Going H-alpha: Positioning the D-ERF:

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WARNING: SOLAR OBSERVATION REQUIRES GREAT CARE AND SAFE FILTRATION.
INSTANT AND PERMANENT BLINDNESS CAN EASILY RESULT FROM SIMPLE MISTAKES. NEVER LOOK AT THE SUN THROUGH ANY LENS, MIRROR OR INSTRUMENT UNLESS IT HAS BEEN FULLY TESTED AND APPROVED FOR SUCH USE. YOU FOLLOW ANYTHING MENTIONED HERE ENTIRELY AT YOUR OWN PERIL!

For a lens with a focal length of 1200mm the solar image diameter is ~Fl/107 = 11mm Ø at its focus.

The PST has rather small optical components which will vignette the light cone from the objective.

The etalon is sandwiched between negative and positive lens components with apertures of ~20mm.
The etalon requires an objective focal ratio close to f/10 thanks to the power of these 'correcting' lenses. The converging, focused sunlight, from the telescope objective, enters the negative lens in front of the etalon to send parallel light through the etalon. Once it exits the etalon, as H-alpha,the light is then refocused by the positive lens which follows the etalon.

Provided the lenses are matched in focal length and the etalon has no power, none of this should really affect the  original focal length of the telescope objective. Though one wonders about the so-called "glass path" of these components they are unchanged from the PST.

A 150mm f/8 objective needs to be  stopped down to f/10 or 120mm aperture to suit the etalon's parallel light requirement. 1200/10 = 120mm. The aperture of the 6" Celestron can be easily reduced to 120mm simply by moving the first Celestron baffle @ 122mm Ø to the objective end of the main tube.

The position of the D-ERF, within the main tube, is limited by the 85mm aperture of the baffle on which it is mounted.  From a full scale drawing [involving map pins and fine cotton] I can confirm the D-ERF can be placed anywhere from 24cm [or further] away from the objective end of the main tube. Or 34cm minimum distance from the objective. This is only true when dealing with a 120mm f/10 light cone. NOT the original 150mm f/8 lens or any other. The Celestron fixed [non-collimateable] cell is ~10cm deep.

Remember that the completely unfiltered, Sun's image is sent right back, by the front surface of the highly reflective D-ERF, towards the objective. Where its normal focal length is considerably shortened by passing back through the [converging lens] objective. An intense image will be present in mid air in front of the objective.

Fortunately one's head will usually block the sunlight falling on the objective. BUT, don't ever make the mistake of looking into, or at the objective, at close range in sunlight. However tempting! Just holding your hand in front of the objective may produce a nasty burn if some sunlight can still reach the lens! Imagine what that focused heat would do to your eye! You have been warned!

Where a sub-aperture, internal D-ERF is used, a proper dewshield might be a good idea. If nothing else it will help to reduce the risk of sunlight falling on the objective lens when a head, or hand, intervenes.

The final blocking filter of the PST is only 5mm in diameter. This makes for a rather small field of view. The true field of view of a 1200mm telescope of 120mm aperture at f/10 using a 12mm EP at 100x would otherwise be 0.6° with an exit pupil of 1.2mm. The Sun has an angular diameter of 0.5° on the sky.

Larger blocking filters [BFs] are available in specialized star diagonals such as the BF10, etc. Their cost is remarkably high! $500US or 800 Euros in Europe at this time.[Early 2018] Presumably the high price means they contain a larger ITF as well as the larger BF [10mm] plus the diagonal itself. The 10mm BF offers a full sun disk, in the field of view, with telescopes under 1000mm in focal length. This suggests using a 100mm objective working at f/10. The TAL 10cm has been used to produce such an H-alpha instrument.

Do not 'mix and match' filters from competing solar telescopes without their manufacturer's direct approval. They each use different combinations of filtration which will probably not protect you in an unauthorized assembly of badly mismatched parts.

Three different 2" Ø tubular extenders. The 40mm provided a poor H-alpha image while the 80mm was rather better. I'm hoping the WO 60mm will be just right for a 20cm inside focus position for the PST etalon. Rather oddly it is described as a 2" extender but no mention of its active length was given. The position of the etalon, within focus, will alter the parallelism of the light passing through it. With a likely deleterious effect on image quality and tuning within the H-alpha passband.

Most modern APOs are rather "faster" than f/10. So will lose aperture when reduced to f/10. iStar offers a 150mm f/10 achromat, amongst others, for increased aperture, but it will still not show the Sun's full disk in one view.

A large Baader D-ERF [Energy Rejection Filter] will be required in all cases. The BF10 must NOT be used as the sole means of filtration with ANY objective. A primary heat rejection filter MUST always be used. Or instant blindness WILL result.

I wish I could report an update by now but the weather has been horribly cold, cloudy and very windy simultaneously. It was 23F, -5C in my workshop and I'd rather not let my breath condense all over everything. Just handling metals even in grippy, industrial gloves is unpleasant at these temperatures. Just putting the gloves on in the first place takes some stoicism if it quickly leads to fingers aching with cold. I do have a variety of industrial gloves but none is really meant for these low temperatures.

Friday and a balmy 28F, -2C in the shed meant I could brave the cold to re-position the D-ERF and other baffles. D-ERF is now better squared on at 27cm from the objective end of the tube. The largest baffle is now just inside the tube to stop down the 150mm f/8 objective to 120mm @ f/10. The sky promptly turned dark and cloudy to celebrate the moment after fleeting sunshine earlier in the day.

Sunday: It has risen to 26F, -3C with a roaring gale as I prepare the H-alpha solar telescope for fine adjustment. I quickly checked the focus with just the D-ERF in the system. The sharpest solar image seems to be at 10" from the plywood backplate. Whoops! I thought it was much shorter than that. It's lucky I now have a good range of extenders! Though I might as well put the Vixen 2" focuser back in to give me some fine adjustability of the etalon position.

I was still seeing the sun's image, when I blinked and closed my eyes, from persistence of vision. This was just from very briefly focusing the bright red image on a bit of plywood. The D-ERF doesn't seem to reduce visual brightness by very much. I must hope it's doing its job.

Click on any image for an enlargement. 

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