It was a nice cold clear night here in Adelaide so I set up my meade ETX-90 up look at Mars. But the 90mm OTA entry quickly fogged up making for poor focus. I was reluctant to wipe the screen but did it with a soft tissue, and now I could focus.
I assume this is a common issue for telescopes so I would like to know the best solution?
Chris, find yourself a stand of sorts, a chair or similar will do and get a 14 dollar fan from Big W or similar and just blow that wind across your set up, it works a treat.
I use resisters in series and fit them into the the spot the front lens cover goes, using small pieces of velcro. A motor speed controller then limits the current through the resisitors.
Yes it's a common problem but there are cheap solutions. I have no doubt I'll be scorned for suggesting what I've used but I recently was out in the feild and it was -2c and everything dewed up and froze, even parts that are connected to me and yet I didn't get one bit of dew on my lense. So here we go.....
Go to a craft store and pick up a black sheet and that looks like thick card board but is actually a plastic material that is flexible. Make a tube about will go over the end of your OTA and extends past by a couple of hundred mm.
The bit I'll be scored for....... I bought an outdoor dog vest at the reject shop for $20 and wrapped it around the OTA...... Not one bit of dew since...
Chris,
With a sct you'll definitely need a dew shield.
Easy to make from some high density foam , Velcro and felt from a hobby or thrift shop.
I almost always set up on my concrete verandah this time of year, too much rising dew from the lawn areas.
I don't think this make any diff - the ETX-90 is a Maksutov-Cassegrain. From Wikipedia this looks to be very similar to the Schmidt-Cassegrain. Can't all telescope designs accumulate dew?
Quote:
Originally Posted by Wilso
... too much rising dew from the lawn areas.
I didn't know dew rises, and I was set up next to a lawn very wet with dew. So now I need a to build a second storey on my house with a verandah !
I think the answer is yes, it does take a while. This is only a problem if I have left it powered off.
But, I normally turn it on when I set up, on quite a low setting, so the corrector plate doesn't get down to the dew point. The couple of times I have had it on low, then had to turn the power up, it has cleared quite quickly, just a couple of minutes.
I don't think this make any diff - the ETX-90 is a Maksutov-Cassegrain. From Wikipedia this looks to be very similar to the Schmidt-Cassegrain. Can't all telescope designs accumulate dew?
Dew is a problem for all telescopes, but SCTs and Maks are especially vulnerable due to their glass face plates. Refractors tend to come with with dew shields and open reflectors (like a classic newt) have one effective built in with the primary being all the way down.
Dew heaters are a pretty comprehensive solution, but expensive. They also mean more cables and fiddling around if you are just doing a quick viewing session.
To me they are over the top for most visual astronomy. Good if you plan on going all night out at a dark site, but at home there are cheaper options that are still effective.
For a start I suggest making a dew shield (foam plus Velcro is a great suggestion). It won’t prevent dew for ever, but it’ll keep it at bay for several hours. A big difference for a mak that doesn’t come with one!
The other great solution is the humble hair dryer. When dew starts forming on that front plate, warm it up with the hair dryer and problem solved. Essential this works the same as using a dew heater (same idea, keep the glass above dew point), but you probably have a hair dryer and an extension cord at home already.
Hi Chris, having had SCTs and maks for years the solutions are:
1. Fan, moving the air. Alternatively set up the scope where there is a slight breeze - this will also keep the dew off.
2. A 12V hairdryer will blow warm air to remove dew and also warm the corrector enough to keep it clear for an hour or two; you can find these on eBay. Simple and effective.
3. Heating the OTA as previously with heater straps and battery;
4. Adding a dew cap - a long lightweight tube extending forward from the scope typically 1.5X the aperture, these are usually made of foam rubber or plastic and slip over the OTA. The stuff used for yoga mats works well. This helps keep dew off (it is formed by sinking air).
5. Keep the scope indoors (and warmer than the night air). While observing, wrap the OTA with a very good insulating material - ideally Reflectix - to minimise the heat lost from the OTA.
6. Repaint the OTA white. The dark blue and black colours which Meade and Celestron used are about the worst possible choice because they cause the scope to radiate heat away quickly which is counter productive.
Regarding point 6 the internal air current (“plume”) frequently seen in SCTs and maks is a result of the thermal gradient between the warm air inside the scope and the outside air. Reducing the gradient will stop it and this can be achieved 2 ways - either cool the scope quickly (but the corrector will fog) OR insulate the OTA to stop the heat loss.
7. Regarding heater straps, these are most effective if used to warm the barrel of the OTA and this is in turn wrapped with some form of insulation - which could be a dewcap.
In retrospect I did everything wrong. Applying my novice thinking that a telescope would work best if its temperature matched ambient, I put it outside at dusk. When I went out at 10pm I was surprised (and a bit concerned) to find it covered in dew! haha ... guess this is called 'learning curve'
Anyway I wiped it dry, remove OTA cap and aligned. With OTA pointing at Moon/Mars soon enough the pupil (if that's wot its called?) was misty .... now I know it's better to keep OTA warm!
I'm going to add heater + shield, plus have a hair dryer handy.
I'm not up for painting it white ... well yet anyway. But interesting point, I guess that's why so many pro scopes are white.