CN FORUMS ARCHIVE Telescope Reviews: What's the point of a Schmidt-Newtonian?

Hey all, I was maybe thinking of getting an MCT as a later addition. Since I occasionally go to Russia, I thought that it might be the place to get a good Mak scope without going through an import dealer. An Intes-Micro MK76 or something similar would be nice. I have to look up what LOMO offers, as I will be very near their factory in St. Petersburg. Maximizing on the fast/bright side for now was a big plus, as long as the magnification wasn't hurt too bad. At f/4 speed, I'm not hurting for brightness. That's equal to f/8 at 1624mm. Since my coma is half, I can compare to a newtonian twice as long. Beats the Orion SVP 6LT quite handily in that regard (for quite a bit more coin).

Closing the tube was listed as a plus by Meade. I totally agree. If you thoroughly insulate the body (which is ugly, adds to cost, and does not sell scopes......so Meade will NOT do it in an econo scope), then you stop significant internal currents. Preventing need for disassembly for internal cleaning is another big plus. Fans add vibtration, which hurts image quality at high magnification, and if you have such a slow cooldown that everything stays close in temp inside, then that's good. Most heat loss would then be through the front element since it is the only uninsulated part of the tube, which gives you a FREE dew suppression system for several hours as long as it's backed up with a well insulated dew shield/lens hood. It's something I will try.

There was a thread that told how to maximize reflector performance. Insulating the tube internally with cork sheeting (this looks best from an outside view) or making the tube out of heavy round cardboard will insulate the tube and slow down tube current generation. Steel and aluminum tubes are bad, bad, bad for temperature stability. Scope tubes can be wrapped from the outside with sleeping bag ground pad insulation material, an old wool army blanket, or other suitable material. An open Newtonian will still need cooling fans in spite of insulating the tube. An insulated tube should need less air through it, though, as current generation is less.

I knew about the curved vanes. The diffraction is still there, but just spread equally over the entire image. The central mirror in the SN-8 is not as big as you fear. It looks about 25% of aperture, not 35%.

Internal air currents are caused by internal temperature DIFFERENCES. This is caused by UNEVEN COOLDOWN of internal components and being open to outside air at a different temperature. If you seal the tube and slow cooldown, then air currents are proportinally slowed. Therefore, no need to cooldown to ambient air temperature. At some point, the currents will become unnoticeable. That point will get a name from somebody, and be another point for discussion. When a scope is cooled down, the point is to equilibriate the temperature of all components, along with the outside air. A highly insulated OTA will be self equilibriated internally as it VERY SLOWLY cools down. It can do this by thermal radiation without the need of swift convective air current cooling. Anybody who fooled with vacuum power tubes knows that the tube plates are in a hard vacuum and have no air cooling possible. They cool by thermal radiation off of dark plates. Very little is off the hot tube envelope, or the tube would be ruined quickly. The tube's glass envelope should be kept clean to allow this thermal radiation to occur. The front lens of a compound telescope will be a major radiant heat loss point when the rest of the OTA is well insulated. I am guessing that dew can be kept off for many hours as long as enough internal heat energy is available. It will also lessen the need for any external dew suppression energy needed later, so a very small battery powered unit will be sufficient. Meade places the dew heater around the perimeter of the glass lens in their new RCX400 series of SCT's. A few wraps of electrically insulated nichrome wire around the end of my SNT should give me a dew heater for almost no cost. If you wrap the ENTIRE OTA under the outer insulation and install a temperature sensing circuit to control the heat supplied, then you can keep the OTA at a constant temperature and forget about any cooldown whatsoever, or allow cooldown only to a point to prevent dew from forming. This varies with relative humidity, but cheap electronic sensing, servo and logic circuits already exist that are easily implemented. If somebody hasn't already done this, I am REALLY surprised. All the technology is just sitting there.

I view my new scope as a Modkateer platform. Getting it to outperform my brother's 10" Orion reflector on planets shouldn't be too difficult. He has added a cooling fan, but admits it is still far from optimal for the planets and lunar observing. Turbulent air is so bad where he lives (he typically looks over a couple of miles of open ground, then a river reservoir with cooler air over it) that he needs a scope that cuts a narrower optical path. Plus, he lives in a small town with light pollution. Something like an Intes MN56 or MN66 or Orion 5" Mak would serve him better for the planets and moon.

I have a Zuiko 65-200 f/4 zoom that has as much or more contrast than any of my Zuiko prime telephoto lenses. I can shoot straight into the sun with less flaring than with my prime lenses. Since the zoom has about twice as many elements, I would not think this would be the case. Obviously, other things come into play. I am thinking that my lens and mirror coatings and alignments will play a much bigger part than vanes or central obstructions or how wide the aperture, etc. The fact remains that Astronomics rated the SN-8 as great for planets while the SN-10 was knocked down 3 points to fair. I called before I bought and the SN-8 was recommended. If most 10" Newtonians perform like the Orion 10" does for planets, then I do not want a 10" Newtonian. Their ratings for the SN-10 would match my ratings for the Orion 10" from what I saw with my own eyes. It, of course, does better for DSO work.

Cooling down a rear mirror while heating up the front lens element is not what I call an elegant solution in an SCT. Keeping everything at a relatively constant temperature (indexed to relative humidity) sufficiently high enough to prevent dew formation looks like the more elegant solution to me. It might not LOOK elegant after I'm done, however. I am guessing that few optical design departments know how to integrate electronic sensing, servo and logic circuitry into something that would work right without hiring an outside engineer. My years of biomedical engineering might pay off here, and this is simple stuff compared to what goes on inside medical equipment. The SN-8 is a much better platform for experimenting with my ideas due to its sealed nature. Any MCT or SCT owner can run with those ideas as well. We have foam rubber insulators for our beer cans, why not for OTA's?

Now I know what to do with that pair of Orvis neoprene waders that don't fit me. Will add impact resistance as well.....and the olive green color will be military approved.