Cassegrain dirty/smudged inside after 1 month?

[graemlourens]

Hey all!

I am very careful with my new telescope (Nexstar Evolution 6") and dusting it regularly (manual air pressure, no can, no touching any optics as i know one should not clean too often)

Yesterday i was dusting in full sunlight, and saw that somehow on the inside of the corrector lense there is a throughout 'smudge'!

I can not explain how this happened are frankly are pretty worried.

The only incident that 'happened' was my first night out, there was a lot of dew, so i took the telescope home, didn't close it or touch it (face down so no dust accumulates) and let it 'dry' over night.

But i can not imagine that the dew went 'inside' the telescope through the eyepiece opening (that was maybe open like 2 mins in total!)

How is this possible / is it normal?

I guess it will not affect my viewing yet, but would like to know the cause of this so i could be more careful in future.

Kind regards, Graem

[RT65CB-SWL]

Hi Graem / 'graemlourens' and welcome to SGL.

It maybe dew, that is one reason SCT and Maks require a long cool down before use your SCT.

Is it new? If "Yes!" then I would have a word with the dealer from where you purchased it. If you end up 'opening it up' and 'doing it yourself' you may end up voiding the warrenty/guarentee.

Also maybe worth sending an email to Celestron.

[graemlourens]

Jep its (relatively) brand new (1 month) and has been in the field only 1 time! (sadly weather in warsaw is terrible, so i was only able to practise short periods at home)

So how to prevent this? Not using the telescope (and keeping it covered) for 30-60 minutes before using it, would that do the trick?

And the only possibility now probably is that a professional opens and cleans it? (I don't like the idea at all .....)

Kind regards, Graem

[Tinker1947]

I had this happen and using a Desiccant Cap and changing the Gel Sachets as soon as they went green cleared the problem....

http://www.firstlightoptics.com/dew-prevention/flo-125-2-inch-desiccant-cap.html

[graemlourens]

Hey Tinker.

Thx for the hint.

But would this only prevent it from happening again, or would it solve the current problem without having to open&clean it?

Kind regards, Graem

[Northern Soul man]

Hi Graem, a very warm welcome to the SGL - I think everyone worries way too much about the conditions of their optics - Its a natural feeling as we want our scopes to be the best they possibly can - you have to take into account that if they were used in a totally "clean" environment, they would never get contaminated - but this is far from the "normal" use a scope gets.  

Its just down to a change in temperature that dew forms - you say you were looking in direct sunlight - this in itself will tell you that the closed tube may have been "warmed" a little due to the sunlight - especially if you moved the scope from a slightly cooler location into direct sunlight.  I try to keep my SCT as close to outside temperature as possible to minimise this change in temperature - your never going to avoid this - its just how it is - moving the scope from warm to cool is going to produce moisture in some form - whether its just a very fine "mist" on the optics to a full blown water droplet - its just mother nature.

I keep the scope in its box that it was delivered in and just take out before a session - when finished I just return it, leaving the back "cover" open to let the warm air dissipate, then cover the next morning.

These closed tube SCT's are prone to temperature fluctuations - its just their design - as soon as dew is present - just try these tips to minimise the problem - you may have heard that these SCT's are "dew" magnets - its a fact, just spend a little time controlling it and don't worry about it - I know its difficult - but best not to worry and just enjoy the hobby.

Paul.

[Peter Drew]

I have this problem sometimes with my 16" SCT if the dewing is bad, it takes a long time for internal misting to clear. A dewshield or heater band will help to avoid this issue. 

[Tinker1947]

Hey Tinker.

Thx for the hint.

But would this only prevent it from happening again, or would it solve the current problem without having to open&clean it?

Kind regards, Graem

It will clear but not over night takes a few weeks, so just keep the gel sachets fresh and your get a nice clear mirror....

[graemlourens]

Hello Paul & Peter

@Paul. So you would recommend to just ignore the dew that i now have, as its not going to really affect my astrophotography much?

I keep my Scope on the mount in the studyroom close to the window, there is nearly no heating there so its far below room temperature, but still it gets affected by sun. Do you think this happened due to that its in a room all the time with temperaturedifferences even if its 'closed' all the time?

@Peter I bought a Dewshield immediately the next day But do you think that internal mist really came from that first dew that 1 time?

What do you mean with 'a long time' ? days or weeks? (as its been 2.5 weeks since the incident)

@Tinker ok thx, will buy and see if it disappears, would be great as i really do not want anybody to take this expensive thing apart

Regards, Graem

[Northern Soul man]

Hi Graem - Thanks - its quite normal to be worried - the scope in an ideal world will be free from dew - you need to try to minimise the change in temperature where you store your scope - I have mine in an outside shed - as the temperature rises in the day and falls at night - if you store the scope inside then the warmth from the day will be a large temperature difference when taking out at night - this is the problem SCT owners have always had.  

Its really nothing to worry about - it will not affect your observing at all  - not sure if you can maybe try to store the scope in an outside un - heated building if you have one, but if you can't, just keep it away from direct sunlight and just place outside 30 mins before you start observing.  The heater band and dew shield are a real benefit to the SCT - I place mine on the scope and just leave on full power.

Its just the change in temperature that produces the dew - as said - try to keep this change to a minimum and remember when bringing the scope back in - this is when the dew will form - so leave the back cover off the back of the scope (where the star diagonal fits) and point the tube downwards to let the moist air rise from the scope - leave uncovered over night and check the next day - if all clear you can then replace the cover.

Really though Graem don't worry too much - I know its hard - I've owned SCT scopes for around 7 - 8 years and have found the best advice is not to worry and let it spoil your enjoyment of the night sky.

Paul

[RT65CB-SWL]

Jep its (relatively) brand new (1 month) and has been in the field only 1 time! (sadly weather in warsaw is terrible, so i was only able to practise short periods at home)

So how to prevent this? Not using the telescope (and keeping it covered) for 30-60 minutes before using it, would that do the trick?

And the only possibility now probably is that a professional opens and cleans it? (I don't like the idea at all .....)

Kind regards, Graem

30-60 minutes is about right!

If you want a cheap desiccant cap; you need some empty 35mm film cannisters --> drill a few small holes in the base of the cannister --> add a few desccicant sachets --> replace the cannister lid --> pop in your 1.25" e/p holder --> done!

[graemlourens]

Thx Paul.

I'm sadly one of those people that are always worried that they broke the equipment, and now the results will suffer

Its hard for me to accept that this will not affect the viewing/photo-results (as dew is water and must bend the light extremely i think), but i will have to trust you experienced astrophotographers.

So i guess it probably happened that time that there was a lot of dew, and i closed up the eyepiece-whole, so all the air was trapped there... But i never thought that the moisture even could get there, as its open so briefly for chaning eyepieces, but now i learn that i was wrong (amazing how many mistakes a beginner makes with telescopes!!!)

Kind regards, Graem

[Northern Soul man]

Hi Graem - the thing to remember is that the point of focus is a long distance away from the front corrector lens - its just at the back of the scope where the EP goes into the diagonal so the "view" of  the corrector lens is way outside focus - you may get a tiny amount of the  " dimming" of the light due to the "mist" (probably less than 1% of the total light transmission) - but more than this - the atmosphere will degrade the image far more due to the "dirty" atmosphere we are looking through than a "misty" corrector lens ever will, so don't let it spoil your hobby.

Nice thread.  Paul.

[graemlourens]

thx Paul, i didnt realize that fact of that its far from focus and therefor not that relevant.

The time i had dew in the lense i could barely see the moon, thats why i'm very worried about dew inside (but obviously i had a lot on the lense that time)

stupid question: taking a hairdryer and gently blowing up the eyepiece tube, would that help getting rid of the current dew?

[Tinker1947]

thx Paul, i didnt realize that fact of that its far from focus and therefor not that relevant.

The time i had dew in the lense i could barely see the moon, thats why i'm very worried about dew inside (but obviously i had a lot on the lense that time)

stupid question: taking a hairdryer and gently blowing up the eyepiece tube, would that help getting rid of the current dew?

This may just shift any dust about that's inside, and add more from the dryer itself....i wouldn't do it....

[graemlourens]

it also sounded very dangerous to me, and your reasoning is logical

i'll try with the demosturizing caps.

but do you think after 2.5 weeks that will still work? it looks like dirt for me and less moisture, but cant imagine after dewing up once inside that its so dirty already...

last question:

so if i have a session outside and even with dew cap (from celestron, not heated) it slightly dews up, after the session i should NOT cover either eyepiece hole, or corrector lense until the next morning right? but if facing down, will not a lot of dust gather on the mirror inside? or is that less bad than dew (i guess?)

thx for all your help, very comforting to know help is just a few clicks away.

[RT65CB-SWL]

I forgot t mention that removing the corrector plate has to be replaced in the same position it came out too!

I think there is or series on YouTube where one is stripped down and re-assembled - not for the faint hearted.

[Northern Soul man]

HI Graem, I would just not worry too much about it - the amount of dust that's going to fall inside the scope is going to be minimal - you need a "hole" to vent out the moist air that builds up inside - the more you use the scope, the more its going to get contaminated - its just how it is, these scopes are a closed tube assembly and its inevitable your going to get something in there.

Don't use the hair dryer, as mentioned, it will just circulate more dust around the inside of the scope - you could probably cover some of the hole - just leave enough of a gap to vent the moist air out.

Yes, leave the eyepiece end cover off, also the corrector cover and let nature do its stuff - I have left my 2 SCT's like this for the best part of 8 years and have never had a problem - after 12/18 hours just go and check the scope - have a look at the mirror and the corrector - if clear - re cap - its then ready for the next time you observe.

I wouldn't try taking the corrector off at this time - your going to have to do it way down the line - there are videos on the net to remove the corrector for cleaning  -but at this stage you would void the warranty - so best leave it on.

Just with general use - your going to get things inside the tube - your not using it in a sterile environment and there is no special way in which to store to minimise contamination and dew (only the advice mentioned from all the posters to this thread) - remember - all of us who own an SCT are the same - we just use them and store them - use them and store them again, its something we all come to live with - as soon as the scope is used a few times, the contamination begins - its all well and good for the manufacture of the scopes to be assembled in a clean environment - but during use we all do things the same way.

Paul.

[ollypenrice]

This is out of hand. Too many worries being raised! Scopes are tougher than you think.

Stand it upright, corrector down and the EP removed. Put a cotton cloth over the hole and stand the scope near (not too near) a mild heat source like a radiator. Hot air rises, as mentioned above, and all will be well in no time. Hot air rises.

As for dust, forget it. Does the secondary mirror in the middle of your lightpath ruin your view?? 

Olly

[graemlourens]

Hey Olly and Paul.

Thank you for your replies.

I am comforted and will just let the scope be. Important for me to know not to close the eyepiece after dew was present (even after reading a lot i did not know that). So all good.

Obviously i still don't like that my corrector lense looks dirty inside, but i'll just have to ignore that and go against my gut-feeling.

But generally i have now learnt to not worry too much about dust and those kind of things.

Thx all for your help!