Deutsche Version
CAUTION
Sun observation IS potentially hazardous! Observation of the sun by inappropriate telescopes or optical equipment not intended for, may worst case cause IMMEDIATELY BLINDNESS!
The following description of modifications on my Coronado PST never should be used by persons without detailed knowledge about the operational principles of the PST and its internal filters.
I'm not responsible for anything, anything you do is on your own risk!
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Overall picture: Actual version of my PST as of 08/17/2010 - Baader-Halpha-CCD Filter 35nm, 2", acting as ERF in front of the objective lens. Eyepiece-sided tubing with original ERF removed. 10mm block filter from BF10. Baader binocular with 1.5x Barlow lens corrector. 40mm Plössl eyepieces. Sun shield howebrewed from black and white cardboard and brass clamp. Individual modifications are described in more detail below. Update: Despite of the rainy German summer of 2011 I was able to do some observations this year. My eyes are still well which proves the concept of the mod. |
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During my ATT 2010 visit in Essen (Germany) I got a Baader binocular at a nearly crazy price. Immediately I decided to put it into my PST. First try about was with help of a 3x Barlow lens, but... the resulting magnification was way too large. However, just one view through the bino left me with one thought which is well-known to all bino owners: "never again without it...". Since I bought my PST in 2006 its warranty was nearly over. Thus I decided to take it apart. I knew about the oil filter tool trick but that didn't work for me. Instead I found out an old leather belt gripped by pipe tongs will help to unscrew everything without leaving a single scratch on it. The photo on the right side shows the components of the PST. The "new" layout the objective lens no longer has an ERF coating on it; instead there is a small 15mm ERF in the small pipe between the eyepiece holder and the PST "box". Thus the small pipe must not simply be removed to gain some more focus range! If doing so nevertheless, one MUST take care of another proper ERF in front of the objective lens. I guess doing the latter also will make the etalon's life happier and longer. In unmodified newer PSTs all the unfiltered light energy (and that's a lot!) from the sun passes through the etalon filter. |
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As a new ERF a 2" Baader filter is used (see below for details). The question was how to fit this to the M52 thread at the PST's objective. Couldn't find any adapters so I had to homebrew something: a short piece of threaded 2" eyepiece tubing was wrapped with adhesive tape and pressed into the PST objective thread (see left). The photo still shows a bit of the red adhesive tape (filter not yet mounted). In the end I had a 2" filter thread in front of the PST objective lens. The Baader filter of my choice is a 2" Halpha CCD filter with 35nm pass band. If the transmission diagrams on the Baader page are compared, it is slightly better than a C-ERF. It blocks outside of the pass band down to 0.1% (info from another website, I lost the link, sorry). The latter corresponds to ND 3 which is important in the infrared because the Coronado block filter is, well, very open in this wavelength range. See diagrams at sonnen-filter.de (german). To be on the secure side I made some tests on my own. I have a medical heat lamp which warms up skin very fast in 30cm distance. Putting the Baader filter in between, the sensation of heat on the skin stops. Of course, red light still passes the filter. In my little laser lab I also have a quite powerful infrared laser which I took for some measurements at 810nm wavelength. Result was a transmission of 3 * 10-4 - that's quite dark (approx. density 3.5). |
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Here we have a picture of the whole ERF mod: the Baader Halpha in front of the objective lens, the small ERF pipe below the eyepiece holder removed. The gain in focusing length now is sufficient to reach focus with the bino and a 1.5x Barlow screwed into its 1.25" adaptor. First view through the eyepiece showed a very obvious advantage: a strong reflection from the removed original ERF was gone. The reflection of the Baader filter was much much weaker and far away from the sun's image. I found out there are also disadvantages about this configuration. The original 5mm block filter was designed to let just the sun's image pass near the focus. Now, closer to etalon and objective lens, it cuts away part of the light cone. Thus, the field of view shows signs of degradation if not watching sun's surface close to the optical axis. Near the border of the field of view, resolution and Halpha detail suffer dramatically (but see below for a fix). A minor disadvantage: the bino at a 90 degree angle absorbs lots of sunrays and quickly becomes warm. |
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The sunray problem is easiest to be fixed: use a shield. Two pieces of black and white cardboard (size about a normal letter) glued to each other. A clamp can be made from a sheet of brass bended like a big version of an electrical micro fuse holder. Glue or use a screw to fix the clamp to the shield. The brass clamp then is put to the eyepiece holder of the PST (see first photo on this website). Take care for the little nylon screw on the eyepiece holder! |
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The problem about the small 5mm block filter didn't let me sleep. I decided to go for a bigger one and purchased a used Coronado BF10. Despite of its name the Coronado block filters used with the Solarmax etalon filters, don't consist of a block filter only. There's also an ERF into it (like it is in the newer PSTs eyepiece end, 15mm diameter). Fortunately, the true block filter part has exactly the same mechanical dimensions for both PST and BF10 so it can be swapped without any problem. Note that the latter is not the case for the ERF which is inside the 1.25" tubing at the BF10 - too small for PST. Now the field of view of the Baader Bino with two 40mm eyepieces fits very well to the size of the 10mm block filter pass. Over all the field there are again rich Halpha structures and full resolution. In the end I have a wonderful PST with 10mm block filter - and a complete surplus Coronado BF5... :) |
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Future plans...
Well I still have that nice Vixen NP mounting from my old 114er Newton hanging around. Wouldn't it be great to put a PST with a really big optics on top of it? 90mm sounds nice.
You see: story has to be continued. Stay tuned...