Dew - ways to prevent it, / left dirt on mirrors

[Aenima]

Dew is a problem when long sessions leave my scope dripping, and it has reached both mirrors leaving them blotchy and dull.

Does anyone have a remedy for either issue?

I have never cleaned a mirror before, not really wanting to dismantle anything - is there another way?

Will a fan of some kind -eg desk fan - moving the air help with dew? Does it need to be heat?

Regards

Aenima

[Pete Presland]

you could try a hair dryer

[NGC 1502]

Looks like Newtonian scopes in your sig.

If you are certain the mirrors need cleaning, a very careful wash in mild soapy water using pure cotton wool with no pressure, then rinsing with distilled water is the standard method.

But it takes a lot of crud on mirrors to affect the view. However some say dirty mirrors dew up quicker than clean ones. Don't check the mirrors for cleaning by shining a torch down the tube at night, most mirrors look dirty if viewed in that way. Have a look during the day. If you are not sure whether they need a clean, then they probably don't.

I've had the secondary mirrors on my Newts dew up in very cold & damp weather, so I use a camping mat extension to the upper end of the tube, and fit it at the first sign of dew. Usually that is enough. If dew does start to form I have a 12v dew gun that I point down the focuser after removing the eyepiece. The ultimate is a secondary heater to keep it a fraction above ambient temp, but my dew precautions work for me.

Regards, Ed.

[NGC 1502]

Hi again, as we cannot edit our posts now, I'd like to add that to delay dewing of the primary (and to bring it to ambient sooner) a 12v fan blowing on the back of the primary keeps the air moving and this helps prevent a Newt primary dew up in very damp conditions.

How much of a problem all this is to you should help decide whats best for your circumstances.

Regards, Ed.

[Aenima]

Cheers Ed.

Thank you, quite well timed there as tonight may be clear for a while here and yes, I did look at the primary at night - with a torch, my problem at the time being misted up secondary, even though after a few months of dew has left blotches i'm hoping to put off cleaning until absolutely necessary.

So even though dew is reaching primary along the tuibe of a 200p, a few inches of extra distance makes a lot of difference? I had previously imagined the primary 'out of reach' from dew being huge as it is, but definitely re-thinking it now though, ta.

Regards

Aenima

[Aenima]

Hi again, as we cannot edit our posts now, I'd like to add that to delay dewing of the primary (and to bring it to ambient sooner) a 12v fan blowing on the back of the primary keeps the air moving and this helps prevent a Newt primary dew up in very damp conditions.

How much of a problem all this is to you should help decide whats best for your circumstances.

Regards, Ed.

Ah. Thanks yes I have a medium sized desk fan available and was curious about the moving air being helpful for the scope, in mind of the dew problem.

Could I just bring the fan out and aim it across the scope? or along it?

Regards

Aenima

[NGC 1502]

Ah. Thanks yes I have a medium sized desk fan available and was curious about the moving air being helpful for the scope, in mind of the dew problem.

Could I just bring the fan out and aim it across the scope? or along it?

Regards

Aenima

You could use a desk fan but be very careful about using mains voltage outdoors in damp conditions. Also you would have to keep moving the fan to keep it blowing directly on the back of the primary.

I use the camping mat tube extension to slow down dewing of the secondary. I think its unlikely to help with primary dewing in severe conditions. I've only had my primary dew up a couple or times when everything is dripping wet.

Regards, Ed.

[sam mayo]

i have the 250px which i found to dew up bad so i used a camping mat and velcro/ducktape and it worked really well,i think i went over board i made a dew shield for the spotter and the focuser and all of the ota for when is bad, it works perfectly so havent changed it.

cost me £7 in total and an hour of bad diy skills.(think most people just use a dew shield and slightly extend the ota as mentioned above)

thanks sam

[Aenima]

Thank you guys,

A dew shield or three sounds like a good plan. I do find all the glass parts dew up so putting small one on finder etc. will be well worth a shot.

I have a goto mount so always have an extension nearby, i find usually that plastic and other stuff takes alot longer to get dewed upon than the glass and metal but i always approach the plugs fully aware of the electrical issue, wish me luck hehe-- ZZT!

Cheers,

Aenima

[sam mayo]

i did see in a picture that some one had taken a fan out of a desktop pc and attached it t the back of the primary (outside of the ota)then added a regulator and a small battery but i have wonderd if blowing cold air on the outside(back of primary)would help,would be intrestid to know

[Alan_B]

My Orion 12" Dob comes with a fan attached to the rear but I think it's mainly to assist cooling . Must say though that I've never had a problem with the primary dewing up

[umadog]

Primary mirrors are the last thing to dew. It is most likely to happen

on very damp nights, nights when a warm breeze suddenly appears, if

you've been out for a very long time, or if you've spend a lot of time

observing at high elevations. The primary fan is, as mentioned above,

for cooling and won't help with the dew. The dew shield is probably

the best you can do for the secondary. The only other option is a

secondary heater, but that's somewhat involved if you want to do it

right. Doing it wrong will mess up the image.

Dew dissolves particulates that have landed on the mirror leaves them

fairly well attached when it dries. Some dissolved stuff can attack

the coating over time. Dirty mirrors are more likely to dew over than

cleaner mirrors.

The coating will last longer if the mirror is clean. Cleaning

regularly will, therefore, prolong the life of the coating so long as

it's done carefully. Cleaning the mirror isn't difficult. You can

simply remove it (on almost all telescopes you'll have to remove it)

and soak it in water then rinse down with distilled water. That

process will get rid of most stuff and the coating will be safe. You

can safely use ethanol too.

[Aenima]

Thank you for the above replies, much appreciated.

I am obviously reluctant to dismantle my scope and unsure of how to remove mirrors without risking damage or not being able to put it back together again. Is there a way, with sky-watcher newts, to remove the primary in its cell and pretty much keep its position while cleaning - I doubt the collimation will hold but thats the general idea - and the secondary in its spiders?

As you may have guessed I havent taken a scope apart or removed any screws even, so the structure and build of them is still a mystery to me. I was thinking of getting some blowers or compressed air but looks as though these options are insufficient for my current troubles.

Thanks again,

Regards

Aenima

[umadog]

Taking out the mirror is part of Newtonian ownership, I'm afraid. You need to remove it from the cell to clean it. Collimation is something that needs tweaking nightly so the fact that it needs redoing after cleaning the optics is a given.

[Aenima]

Thanks, yes I guess its necassary to remove the cell and detach the mirror. I knew obviously the collimation will need redoing, I just didn't want to undo everything without knowing if I can put it all back again.

I was hoping to work out the simplest safest way to do it, and undo the least amount of screws etc as possible. Maybe there is a how-to on the net or something?

Regards

Aenima

[umadog]

It should be pretty self evident. The cell is held onto the tube by 3 or 4 screws, probably around the bottom edge of the tube. You undo those and pull away the cell. I usually stand the scope upright, undo the screws, then lift away the tube assembly.

Once you've done that, you'll see the mirror is held in place by 3 tabs. Those tabs have screws. You just undo those screws, remove the retaining tabs, and then the mirror can be lifted out. In most of these scopes these screw heads are located on the same side as the optical surface. So there's always the possibility that the screwdriver could slip or fall onto the aluminised glass. To guard against that you should place a soft towel over the mirror as you work. That's basically all there is to it. There really aren't very many components and they are put together in a very simple way so you can't go wrong unless you drop the mirror or drop something onto the mirror. Remove jewellery and watch before starting. You should have no problem getting it all back together. It's scary the first time but after that you'll think nothing of taking it all apart.

[Cath]

I've always wondered why we don't have the option to have a flat glass (or such like) front on closed tube reflectors so as to stop dust/water vapour ingress. Some SCT's have one (like my 6SE, although that is part of the optics I know).

I realise it would cut down a small amount of light (a couple of percent at most I'd have though) but my 6SE SCT seems to do just fine with it's front correcting lens.

[Aenima]

I've always wondered why we don't have the option to have a flat glass (or such like) front on closed tube reflectors so as to stop dust/water vapour ingress. Some SCT's have one (like my 6SE, although that is part of the optics I know).

I realise it would cut down a small amount of light (a couple of percent at most I'd have though) but my 6SE SCT seems to do just fine with it's front correcting lens.

Thats an interesting idea, I realise, like you have, that the extra glass would affect the optics and light path but something that is shaped to correct the difference (again, like the SCT's) might be suitable. I always thought the newtonians were built without glass for cost efficiency and to avoid the problem of colour aberration that refractors suffer, but it does also seem to leave the optics exposed and vulnerable to all kinds of peril.

Isn't there a newtonian cassegrain or schmidt-newtonian? Something like that? Haven't heard much about them untill sky-watcher made one, does it have glass?

Maybe it will catch on as a new type or someone will try it as a DIY project - either that or they will discover the reason why newtonians dont have glass fronts already.

Regards

Aenima

P.S Thank you Umadog for the advice about taking my mirror out. It is a bit scary at first but so was collimation and I got the hang of it eventually. I will be a lot more comfortable doing it now keeping in mind what you said. Ta.

[NGC 1502]

An optical window at the front end of a Newtonian can also be used to mount the secondary, so avoiding diffraction spikes as well as keeping the internal optics clean.

But the downside is the cost of a full aperture optically flat window, longer cooldown, and it would be much more prone to dewing as with SCTs and others.

With optics, everything is a trade off, so compromises are made.

Regards, Ed.

[Aenima]

Cheers, Ed.

Like it. Helpful and informative. -----

Anyway, I have now brought a camping mat, couldn't believe it - a big green lightweight foam one in a 99p store. I had thought at first it might be too heavy to use a camping mat, as I imagined them to be made of rubber and be much thicker, but this stuff seems perfect.

Will be cutting it up before next clear night, i'm assuming it needs to protrude about 3-4 inches from the opening, one layer thick, a good fit and removable? A smaller one for finderscope, too.

Also going to order a travel hair dryer and consider using my desk fan to move air around. All these things should prevent the level of dew getting that bad again, hopefully avoiding having to clean the mirrors as much as possible.

Thanks for the tips posted above - much appreciated.

Regards

Aenima

[Alan_B]

i'm assuming it needs to protrude about 3-4 inches from the opening

It needs to be a lot more than that . I left the width of my camping mat untouched so it projects the full mat width from the end of my dob

[Aenima]

Hi,

Thanks for posting,

So how much roughly did the mat protrude from the end?

Is it not effective unless the full length of the mat is used?

I was hoping to make the finderscope a shield from the same mat.

Regards

Aenima

[sam mayo]

mine over laps about 4 5 inches it hasent dewed up since including long sessions thought it was to insulate the scope to deter the dew but it might have been luck im certainly no expert would follow better advice

sam

[umadog]

Ed's right. A flat optical window on the front of the Newt would create at least as many problems as it solves.

[Cath]

Ed's right. A flat optical window on the front of the Newt would create at least as many problems as it solves.

Well I've never had the actual mirror on my SCT (6SE) dew up, but the front window (correcting lens) will almost always dew up without the dew shield. With the dew shield I've never seen anything but the eye piece dew.