Dew problem

[edarter]

Hi, 

In the UK we are well in to autumn now and on the last couple of occasions the skies have been clear (which is rare!) my 130pds has developed dew on the secondary mirror well before the primary. I've read that dew shields are not needed for newtonians but would fitting one help reduce this problem? Also, do people fit dew heaters near the top of the OTA to help prevent this? 

[Benjam]

A dew hood will hold the dew at bay and give you a little longer time then not having one. Eventually you will succumb to dew unless you have dew heaters. I have a SCT and use a 8” dew band at the top just below the glass and a 2” band, either around my focal reducer or my eyepiece. Haven’t had a dew problem since, a worth while investment. 

[edarter]

Thanks for the reply, I will go the way of the heater, christmas list being written! But would I need one at the back of the newt for the primary and one at the top for the secondary? 

[adoldesa]

Hi, 

When I was using a newton for imaging (SW 200mm f/5) I had a long dew shield which did a good job at keeping dew and stray light at bay.

In general a dew heater is not necessary for the primary mirror and I have seen people mounting a very thin heater directly around the secondary mirror cell taking care to route the cable along the mirror mount to minimise obstruction. For observations another heater around thr eyepiece can save a lot of frustration and it is almost necessary if using a coma reducer for imaging. 

[edarter]

Thank you Adoldesa