "Engineering Drawing" into acceptable form?

Chris / Macavity (SGL)

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"!