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Dew on Newtonian?

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#1 James Cunningham

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Posted 20 November 2012 - 07:58 AM

I just got a 5 inch Newtonian scope. Will I ever have dew problems with it. I only ask because the mirror is at the back and it would seem that the tube would act as a dew shield. Thanks.

#2 Jarad

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Posted 20 November 2012 - 08:07 AM

In general, with a solid tube newt you will not get dew on the primary (except on the worst of nights). The secondary, however, may dew up more easily.

Jarad

#3 James Cunningham

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Posted 20 November 2012 - 08:13 AM

I am new to Newtonian scopes. Where is the secondary?

#4 Mirzam

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Posted 20 November 2012 - 08:28 AM

The secondary is up near the front end in the middle of the tube. You may need to adjust it. :tounge:

I have seen steel tube newts experience dew problems with the primary. The steel gets real cold on a very clear night and cools below ambient temperatures. This eventually transfers some of the cold to the primary. As was mentioned, this is only likely to happen under pretty severe dewing conditions. The solution is to insulate the tube interior with cork or some sort of liner.

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#5 Scanning4Comets

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Posted 20 November 2012 - 09:06 AM

I've never had my mirror dew up on me or even my secondary for that matter. My tube is lined from front to back inside of the tube with a black felt like material. I also have a really long dew shield.

See attachment.

Cheers,

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#6 Achernar

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Posted 20 November 2012 - 09:28 AM

Yes, dew will be a problem for you because while the primary mirror is unlikely to dew, the finderscope and the eyepiece are another story. Diagonal mirror also dew up as well, that is why I use heaters on the secondary mirror, finderscope and eyepiece of my larger telescopes. Nights however where the secondary dews up are often really yucky for observing anyway, but you can use heater strips on the finder and eyepiece to keep dew at bay.

Taras

#7 jgraham

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Posted 20 November 2012 - 11:00 AM

This is a picture of my LXD75 N6 (6" f/5 Newtonian) after a night with heavy frost. Both the primary and secondary were clean and clear, but you can see hown the frost reaches into the tube and could effect the secondary.

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#8 jgraham

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Posted 20 November 2012 - 11:01 AM

This is rarely a problem, but if it happens you can reduce it by adding a dew shield.

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#9 denodan

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Posted 25 November 2012 - 01:52 AM

In general, with a solid tube newt you will not get dew on the primary (except on the worst of nights). The secondary, however, may dew up more easily.

Jarad


I had an 8" Skywatcher, solid tube, never dewed up outside, but as soon as I took it inside it had dew on the mirror, due to it being warmer inside then out, in mid winter, always had to leave mirrors uncapped till the dew was gone. Have the same problem with my Skywatcher 10" Flex Tube Dob, it's fine till I bring it inside.

It is exactly the same as if you wear glass, go from cold to warm and they fog up.

If you finished with your scope and a big temp differnce, un cap your mirrors and let the condensation go, else you will end up with mold on your mirrors in time, if left capped going from a cold outside to warm inside.

#10 omahaastro

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Posted 25 November 2012 - 02:05 AM

If you don't want to mess around with dew heaters... I find hitting my equipment periodically with a $15 DC hair dryer works great... also helps expedite evaporation of condensation.

#11 astro_baby

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Posted 25 November 2012 - 07:10 AM

Depends on the climate really. I have an 8" newt and frankly without a dewshield it woild be useless. The UK is quite damp and a scope will dew very readily. Newtonians will dew on the secondary quite readily depending on climate factors.

Here in the Uk we can have good seeing with bad dew issues.

I'd get a dewshield anyway to remove any stray light. Modern newts tend to have their secondary ver close to the end of the tube so stray light can enter the tube and focuser. A dewshield will kill that problem.

Every single star party I have been to in the UK has seen people who were sure they didnt need a dewshield knocked out on the first night. The following morning there is a run on camping mats at the local camp and hike store as these can be adapted to be a home made dewshield.

#12 Achernar

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Posted 25 November 2012 - 09:47 AM

It's just as bad in Coastal Alabama. Thjt is why my 10 and 15-inch telscopes are wired up with heates strips and a heavy battery to power them. The skies may be milky, but at least I can observe as long as I want without the telescope dewing up. I also use a shroud on my 15-inch, which is an absolute necessity to keep stray light out as well as dew off the primary mirror.

Taras






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