How many of use have bought a telescope before we really knew anything about what astronomical direction we are headed in?
I bought a GSO 8” Classical Cassegrain when I wanted to change from a cumbersome 10” GSO dobsonian. It had sat in a corner for months. I wanted something easier to manage and set up, so when I saw the short CC8 for sale second-hand, I bought it. Out with the dob.
So visual astronomy morphed into mirrorless camera astrophotography, then naturally into CCD cameras - cheaper uncooled, then cooled. Sound familiar?
But the f/12 CC8 is a tricky beast to start astrophotography with! Not the least of which is polar alignment and plate solving at this narrow FOV. Anyway, I’m getting off topic…
All goes well when imaging in the drier summer months, and when doing shorter sessions. But as soon as normal New Zealand weather arrives (frequently), dew appears as if by magic and ruins a good night’s imaging. Only you are not sure what is really happening at first. Is it seeing conditions? Is it collimation? What about the cheap GSO 0.5x reducer I bought - could that be causing poor imaging?
Somewhere when I bought the CC8 I read on telescope sales websites that Classical Cassegrains do not suffer from dew, and unlike Maksutovs and Schmidt-Cassegrains they are quicker to cool down (true). I watched a YouTube video by a guy from a reputable astro store repeat the same no dew claim.
But why was my CC8 seemingly dewing up and disobeying the experts?
On particularly cold damp nights I could see dew frosting around the outer 2-3cm of the primary mirror. I have mitigated this by wrapping a dew strip around the large outer rim of the black metal primary mirror holder. Then I noticed that throughout the cooler damper nights, the images still started to fade after 20 or so 300 sec EXP images (see pic).📷 20260519_014308.jpg
Of course dew is just obeying the law of physics, and no matter what you hear about Cassegrains - “mine never suffers from dew”, or “a dew shield solved it” or somesuch - if the mirrors reach dew point and are open to the humid air, they will dew over. So the question is not whether your Cassegrain dews up or not, but rather what are the atmospheric conditions at your imaging site?
The first experiment was to use one of my guide scope heater bands and wrap it around the cylinder/cannister that holds the CC8 secondary mirror. I did this and tried a night of imaging - terrible! Actually, there was no dew, but the shape of the band and the thick wire across the spider vane created awful diffraction effects around the stars (see single sub image below of M83, artefact in lower left corner caused by something else). But it did prove 100% that a secondary mirror dew heater was needed.
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So back to the topic of the DIY dew heater I built. The first image above shows significant frosting at about 2am. So I started researching, watched a very good video: https://youtu.be/q2vyVyWRCO8?si=BQg-10AjfYJIQhqM and jumped onto Gemini to get some ideas. Long story short, Gemini said that a simple solution is to use the formula Power (watts) = Voltage squared / Resistance. So, a steady 2.5 Watt dew heater ring could be made with five 12 Ohm resistors soldered in series, the exposed wires heat-shrink wrapped, then run from the 12V port of the ASIAIR+. Ok - great!
I ordered the parts and the images below show the simple idea coming together. The parts list is simply:
Five 12 Ohm resistors.
Hi-temp tape to hold the ring in place.
Plastic electrical tape to hold the heat in & stop/minimise thermal plumes in the tube.
Wire, solder, soldering iron, DC 5.5 × 2.1mm plug.
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No, I didn’t have any black cable ties! I’ll replace it eventually.
I was careful to use a wire that was no wider than the spider vane. It terminates in a DC5.5x2.1mm plug that goes into the ASIAIR+
The resistors sit in the small 3mm gap between the secondary holder spacer ring and the end of the spider assembly ring. You have to be careful not to change the threaded spacer ring position, any changes will be amplified at the focuser end, moving the focal plane considerably. I undid mine but put it back where it was supposed to be.
Does it work???
As usual with telescope things, it’s cloudy at present. But I have powered it on and the resistors all warm up nicely. Before I wrapped them with the high temperature tape and electrical tape, they got a bit hot - I could hold them for 10 seconds or so, then needed to let go, so they are just above comfortable to the touch. After taping they were warm to the touch, easy to touch indefinitely, so the heat is definitely going inwards towards the metal secondary mirror holder. I put the telescope outside for 2 hours with the heater ring going this evening. The ASIAIR+ power monitor says that it draws approx 2.5-3 Watts continuous, as expected. No dew but not a really cold damp night to test it with. When it is I will post the result with an image hopefully.
It’s an experiment of course, and if it is not sufficient I will put the resisters in a tighter ring and secrete them inside the secondary mirror cannister directly behind the mirror itself, and drill an exit hole for the wires to the ASIAIR+. But I’m hopeful that there is just enough heat to raise the mirror above dewpoint.
As a final thought for tonight, have you ever wondered why telescope makers leave it up to the customer to figure out and implement a solution to dew?? What I mean is, it is ok for refractors which are relatively easy to use with dew bands, but Newtonians, Classical Cassegrains, Schmidt-Cassegrains, Maksutovs, etc. all have issues with dew. I know Celestron sells a dew ring for the C8 and other scopes, which is great. But GSO, Skywatcher and others all sell these scopes with no dew solution whatsoever. It’s only the higher-end brands like Planewave, ASTROWORX, smart telescopes that have this.
The only solution left for the lower-to-mid-range open-tube telescope users is some generic option from Aliexpress (or 5x Aliexpress price for the same thing from a telescope shop) that may or may not fit, or go the DIY route which I have done.
I hope this is interesting and useful for others.
CS
