Dew filters

[Joe beaumont]

This may sound like a silly question but what is the purpose of a dew filter and are they any good 

thank you 

[fifeskies]

The "dew filter" is an extended tube that fits over the front of your telescope. 

From previous I see you have an SCT telescope. This prevents condensation settling on the front corrector plate which ruins the view when using your telescope outside at night. 

In our damp UK they are very important if you want to get as much viewing time as possible , and perhaps even a heated one depending on where you are observing from (ie is it a very damp site).

They work very well which is why most refractors have quite long dew shields built in, and why heated bands are wrapped round the scope near the front.

 

They are not usually needed for Newtonian types as the long OTA should protect the main mirror from dew settling (in theory... but it still can)

 

They are easy to make DIY from thin cheap camping mat.

 

Edited November 13, 2020 by fifeskies

[Joe beaumont]

Thank you so much for your help 

[jonathan]

More commonly known as a Dew Shield.  There are also dew heater tapes, dew heater tape controllers, or on some telescope designs a light shroud serves to prevent dew also.

It should be noted that all dew prevention equipment can only do so much, a heavy dew will usually defeat dew prevention measures eventually.