Esprit 100 - dew on lens

[Andyb90]

Hi Everyone,

I was out imaging a couple of nights ago with my Esprit 100. I had an Astrozap dew heater tape on the dew shield, positioned over the (front) lens. That was connected up to an Astrozap dual channel dew heater controller with the power set to medium.

Admittedly the heater tape is the 4" version, which is slightly small for the Esprit 100, but I noticed the lens had dew on it part way through the night. I'm not exactly sure how long it took for the dew to form. I'll find out when I process the images.

I'll be ordering the correct sized heater tape for the scope, but I was a bit surprised to get dew on the lens. Is there something extra I should be doing to prevent the dew forming? The temperature did go down to around -5c that night. Under those conditions is it best to always set the dew heater controller to max or even use a couple of heater tapes on the scope?

Andy.

[Xplode]

It's best to use the heater on the telescope tube instead of on the dew shield as with the heater on the dew shield very little heat will be transferred to the lens.

Mounted correctly you should normally get away with setting the controller to less than 50%

Edited January 3, 2017 by Xplode

[Owmuchonomy]

Precisely the reason I purchased a 'spare' hairdryer for my wife this Xmas, the sort that can easily be poked inside a dew shield. Works a treat after a few seconds. I just have it handy every so often.

[ollypenrice]

34 minutes ago, Owmuchonomy said:

Precisely the reason I purchased a 'spare' hairdryer for my wife this Xmas, the sort that can easily be poked inside a dew shield. Works a treat after a few seconds. I just have it handy every so often.

It works a treat for getting rid of the dew but will trash your subs for at least the next half hour, unfortunately. The optics need to be thermally stable for imaging, not subjected to radical changes. For visual it should be OK but we have found, here, that warm air heaters are no help in imaging. Pity!!

Andyb90, Ole is right, get the tape as close to the glass as you can because it is that that you should very slightly heat. You don't want to be heating the dewshield because you'll create currents in it which will damage the resolution while not really addressing the dew. What you could also do would be make the dead simple 'camping mat dewhshield' to put around your metal one and extend it 2x or more further out. This will have an enormous effect.

Olly

[Andyb90]

Below is a picture of my setup. The Skywatcher dew shield is fully extended.

The front lens is approximately level with 'Sky' on the dew shield logo. I removed the 2 screws from the dew shield, but as far as I can see it's not removable.

I could move the front CNC tube ring further back down the OTA and put a heater tape directly behind the dew shield, but it wouldn't be as close to the lens.

This one has me scratching my head a bit.

Andy

[Xplode]

You need to think about what conducts heat the best, the tube is in direct contact with the lens cell, but the dew shield usually has some kind of felt inside it so it doesn't scratch the tube, the felt transfers heat almost as bad as air.

The dewshield seems to be designed in a weird way on this scope, only half of it extends in front of the scope? Normally around 9/10 of the dewshield extends

[NigeB]

Hi Andy,

How long did you get into your session before dewing up?

I have a similar issue with my TOA-150. I have the dew strap wrapped around the tube immediately behind the dew shield, i.e. as close as I can get to the lens cell without being on the shield itself. My cell is closer to the back of the dew shield than it sounds like it is in the Esprit, but nevertheless, the strap is a few cm from the cell, and so the efficiency of this arrangement is poor. On bad nights I can dew up about 4 hours into a session (my observatory is in a hollow, which is is great at holding on to fog - that doesn't help). 

I've considered putting the strap underneath the dew shield so I can get much closer to the cell. On mine there are very small grub screws which hold the main shield onto the locking collar (the silver ring on your Esprit). It's possible your shield may screw into the silver collar. If you're going to try, I suggest you unscrew the big locking screws completely to avoid damaging that nice paint work, then wrapping some rubber bands around the collar, and the shield, to help you get a grip. It's a 2 person job, one to hold the collar with both hands, and the other to try to (gently!) unscrew the shield. If you manage, there's still the question of how to route the wire - aside from getting the drill out (never!) it would probably need to come out of the front, which looks odd and also makes putting the cover on a bit tricky - and you don't want a connector rattling around in there when it's all closed up, so securing the cable and connector inside the shield will be necessary, and that will interfere with your ability to extend/retract the shield.

All in all, I'd say Olly's extended shield suggestion is likely to be the most effective, and one which I'm going to try myself.

Nigel

 

 

[Andyb90]

47 minutes ago, NigeB said:

How long did you get into your session before dewing up?

It was definitely several hours. I had no problem during previous sessions. So I think the colder -5c temperature didn't help.

Must admit I hadn't considered the shield may screw into the collar. I purchased the scope from FLO so will check with them. If it can be unscrewed I might just remove it and use a homemade shield instead with the tape nearer the objective.

For now though I've ordered a 5" tape which is the proper size and will put it behind the dew shield. I'll also increase the power on the heater controller and make an extension for the shield too as suggested.

Andy.

[StarryEyed]

I put the strap over the shield above the lens cell. I then have a fexible dew shield (a cut section of camping sleeping mat ￡1) over the top of the strap which extends beyond the dew shield and held in place with doubled cable ties. This insulates the strap, Its cheap and if I need to take it off cable ties cost little to replace.