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Important information about using of PST Etalon

Take more power from your PST etalon, but check the risk

Adapter for connecting the PST unit to a 2" eyepiece focuser. With this adapter you can use the etalon filter unit on a larger telescope and get more details on the sun. Of course, the adapter has a 48 mm filter thread, so that a necessary ERF or strong UV blocking filter can be screwed in if its a smaller scope up to 4"..

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The adapter is to screwed to the filter unit instead of the original tube. This allows the filter to be attached to any telescope. The built-in sun finder will be covered and therefore cannot be used. Very important: a backfocus of approx. 20 cm (about 8") is necessary when using the original "black box" of the PST! However, we would be happy to advise you on how this can be achieved. We are also happy to help if you cannot unscrew the original tube.

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You can also easily determine for yourself how large the back focus on your telescope is: point the telescope at the sun or the moon without the eyepiece and deflection system. Hold a piece of paper so that the sun or the moon is in focus and determine the distance between the paper and the focuser's drawtube at most insert position. That's the back focus on her telescope.

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Picture taken from Thomas Borowski with a PST filter and FH150/1200 + 70mm C-ERF (build backward into the tube) and a DMK 31 AF camera.
The use of an additional ERF or strong UV blocking filter is strongly recommended (necessary as protection for the etalon filter). With telescopes up to approx. 4" opening and focal ratio longer than approx. 1:10, this filter can be mounted on the adapter, otherwise an appropriate filter in front of the lens is to be preferred and/or the lens is to be dimmed! With larger opening ratios, the light cone is otherwise too large in the etalon and the frame is heated.
A simple way to protect the filter is to use a good normal red filter with 48 mm thread (as for a 2" eyepiece). This works up to max. 4" scopes and f : 10 (mounting on the adapter).
We also manufacture ERF front filters with different sizes to fit your telescope. The cell of the filter can also be used to limit the focal ratio to f: 10.
Together with the rear adapter, the Etalon can also be used alone without the "black box" and the problematic prism inside there and then with a larger block filter. In this case, the light path of the lens must past the lenses only so that no large light cone falls on the etalon's cell or the adapter. But because you can palce the etalon closer to the focus you have more choices.

Some thoughts about heat protection filters and ERF's:
The term "ERF" (= Energy Rejection Filter) or simply energy protection filter was coined by DayStar in the 70s and 80s and was a UV protection filter in terms of function. Strong UV light has a decomposing effect on many substances and thus also on the evaporated layers of an H-alpha filter. Over time, the layers have become more stable, but the load is still considerable and the filter must be protected from it.

Over time, ERFs were increasingly coated with a coating that more or less dampens the near IR range with the idea that the H-alpha filter should be heated to a lesser extent. However, one must also note that the H-alpha filter itself does not heat up strongly, since it reflects the radiation and does not destroy it into the filter-glasses. But you have to keep an eye on the frame - if it is smaller than the light cone of the sunlight, a suitable measure must be taken so that the frame itself cannot heat up excessively.

It is often said that the ERF has to protect the H-alpha filter from too much heat radiation. Otherwise it bursts. This sounds logical and therefore very expensive ERF with IR protection are offered. That's partly true because a red filter absorbs part of the heat and for bigger scopes than 4" a good thing. If one compares the passband characteristic, however, one has to note that many ERFs today actually show a partial filter effect in the near IR range. However, radiation heat does not begin just above the H-alpha line; it ranges from the middle to the far IR range. The critical range is between 1500 nm and 5'000 nm. Unfortunately, diagrams usually only reach up to >2500 nm, nevertheless one sees that the many ERFs are quite permeable exactly in this critical range from about 1500 / 2000 nm upwards - they therefore offer only limited protection against radiation heat (not to be confused with heat radiation).


(Link to image source: www.sonnen-filter.de)

Not that the near IR range is harmless - even too much light in the visible range can be very dangerous. Of course, the eye is also at risk in this wave range. However, the complete H-alpha filter unit of the PST also reliably blocks all harmful light up to more than 50'000 nm, it reflects it to a large extent and does not destroy it. Therefore, such a filter is not heated up too much. The H-alpha filters, like all other products, do not react well only to stronger UV radiation - they gradually decompose as a result. Therefore, an ERF must be used - primarily designed as conventional red glass that safely blocks UV and is not damaged even in the filter effect.

A real "thermal insulation ERF" then looks like this and blocks IR radiation even at over 2,500 nm. However, it lets UV light through quite well and that would destroy the etalon in the medium term.....


(Source of image: Zeiss)

But what can be dangerous for an H-alpha filter is if too much light falls on the filter holder and/or etalon's cell. This can then be heated up to a dangerously high temperature. This is especially the case with optics with a large number of openings, e.g. f : 6 and/or with longer focal lengths. In this case the lens must be dimmed or otherwise it must be ensured that the light beam is not larger than the filter opening of the etalon filter! Because the etalon need an longer focal ration, a focal extension lens is necessary with short focal optics anyway otherwise. Only an expensive heat protection filter alone is not the optimal solution. The PST is designed for a lens with f : 10 - ideally, the replacement lens should have the same focal ratio of the original lens (40 / 400 mm). And of course, individual components of the H-alpha filter system must not be used individually, as this does not achieve sufficient filtering of the sunlight. The etalon must always be combined with a block filter to protect your eyes and at best a ERF to protect the filter system itself.

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