Dob Dew

[frogman]

dew shield for secondry (http://www.astrosystems.biz/dewgrd.htm)

Might get one of these....

Anthony

[atalas]

Good find a Anthony ,that will do the job and the price is right!

Louie :cool2:

[frogman]

how do i find out the size of my secondary mirror ?????

12" GSO

[stringscope]

Anthony,

If you are having a dew problem with your dob and in the interests of simple solutions first, FWIW, you might wish to consider the following before investing in a heated secondary:

1. A general rule of ATM thumb for solid tube dobs is the tube should extend beyond the secondary for at least the diameter of the tube. In your case, assuming the tube is in the order of 330mm diameter you might like to consider making a dew shield from corrugated cardboard, closed cell high density foam etc to ensure the secondary has adequate dew shielding. The advertising photos for the 300mm GSO dobs look like the tube would be lucky to extend more than 150mm past the secondary. I suspect a simple (and cheap) dew shield will probably fix your problem in all but extreme conditions.

2. I understand the GSO dobs have steel tubes. I have read reports of owners lining the inside of these tubes with a thin layer of cork or felt to provide some insulation and assist with the prevention of "cold air" tube currents and resultant " potential overcooling and dewing of the optics. I have experienced dewing of both primary and secondary mirrors in a steel tubed Newtonian during winter in the Central Highlands on Victoria but have never seen dew on the optics of my Sonotube/Formatube dob in very similar conditions here during Canberra winters. This dob has a corrugated cardboard dew shield that extends the tube 450mm in front of the secondary, and this is for a 250mm diameter tube.

3. Heated secondaries have to be carefully managed as they have the potential to introduce significant heat plumes into the optical path. The Protostar web site has some interesting video footage of this effect.

4. Getting electrical power to the secondary can be an interesting exercise as well. To avoid increasing the diffraction from the secondary support structure by running wires along the secondary spider arms, it is preferable to use two of the secondary vanes as conductors. However for this to work, at least one will have to be electrically isolated from the steel tube and from the other spider arms.

Cheers,

[frogman]

excelent points raised stringscope thank you for your input

[frogman]

would it work it i was to have a starmap printed out and laminated.. then wrapped around the tube and held on with velcro ?
Kill two birds......

[iceman]

Excellent points Ian.. I'll definitely have to make a dew shield/tube extension for my dob.

What can be done about dew on the primary mirror?

[rmcpb]

Mike,

Without trying to sound smart don't observe close to zenith on dewey nights and make sure your scope is level between sessions. Also the dew shield discussed above should help any dewing of the primary as well.

Cheers

[iceman]

Rob I'm aware of that to avoid dew, but it's inevitable over the course of the night to be pointing closer to upwards.

I was hoping something like a fan would help to stop the mirror dewing up? (or slow it down at least)

[frogman]

Ice the dan on mine is just an 80mm computer case fan connected to my gel cel

[stringscope]

My 8" dob has a 10" ID formatube tube and the 203mm 1:6 ratio Pyrex mirror is held in a UO open aluminum cell. The formatube product is some form of high density cardboard and is very thermally stable and has good insulation properties.

I have an 80mm 12v blower fan mounted in the rear the tube. I sourced this fan from Jaycar and it is rated @ 34cfm (Cat YX-2512). I use this fan to assist with thermal equalisation of the primary mirror, dew protection for the primary mirror and I like to think that the 34cfm flowing through the front of the tube will also assist with keeping dew off the secondary mirror as well. This fan is installed so it is exhausting air from the bottom of the tube and drawing air in from the top. The rationale is that still night air is slightly warmer and less humid at head height as opposed to ankle height and this is therefore the preferable airflow direction from a dew minimisation perspective. This airflow direction also provides for a greater likelihood of a smoother airflow within the tube. I do accept the prefered direction for mirror cooling is the reverse of this with the fan's blowing air on the back of the mirror. However, I find with this cfm rating the 200X35mm mirror temperature stabilises within 15-20 min.

With this scope I have not had any mirror (primary or secondary) dewing problems in 9 years. Since moving to Canberra, during extended observing sessions (6+ hrs) in winter the tube has been very heavily frosted over at times and this has never caused a problem.

I have attached a photo of the fan installation on my dob.

I suspect the dew problem you are having with the primary mirrors in the GSO dobs might have some or all of the following causal factors:

Uninsulated steel tube is cooling very rapidly and possibly "supercooling" below ambient.

Air inside the tube and in contact with the tube walls cools, becomes denser and sinks to the bottom of the tube.

This cold air will flow around the mirror and cause the mirror temp to potentialy drop below ambient which increases the risk of dew forming in humid conditions.

If there is a plate behind the mirror preventing or restricting this cold air from exhausting from the base of the tube it will then pool around the mirror which will further increase the potential for dew formation.


Possible solutions for GSO dobs:

1. Install extraction fan behind mirror.
2. Insulate tube with cork of cloth.
3. Extend tube to a minimum of 350mm in front of secondary

Cheers,

[atalas]

Guy's you can hold dew off for a while " but it will get you eventually!

The best way to control dew is electronically .

Louie;)

[ballaratdragons]

Anthony,

I use my fan sometimes to cool my mirror down (when I am impatient).

Originally I noticed that I had turbulents in the tube when I left the fan on.

BUT!! I have since noticed that when I am viewing and the slightest dewing starts on the secondary, I turn the fan back on again and the dew does not affect either mirrors.

I think the original turbulents are caused my temperature variations inside the OTA. When stabilised to ambient temps the turbulents dissappear and the mirrors do not fog because of the slight draught blowing up the tube. I was going to make a dew sheild but found that the fan stops the dewing. And we have fog most nights here. My OTA and base get dripping wet but inside the OTA is dry. I am suprised at how long these Duracell 'AA' batteries last! Apart from using the fan for cool-downs I have also left the fan running for 4+ hours many times and they refuse to go flat. I am still going to make a dew sheild anyway, but for now, the fan works a treat. I usually pack up because I get too wet, not the optics! LOL. Funny but true.

I do have a dew sheild on my finderscope. It works great.

[rmcpb]

Mike,

Like I said I was not trying to be too smart in my reply but if we are having a discussion on dew we need to include some of the basics so people new to astronomy can learn some of the more simple methods to avoid dew short of fan installations,etc.

Maybe I could have said it better but that was the intention.

Cheers

[frogman]

yeah i have had my fan running all night (observing) but i still had a SMALL amount of dew on my secondry... mind you it was damn near raining.. just ask the boys that came up ! :)

[Orion]

Scope in a bubble! :P

[GrampianStars]

G'day Y'all
I made my own heater strips
details here:- http://www.dewbuster.com/heaters/heaters.html

& used aircraft tape to fasten on to the inside of my flexible "Orion" shield 1' from the corrector

I have not had any dew problems or curent eddies since even pointing to zenith.

[frogman]

Ok a new solution to my dew problem....$5 camping mat and some sticky velcro. the plastic reflected so i threw it !
try it out tonight

[Brian]

I use the same set up as stringscope (Fans exhausting out the rear of the scope) When things get damp at a star party, I',m usally one of the last to pack up. Unless you observe in severly cold conditions like I some times do, this works very well. I have a piece of surpluss nichrome heater wire coiled behind the secondary hooked up to 12V for backup, when temps get below freezing.

[Thiink]

Any chance of a pic (or two) of the fan from you 12" GSO guys? I want to see how they are setup from the factory.

[frogman]

There you go Thiink

[Thiink]

Thanks mate. Any chance of one taken a bit further back?

[frogman]

yeah youll notice a reddish ring around part of the pic thats where the power plug is supposed to be seated... but the supplied nut and bolts came loose and now i've lost them :)

[ving]

thanks, now can you take it off and put it on my scope? ;)

[frogman]

its only an 80mm computer case fan with a male felale end onit
make one reall easy
Do you want one made for when you come on the 7th

[Thiink]

Excellent, thanks Anthony! I was thinking of building something similar for my 10".

A question for you dew experts, would it be better mounting a fan under the mirror mount as in the above pics, or mounting the fan onto some form of board/"plug" (which would require bigger mirror-locking bolts) to force the fan to draw air from inside the tube/around the mirror, as in stringscope's setup on the first page.

[frogman]

no idea mate.... that setup works for me and you know the old saying,
"If it aint broke......."

Anthony

[ausastronomer]

Mike,

I have a dew shield that I made that extends my tube by 320mm. It does an excellent job as a dew shield and also as a light baffle. Ian indicates the tube should extend past the secondary by at least 1 x the scopes aperture. That is consistent with my thoughts as well being that the tube should extend somewhere between 1 and 1.5 x the scopes aperture past the secondary. My secondary only dews under the most unfavourable conditions and even then it takes an awfully long time for the secondary to dew up. I actually forgot to take my dewshield with me to the April mini star party, you could have had a look at it and judged how effective it is. I really think it a better option than a secondary heater for several reasons including those that Ian mentioned.

In addition to the dew shield there are sound practices you can follow that also delay the onset of secondary dewing.

1. Return the scope to the horizontal position when its not in use.

2. Cap the focuser when you remove an eyepiece. A 35mm film cannister works well for this. Keep the cannister in your pocket and drop it in when you remove the eyepiece.

CS- John B

[ausastronomer]

Simon,

Anthony's is a factory fitted blowing fan. There are 2 schools of thought as to whether the fan should blow on the primary or draw air down the tube. The majority of factory installed fans including the GSO dobs "blow" on the primary. I have an older GSO 10" dob to which I have installed a fan and it blows on the primary. My primary mirror cell has a steel cover plate and I have attached the fan to this. The newer scopes have a different mirror cell without the steel protection plate. I removed the plate to allow the scope to cool faster when I didn't have a fan but used the plate as a convenient method to mount the fan when I decided to stick a fan on it. I used a 12V DC 90cm brushless ball bearing fan from DSE ($20).

I am also in the process of fitting a 12V "air pump" to the scope to gently blow air around the face of the primary to "lift the boundary layer" I probably won't have this finished for another 2 weeks or so. I will post some pics of the entire setup when I have it finished. There has been a large amount of literature over the years about "removing the boundary layer" in tubed newtonians and I am convinced the results can only be positive.

If you care to post some digicam pics of the mirror end of the scope I can advise you about the best way to mount a fan to it.

CS-John B

[Thiink]

Thanks for the reply ausastronomer. I've never noticed it before, but there is already mounting holes for an 80mm fan. The original question is still valid though, should I just mount a fan to that and blow onto the mirror, or should I surround the fan and draw air from the tube.

Thanks. :)

[ausastronomer]

I would get an 80mm Fan from DSE and just mount it to those mounting holes and blow air onto the mirror. The idea of this is to cool the mirror faster so that it reaches thermal equilibrium and the images stabilise more rapidly. This wont help the dew issue and to alleviate that i would mount an extension tube/dew shield to the front of the scope as Ian and I both discussed in an earlier post.

CS-John B

[Starkler]

What some have done is to fashion a ring which seals against the inner wall of the OTA and with a hole with a diameter to just cover the primary mirror clips. This can give three possible advantages.

1: Channeling of the air from a fan behind the mirror to scour the warm air barrier layer from the mirror surface.

2: Act as a mask to hide any possible turned edge on the primary mirror.

3: Remove diffraction noise contributed by the mirror clips.

(None of this has anything to do with secondary dewing).

[iceman]

Geoff can you post any pics of something like that done? I've read about it before but am having difficulty "seeing" it.

[ausastronomer]

Mike,

I have 1 fitted to my scope so I can show you exactly what it is. The only difference being, mine has a slightly larger aperture than the baffle Geoff is talking about. I installed mine purely as a light baffle hence the slightly larger aperture. I previously had installed a mask exactly as Geoff described but subsequently removed it as it caused an issue with tube currents but at the time I did not have the cooling fan installed. I may have a fiddle there again and see what effect the sub aperture mask has in conjunction with the cooling fan and the boundary layer air pump, that may just all work nicely actually with 2 air sources helping to move the tube currents.

CS-John B

[Thiink]

I'm going for a drive to DSE tomorrow (I have to go to Orange, they closed our DSE down here in Bathurst.. :(). Whats the best way to feed the fan the required 12v? I could use one of the DSE 8x AA battery holders and use rechargeable batteries, but that only gives off 9.6v. I know the fans work on lower voltages, albeit at slower speeds, but is that going to be enough to cause enough CFM to be useful? What size/quantity of batteries does the factory fan use? I noticed the Orion branded fan/battery packs run 8x D sized batteries, ouch!

The only other thing I am going to need is a plug so I can disconnect the fan so I don't eventually crush the wires when I sit the tube on the floor inside the house while I am packing up.

[RAJAH235]

Simon, you can get a 12 volt 4 ah Gel Cell,(jaycar part#sb-2484 @ $35.00),which will power the fan for days. It's a small compact battery. The fan runs on about 150 ma., but of course you don't use it all night. As for plugs etc, use a DC plug/socket. eg; part # pp-0511 (2.5 mm female plug) $1.55, & part # ps-0528 (2.5 mm male inline socket). $2.45. or if you want to mount the socket on a bracket, use > part #ps-0524 Bulkhead male. $2.65.
HTH. :D L.

[Starkler]

John I recall a discussion on one of the yahoo groups about this and I raised the concern about having to run the fan all the time without a choice due to tube currents entering the light path.
The reply was that a sub aperture mask isnt required and just needs to be big enough to direct air toward the upper edge of the mirror to create turbulence and it will give the desired effect.

[ausastronomer]

Geoff,

I am aware of that but I want to try the sub aperture mask to eliminate the diffraction noise off the mirror clips, I already have the problem solved for the boundary layer.

CS-John B

[Starkler]

I'll be very interested to hear about the results.

[bytor666]

here's my fan on the back of my 12" gso scope. I then put a round piece of "foamie" on the back to keep stray light out.
I tested the fan about 2 weeks ago & it ran on USED batteries for 7 hours!!!!!
--------------------------------
Mark
12" gso scope