Maksutov insulative dewshield, and then pimping it..
Hi all,
I struggled a bit trying to work out which forum to put this in. Could have been Equipment, DIY, or this one. It deals with contemporary thinking behind Mak and SCT insulation/dewshield, DIY modifications, and a bit of artistry. Not entirely one or another exclusive theme, so I've opted to place it here.
Popular thinking about Catadioptric telescope "cooling" and dew control is to wait until the OTA cools before attempting high magnification viewing or photography. However, new thinking takes a different approach. Instead of waiting for the OTA to cool, or for that matter force its cooling, the new approach is NOT to let the OTA cool. This doesn't mean to apply heat, but instead to insulate the OTA in order to dramatically slow its cooling.
The problem Catadioptic scopes have is the tube cools very quickly, however, the primary mirror, its cell and the baffle tube do not cool anywhere as fast, so the surrounding air inside the OTA experiences a significant temperature differential between the warm primary/cell/baffle and the cold tube. This in turn generates those dreaded tube currents.
Now, if the tube is NOT ALLOWED to cool quickly, but rather the OTA is insulated, then the air inside the OTA does not experience a temperature differential, and those dreaded tube currents are not generated. Best of all, this also means that you can begin to use the telescope immediately as there is no need to wait for "thermal equilibrium" of the OTA as there are no thermal currents to wait to abate.
The insulation method also sees this material be extended beyond the corrector plate doubling as a dewshield. The insulation/dewshield combination also helps to extend the dew-free period of the corrector, if not totally eliminate dew formation.
With my 7" Intes Mak, I made this insulation/dewshield out of white Corflute. Colour selection is a critical part of this combination. Black is the very worst colour for our gear to be painted. It loses its heat the fastest and cools to below ambient temperature, making it attract dew and even frost long before any other section. Black as the colour of any insulative material will actually be counter productive with what we are trying to achieve.
The Corflute was prepared to facilitate its wrapping around the OTA by having every second channel cut on one side. The Corflute will then very easily wrap around the OTA with the cut section facing out. The part of the Corflute that doubles as the dewshield, the inside was lined with black felt. The Corflute also had notches cut out to accommodate various items, like finder scope blocks, handle and anchor points of the dovetail plate. These slits also make rolling up the Corflute to stow very easy.
The result is quite remarkable. This Intes Mak is a heavy item, with a very thick aluminium tube, and the meniscus corrector lens is very thick. The heat differential would happen for a long time if the OTA was allowed to cool unchecked. But with this simple Corflute dewshield, I've been able to commence high magnification viewing straight away on setting up. And so far I'm still to be overwhelmed by dew on the corrector plate.
Alex.
Last edited by mental4astro; 22-12-2018 at 12:11 AM.
I've also noticed that despite the excellent internal baffle rings inside the OTA of this intes, occasionally with the Moon I get an internal reflection. I've been wanting to investigate whether further modification of the dewshield is possible in order to further control these internal reflections and even further improve the length of time the corrector stays dew-free - so far even though dew has not defeated me, by the time I've finished my lunar sketches this scope, dew was just beginning to form. With my SCT, I would have had to used a hairdryer numerious times.
With this in mind, I made a second dewshield that fits over the Corflute, and fit a baffle ring to its aperture opening. This secondary dewshield can fit all the way down along the dewshield part of the Corflute, and it can also slide further up the Corflute in order to extend the dewshield and seek to reduce reflections and further improve contrast. This is the first part of the pimping process of the dewshield.
The inside of this secondary dewshield is painted matt black. The outside of course is varnished and painted white. However, just a straight white finish isn't particularly attractive or inspiring. So here was presented an opportunity to let the creative juices run with some decorative elements. The inspiration here was the two major forces that opened the space frontier, the USA and Russia.
I've always had a fondness for old Soviet propaganda posters. The design elements are striking & forceful. By design they had to be dramatic in order to catch one's eye and inspire patriotism. Font selection was also important and carefully considered for the message being transmitted.
What I did not want to do was just rip off and existing design. I had to consider the overall white colour scheme of the dewshield, so I came up with my own take on the design elements that are often seen in old Soviet posters. Though I did want to use a well known Soviet era element, the "Monument to the Conquerors of Space". I also did not want to use an inflamatory propaganda slogan, so I came up with my own quirky "Soviet-esque" caption that translates to "Human Cosmonautics" - a phrase that is neither English or Russian, but still has that Russian Soviet edge to it.
On the other side of the dewshield, I put what for me is one of the most significant spaceprobes launched by NASA, Voyager. Just the probe, stark and prominent. Its own statement of strength.
The pics below show both decorative elements, and the secondary dewshield both retracted and extended.
First light with this whole set up was most promising. Short lived though as clouds rolled in only one hour into it. The forecast for next week is much more favourable, so I'll be able to give this rig a proper thrashing.
Alex.
Last edited by mental4astro; 22-12-2018 at 12:14 AM.
Craig, yes I would like to. So far the way I've been able to rip the Mak to 385X and the image is tack sharp immediately on setting up is something I've never been able to do before. I certainly was not able to do this with the uninsulated Mak, and certainly not with an uninsulated SCT. This alone is proof for me to the benefits of this different take on scope preparation.
Alex.
Last edited by mental4astro; 22-12-2018 at 12:26 AM.
I still have my Meade SCT. I've been slowly working on giving it the Steampunk treatment. Hoping to get a bit more done on it this break. Something that no one is expecting!
Very interesting about white vs black heat loss. I did a search and most info is in agreement with Alex but it seems there are also situations that have the opposite result.
I prefer the traditional white tube over all other colours so I'll be doing my black C8 in white material.
This is mod so easy and cheap that it is really worthwhile doing.
Unfortunately my telescope is on loan and far away for a few months. I hadn't actually been using them for a few years.
Certainly works Alex ..I tried it on a 150mm Mak I have, and straight into sub arc doubles (when seeing allows) , with NO LONG cooling needed anymore..thanks Alex.
Philip, using the material you are for your SCT is a very good idea! The tube thickness of SCT's is very thin, as is the corrector plate. An SCT needs better insulation because is has such a small heat mass in its glass and tube. A Mak has a thick corrector, so it holds its heat better.
This idea is by no means mine! It's been developed by many other amateurs around the world who have been thinking differently about thermodynamics and how it applies to our scopes. Wavytone here in Oz has been an early adopter of this technique. Thing is some common ideas are often sorely mistaken in practice, and unless someone challenges the status quo, the same poor practices are continued and better practices are not adopted.
This technique by no means will totally eliminate dew from forming. Dew formation has so many variables, depending entirely on your location and the prevailing conditions of the night. However, in many instances it will stop dew, or slow its formation. And when dew does form, a little hair dryer set on is lowest heat setting is enough to dry whatever dew has formed. Just don't blast the optics with intense heat. If your hairdryer has a no-heat setting, this is even better.
Thought a little update would be in order about my experience so far with this insulated mod to my Intes Mak.
Where I live, the backyard is lush with turf, a healthy vegie patch and a chook run. Add to this I live across the road from a park that is now irrigated. And just a couple of km from the beach. Dew forms here at the drop of a hat when the weather app's say the dew point hasn't been reached. I have heavy dew form on the underside of the tin awning of the veranda in the backyard.
I set up my Mak at 5pm, just before sunset. I put the insulating dew shield on at that time too. I came back out after the clouds had cleared a couple of hours later, and dew was settling on everything, except the optics. It was only by the time I was packing up at 10:30 that the corrector plate of the Mak started to form the softest coat of dew. I had a crystal clear three hours with the scope sketching the Moon, and the scope only yielded to the conditions well after I had finished. And by this time too, the dewshield had dew running off it too. If I had been pushing on into the night with the scope, I would only have needed a gentle application of a hairdryer to clear the soft dew on the corrector. For the location I live in, this is amazing!
I'm very happy with this simple and inexpensive mod.
Flocking the inner baffle tube reduced any flaring when looking at things like the moon off axis. I also used flocking board on the interior. Internal reflections was reduced by so much that I removed the secondary baffle and effectively made the secondary shadow smaller.
I've been looking at the Corflute board I have for my MAK for the last 3 months. Ill get round to trying this.
.
