Primary Mirror heating

neilallison (Neil) · #1 22-05-2014, 01:41 PM

Hey guys

I'm looking to make a primary mirror heater for my 8" Newtonian using nichrome wire.
Does anybody have any suggestions as to the best way to fasten it onto the back of the mirror?

Thanks in advance
Neil

Joshua Bunn (Joshua) · #2 22-05-2014, 02:37 PM

Just a few thoughts, Probably use heater pads of some sort and stick them to the backplate so they radiate to the mirror, don't heat the mirror directly.

Josh

neilallison (Neil) · #3 22-05-2014, 02:56 PM

Thanks for that Josh.

I was thinking of maybe having a thin rubber mat between the back of the mirror and the heating wire. Do you think that would be Ok?

julianh72 (Julian) · #4 22-05-2014, 03:05 PM

There's a neat trick used in the 3D Printing hacker community to make a heated bed-plate using a printed circuit board (PCB), with one or more copper traces mapped out over the surface of the PCB as the heating element, and then using the smooth reverse (non-conductor) side as the "working face", so that the PCB acts to even out the surface temperature. For best results, use a sheet of thin cork or similar on the conductor side to retain heat, to ensure the heat flux goes to the hot working face, rather than just radiating away as waste heat.
http://reprap.org/wiki/PCB_Heatbed

There are many variations on this theme - glass or aluminium sheet as the plate, nichrome wire or power resistors as the heating elements, etc. This web site might give you some ideas:
http://reprap.org/wiki/Heated_Bed

barx1963 (Malcolm) · #5 22-05-2014, 03:06 PM

Neil
Is there a reason you are trying to heat your primary? Putting heat into it will create tube currents which will seriously disrupt your views.

Malcolm

Joshua Bunn (Joshua) · #6 22-05-2014, 03:13 PM

Quote:

Originally Posted by neilallison

Thanks for that Josh.

I was thinking of maybe having a thin rubber mat between the back of the mirror and the heating wire. Do you think that would be Ok?

Maybe, I havent tinkers with this before, Im going by whats on my scope from the manufacturer and from ideas ive gained along the way. Probably a good idea to avoid sticking things to the mirror to avoid restricting its movement when changing temps. And maybe a fan would help to, to avoid tube currents as Malcolm says.

neilallison (Neil) · #7 22-05-2014, 03:17 PM

Hey Malcolm

I have a heater on the secondary and the guide scope so I wanna get the whole unit heated so I can go a whole night without getting fogged up.
I only intend to put the smallest current through the primary heater.
I also intend to put a fan on the back too just to keep the air moving.

neilallison (Neil) · #8 22-05-2014, 03:20 PM

Quote:

Originally Posted by Joshua Bunn

Maybe, I havent tinkers with this before, Im going by whats on my scope from the manufacturer and from ideas ive gained along the way. Probably a good idea to avoid sticking things to the mirror to avoid restricting its movement when changing temps. And maybe a fan would help to, to avoid tube currents as Malcolm says.

I haven't had a proper look at the mirror cell to be honest so I'm not sure what options are available to me in regards to mounting the heater. Maybe I can mount it on the back of the cell rather than on the mirror itself.

Rick Petrie · #9 22-05-2014, 03:22 PM

I tend to agree with Malcolm here Neil. Have you got a fan at the back of your primary? What you normally are trying to achieve here, is for the primary to cool down to ambient temperature and avoid any difference of mirror temperature in relation to the outside temp.
If you are experiencing dew problems, you are better off directing the flow of outside air around and across the face of the mirror by using a fan. This will reduce the buildup of dew and not seriously affect the mirror as in heating it.
There is a post link here of a method used by an IIS member which I have similarly integrated into my telescope and it works a treat. You seriously don't want to heat the primary mirror in any way.
http://www.iceinspace.com.au/63-400-0-0-1-0.html

neilallison (Neil) · #10 22-05-2014, 03:40 PM

I haven't got the fan installed yet. Maybe I should install that 1st and see how it goes.

Thanks for your input guys

barx1963 (Malcolm) · #11 22-05-2014, 06:37 PM

Definitely try the fan first. I assume the scope is a solid tube one? I have yet to have an issue with dew on my primary in any scope, and I live in Colac which has some very dewy nights. Even my little 8" travel scope which has absolutely no tube has yet to fog up.

Malcolm

neilallison (Neil) · #12 23-05-2014, 02:22 PM

Quote:

Originally Posted by barx1963

Definitely try the fan first. I assume the scope is a solid tube one? I have yet to have an issue with dew on my primary in any scope, and I live in Colac which has some very dewy nights. Even my little 8" travel scope which has absolutely no tube has yet to fog up.

Malcolm

Hmm. Maybe I'm worrying about nothing then. It is a solid tube scope (Vixen R200SS).
I have almost got the fan assembly completed now. hopefully get to try it out over the weekend.
I'll let you know how it goes.

MrB (Simon) · #13 23-05-2014, 02:25 PM

Another vote for fan first.
Like others I've never had a dew problem on the primary with a fan.
The secondary is another matter, I'm still working on a fix for that .

barx1963 (Malcolm) · #14 23-05-2014, 04:07 PM

Simon
I used a Kendrick wrap around secondary heater on my 12" dob. It is on this page http://www.bintel.com.au/Accessories...7/catmenu.aspx
Needs a good controller though (Digifire 7 was my choice) as you only want a tiny amount of heat and the Digifire make setting 10% a breeze!

Malcolm

MrB (Simon) · #15 23-05-2014, 05:08 PM

Cheers Malcolm, I have exactly that, the wrap-around and the Digifire7.
I just haven't got around to installing it yet with work and a 3 week old bub and, of course, the endless cloud

neilallison (Neil) · #16 25-05-2014, 02:47 PM

I now have my fan installed (non-destructive mod) so hopefully, I'll get some time next weekend to try it out.
I will let you know how it all goes.

Cheers guys.

multiweb (Marc) · #17 25-05-2014, 03:38 PM

I did have a small nichrome wire glued to the back of my newt primary in a spiral pattern years ago and I was wondering why the focus went south even with very little heat applied

So not so good an idea. Long dewshield and fan seem to do a better job.

Varangian (John) · #18 30-05-2014, 11:05 AM

Quote:

Originally Posted by neilallison

Hmm. Maybe I'm worrying about nothing then. It is a solid tube scope (Vixen R200SS).
I have almost got the fan assembly completed now. hopefully get to try it out over the weekend.
I'll let you know how it goes.

Just for the record I have never had (or never read anywhere during my research on dew control) of a primary mirror fogging up. Has this happened to anyone? I've struggled through the worst dew conditions I can imagine and everything else went dewy but the primary as the solid tube OTA effectively acts as a dew shield for the primary

barx1963 (Malcolm) · #19 30-05-2014, 02:38 PM

The only mirror I have seen gog up was a 16" Lightbridge at Snake Valley a few years ago. Only other times are on mirrors under a cover in the morning.

alistairsam · #20 30-05-2014, 02:52 PM

hi,

I've had my primary dew up, but mine is a truss and I added some flocking material about 12inches high from the primary. That stopped dew formation for sometime, but then it would still dew up if I'm imaging high up. so I flocked the interior of the whole truss and now have no dew on the primary.
so adding a foam extension to the top of your OTA will reduce it a lot as the tube would need to be near vertical for dew to fall on the primary.
and the fan would also help by forcing air upward and out of the tube.

Cheers
Alistair