4 replies to this topic

[jmillsbss]

I recently acquired the 10" Imaging Newt from AT.  Since it came from a fellow member, it has a few mods already, some I'll keep, others will probably be reconfigured.  The scope is the truss tube model here:

 

https://astronomics....809e8ac57&_ss=c

 

One of the mods I'll keep is the addition of the secondary heater.  After taking the secondary chassis/holder apart and seeing how it's made, (I had to reassemble the secondary when I received the scope) I'll probably switch it out with an Astrosystems version like my other visual Dob Newt, perhaps with a curved spider.  We'll have to see how much I Iove, or hate, the diffraction spikes.  I've done a little observing with the scope and the views are really good with a coma corrector, so I'm not swapping out the secondary mirror until I can get some images with the telescope.  However, I think the structure could be improved and will help what many on the boards have said about the slipping of collimation with long tracking.

 

The question is about the possibility of a dew heater on the primary, though.  I included an image of the scope so you can see that it is like so many of the truss model CDKs and RCs with the lower structure is only a barrel of sheet metal, 9"-10" deep.  I wonder if the AstroZap shroud around the center truss sections will be enough to protect the primary from dew issues?  The scope also has 3 small, really small, fans.  They look diminutive but move a reasonable volume of air.  My 120mm fan on my other Dob/Newt, a full tube type, however, moves considerably more air, so much that you can feel air movement at the secondary of that scope.  I have said for some time that I wanted to add a boundary layer fan to that scope but simply haven't done so.  I have the fan and all the wiring to complete the project, but someone's gotta start it for me as I just don't seem so motivated!

 

So will the modest amount of air from the 3 rear fans keep the primary dew-free?  Should I consider a boundary layer fan as well?  If a heater would be the best, does anyone have any resources or ideas that might help me get a plan together?  I know there aren't a ton of these scopes out there but I figure the CDK and truss tube RC guys and gals could chime in with their experiences?

[unimatrix0]

You may not want to heat the primary mirror.

I am just talking about in theory of how during summer I have to wait for the mirror to be ambient, because of being too warm and the outside air being cooler. 

 

IMO- heating the primary mirror could create turbulent air inside the tube and cause issues similar to very bad seeing. 

[AstroApe]

I wouldn't heat the primary. Keep the fans running and use a shroud should keep the primary dew free under the vast majority of circumstances. 

 

I've had the primary dew up on my 12" solid tube Dob before, but it was under very humid conditions, no fan running, and the tube was aimed fairly high towards the meridian for a long time... I just stuck a 2" dew heater strip behind the primary for about 10min on a low setting to get the dew off, then ran the fans for about 15-20min to get the mirror cooled back down, then just left the fans running to keep the dew off.

 

EDIT:

BTW, if you do decide to go with a primary heater, Kendrick Astro Instruments has a large assortment of sizes to choose from. They use to have fairly large heater pads that fit between the mirror and cell, but it looks like they've gone to a "red rope heater" which is basically a long thin heater strip that you run around the bottom of the primary.... it's a pretty interesting design that looks as if it'd be handy for all kinds of unique heating needs. Here's a link to them (bottom of page):

https://www.kendrick.../newtonian.html

 

 

Edited by AstroApe, 13 January 2025 - 11:08 AM.

[Knasal]

I wouldn't heat the primary. Keep the fans running and use a shroud should keep the primary dew free under the vast majority of circumstances. 

^^ This. I own 2 Dobs and have had them out in extreme dew conditions, and never heated the primary. As has been noted, I’d think you’ll be creating heat in the tube that will cause air turbulence.

 

Kevin

[jmillsbss]

You may not want to heat the primary mirror.

I am just talking about in theory of how during summer I have to wait for the mirror to be ambient, because of being too warm and the outside air being cooler. 

 

IMO- heating the primary mirror could create turbulent air inside the tube and cause issues similar to very bad seeing. 

 

 

I wouldn't heat the primary. Keep the fans running and use a shroud should keep the primary dew free under the vast majority of circumstances. 

 

I've had the primary dew up on my 12" solid tube Dob before, but it was under very humid conditions, no fan running, and the tube was aimed fairly high towards the meridian for a long time... I just stuck a 2" dew heater strip behind the primary for about 10min on a low setting to get the dew off, then ran the fans for about 15-20min to get the mirror cooled back down, then just left the fans running to keep the dew off.

 

EDIT:

BTW, if you do decide to go with a primary heater, Kendrick Astro Instruments has a large assortment of sizes to choose from. They use to have fairly large heater pads that fit between the mirror and cell, but it looks like they've gone to a "red rope heater" which is basically a long thin heater strip that you run around the bottom of the primary.... it's a pretty interesting design that looks as if it'd be handy for all kinds of unique heating needs. Here's a link to them (bottom of page):

https://www.kendrick.../newtonian.html

 

 

^^ This. I own 2 Dobs and have had them out in extreme dew conditions, and never heated the primary. As has been noted, I’d think you’ll be creating heat in the tube that will cause air turbulence.

 

Kevin

It doesn't make sense to me to heat the primary either.  Just that I've seen some scopes like this design where primary heaters were used.  I'd be more inclined to increase the volume of air across the mirror first.  What about the addition of a boundary layer fan?  It likely doesn't have to be a large fan?  I've never heated my secondary in my tube Dob/Newt and only once did I fog/dew the secondary, and this was before the addition of the high flow 120mm fan I mounted on the back of the primary structure.  That fan alone circulates the air well enough to prevent dew, in addition to speeding the cooling.

 

I maybe should have said, but the scopes all live out in the observatory now, and are generally ready to go within just a short time after rolling off the roof.  Certainly faster than moving from a heated space to the cold and damp outside air.  Still, after a few hours under the stars, the colder metal and glass can't always keep pace with the rapid cooling of the air.  There have been nights where it was very dry and others where it was thick as chicken noodle soup.  Still gotta have heaters and fans, only not as long to acclimate before starting.

Edited by jmillsbss, 13 January 2025 - 11:40 AM.