Herschel wedge mod
Posted 26 June 2005 - 10:52 AM
After reading some other posts on Hreschel wedges, informationon the web, and in the Solar Observers Handbook, I decided to try a Herschel wedge for white light / Continuum filter observation. A Herschel wedge was stated to offer the best contrast and least scattered light of any method of white light observation.
Currently the only sources of Herschel wedges seems to be Baader and APM. It is my understanding both units use a Herschel prism sourced from Zeiss. The Baader unit also incorporates a preforated plate(s) of stainless steel with light abosrbent coatings, making it suitable for use at public observation events. The APM unit has a reflective surface for the exit beam which is quite bright, and can be viewed directly -- not a good choice at public events where the curious might try and peer into the wedge.
However, the Baader unit cost alomost twice as much as the APM unit, and was not available until the end of June (or later), so I went ahead and purchased a 2" wedge and ND 3 filter (needed for all Herschel wedges) directly from Markus Ludes of APM.
The views are the best I have ever had! On days with good transparency the sun's disc is set in an almost black background, the contrast is tremendous, and allows the Baader Continuum filter to reveal even more subtle photospheric detail.
That exit beam, however, concerned me -- see the photo below. So I decided to make a removable device that would both shield and absorb this beam (continued next post).
Posted 26 June 2005 - 11:11 AM
This project would have to be cheap, using free or surplus materials, and effective. I ended up finding a thin peiece of sheet aluminum (free) which I figured I could cut and form to fit over the bottom of the wedge. I would paint the interior black to absorb the light. To get rid of any heat build up, I purchased a surplus CPU cooler (heat sink and integrated 12V DC fan) for $2.50, and a tube of thermal joint compound ($4.95), a DPDT switch to control the fan ($1.25) and 2.1mm DC coax power jack (2 for $2.40) and a 12V 2.1mm power cord ($2.50) from All Electornics.
So for less than twenty bucks and about 3 hours time, here's what I produced (continued next post):
Posted 26 June 2005 - 11:21 AM
Posted 26 June 2005 - 11:37 AM
Posted 26 June 2005 - 01:55 PM
Posted 26 June 2005 - 03:16 PM
Thanks for the feedback.
High clouds have moved in, and I just put the scope away after 5 hours of observing. Judging by feeling the metal housing of the wedge, the temperature never rose above ambient -- about 75 degrees F.
Posted 26 June 2005 - 06:44 PM
Posted 27 June 2005 - 03:10 AM
I guess you can make this yourself too. Just glue it in place.
Posted 27 June 2005 - 07:18 AM
Posted 27 June 2005 - 07:26 AM
Posted 27 June 2005 - 09:24 AM
PIII CPU cooler -- CF-177 $2.50 (cut the side "wings" off)
Thermal Joint Compound -- TG-2 $4.95
DPDT on-off-on switch -- MTS-8 $1.25
DC "co-ax" 2.1mm power jack -- DCJ-21 $1.20 ea.
12V 2.1mm 6 foot long power cord -- CLP-74 $2.50
Tools used: Dremel tool, file, sandpaper, and drill. Find some scrap sheet aluminum, add four small bolts w nuts and washers to mount the CPU heat sink, and some black spray paint. Be sure to sand the sheet metal perfectly flat where the CPU cooler goes for best contact (and use the themal joint compound between them)-- it warped a little bit after I bent the side "flaps" that secure it to the wedge. Away you go!
I don't know what brand of we[d]ge you use but the Baader wedge uses some kind of light trap that is made of triple folded perforated sheet metal. No dangerous refelction from behind with this one.
I guess you can make this yourself too. Just glue it in place.
I considered this, but I didn't have a clue as to what the "special" anti-reflective coatings Baader applied to their preforated stainless steel sandwich, but I knew I was going to be pushing the envelope if I tried to relplicate it (other than just using flat black paint!)
Here's the reasoning I used in designing the con-TRAP-tion I made, and after the first use I am confident that there will be little heat build up in the almost completely closed-off aperture of the APM wedge (I left about a 0.125 inch (3.2mm) opening at the front -- towards the telescope -- edge to allow for air movement.) This absorber is very easy to make, and has NO reflected image of the sun to view whatsoever (see Baader wedge below).
Holding my hand in the unfocused reflected/expanding light cone exiting the wedge showed the energy level to be falling off -- if you think about it the only reason a magnifying glass works to ignite paper, etc. is that the sun's disc is focused into a tiny image (area). As you move the lens and the image defocuses to a larger diameter, the energy level becomes less concentrated -- is spread out over a larger area, and becomes harmless. When it reaches the size of the lens itself, it is no more intense than the light entering the lens...
This same principle applies to the light entering the telescope. Covering the entire exit area of the wedge with a heat-conducting piece of metal not only increases the radiative surface to expend the heat to about 10 square inches, the CPU cooler's heat sink area adds another 4+ square inches. The area of the objective is a little over 12 square inches. And the Light hitting the sheet metal is already de-focused. Therefore, the surface area radiating any de-focused light (converted to heat) is larger than the objective gathering the light, and the wedge remains cool -- running the CPU fan over the heat sink is the "icing" on the cake so to speak.
That's my reasoning, anyway. Hope I'm not mis-stating any physical principles. Maybe no one has tried it before because it doesn't seem that intuitive, but it seems to work.
Part of my plan also included adding a TEC: If the CPU cooler didn't work by itself, the DPDT switch allows for the addition of a small Peltier thermoelectric cooling module (~ $15, making the total cost around $30) -- but at this point a TEC seems to be completely unnecessary.
BTW, here's a pic of the Baader wedge showing the preforated/coated stainless steel, triple folded, light and heat diffuser. By all reports the Baader wedge is excellent -- while more "elegant" than my sledge-hammer approach, the diffuser seems to add about $200+ to the cost of wedge ownership (when it becomes available):
Posted 27 June 2005 - 04:23 PM
I guess the sandwich is just painted black.(or black anodized aluminum) probably a heat resistant paint would do the trick to. (Hammerite has one for up to 600°C)
I own the baader myself and I'm very pleased with it (at 500€ it better did) The main advantage is it's a part of a very modular system that allows the user quite some freedom regarding it's use. In short: it's a fine piece of equipment but indeed very expensive.
Posted 28 June 2005 - 06:52 AM
Posted 28 June 2005 - 07:56 AM
Posted 30 June 2005 - 09:56 AM
Posted 30 June 2005 - 10:30 AM
I used Rustoleum flat black spray paint -- I think it was about $5. Any flat black paint will do, as experience shows the temperature will not get much above the ambient temperture outside. If you want to get really fancy you could have the parts anodized black...
P.S. I also covered the internal parts -- the switch and connector -- with black electrical tape.
Be sure to post some pictures of your absorber and let us know how it works!
Posted 30 June 2005 - 12:32 PM
Posted 30 June 2005 - 03:33 PM
I'd have to agree the Herschel wedge is the "white-light filter" of choice for refractors
But NOT a SCT or any other reflector.
Posted 30 June 2005 - 03:59 PM
Posted 30 June 2005 - 04:08 PM
Posted 01 July 2005 - 06:11 AM
Posted 03 July 2005 - 01:23 PM
Now that it's closed up, how hot does it get in there?
I was curious about Armand's question -- and decided to use an old automotive electronic thermometer in order to get some quantitive data. I shortened the "outside" sensor lead to about 5", and placed it in the wedge. Then I used double-stick foam tape to attach the read-out meter / "inside" sensor on the back of the wedge. After the scope had been up and tracking the sun for five minutes, I took a reading every five minutes (except for one bathroom break!). Here are the results:
TIME "IN" "OUT"
09:20 74.5 71.4
09:25 74.3 71.8
09:30 ---- ----
09:35 75.2 73.6
09:40 74.8 74.3
09:45 74.7 74.7
09:50 74.1 74.7
09:55 75.7 75.7
10:00 75.4 74.7
10:05 76.8 75.6
10:10 78.1 76.3
10:15 78.8 77.2
10:20 78.1 76.8
It is interesting to note that the sensor inside the wedge (the "OUT" meter figures) are actually below the ambient air temperature(s) most of the time! I didn't expect that without the use of a TEC...
Here's a picture of the sophisticated device:
Posted 04 July 2005 - 08:05 AM
Your contraption and the temp measurements of the heat-shield interior are outstanding. I'm considering getting an Intes Herschel wedge myself, as the H-alpha stuff, and even the Baader Herschel wedge, is way too expensive for me. So this heat sink jammie may be a worthy addition.
I'd be interested in knowing if the "in" "out" temperatures would be significantly different without your "fan-sink" attached?
What's the possibility of you re-running the experiment without it? :praying:
Posted 04 July 2005 - 01:01 PM
Here ya go: Today's readings, starting with the fan Off, then turning it On (remember the OUT reading is inside the wedge, while the IN reading is actually outside the wedge in the display / contoller).
TIME "IN" "OUT"
09:25 81.3 86.9
09:35 82.2 88.3
09:45 84.0 91.6
09:55 84.4 91.0
10:05 86.7 93.7
10:20 84.4 88.2
10:40 84.9 88.5
It does appear the fan has a cooling effect, reducing the difference between the ambient temp. and that inside the wedge. Whether or not a fan is used appears how comfortable you are without one. The temperature difference without it does not appear that significant, just lets me sleep better I guess, and it's very inexpensive to add later if you so desire.
Be sure, however, to leave a little opening as I did at the front edge to allow a bit of air circulation -- otherwise I imagine it might become much warmer inside the wedge: