Dew heater and boundary layer fan for larger dobs?

[Ships and Stars]

Hi all,

I picked up my 20" Stargate dob near the end of viewing season here in Scotland around April, so haven't spent a great deal of time with it, but want to get it ship-shape for August onwards.

First question if I may - I definitely need to add a dew heater on the secondary mirror - the primary seemed ok the few times I've used it, but the secondary always dewed up in a hurry. How do you reliably power a dew heater, three or four fans and a GOTO Synscan system away from home, especially in the -5 or -10C temps I regularly get at my dark sky spot in Scotland? I'd guess a regular 7Ah power tank is a no! One obvious option I have available is the large 105Ah leisure battery in my campervan. I was thinking I could run a long cable from the van to the scope and have plenty of juice to power everything. At home I'd just use a 12v power supply like the ones from FLO. Ironically, my 105Ah leisure battery is £10 cheaper than the 7Ah Celestron orange power tank.

Second question revolves around cooling fans (none at the moment). I'll add three computer cooling fans on the back - there are already mounting spots for those. The bigger issue I see is aiming a good, vibration-free boundary layer fan at the face of the primary on a truss tube dob with the dew cover in place - I was thinking some kind of standalone fan sat near the base with a small plastic duct pointed at the mirror, perhaps through a small window cut into the dew cover and an exhaust port cut opposite. I've read a boundary layer fan can improve mirror-related seeing issues more than rear fans. I've also read a 1C difference between air and mirror temps is acceptable, and less than 0.2C temp difference the target for optimal mirror temps.

Thirdly, this begs a related question - if you're using a dew heater on the primary, wouldn't that heat the mirror up and impact views? Obviously not as much as a dewed-up mirror...😀

Any thoughts on any of this welcome - no need to address all of my musings here. References to existing articles welcome - I'm guessing most of these questions are frequently asked, except maybe regarding the boundary layer fan aimed at the business side of the primary...

Cheers!

Edited June 26, 2019 by Ships and Stars
clarify

[jetstream]

Lots of ideas out there but Mr Greer has done a lot of testing- this site has much good information. Microblur, boundary layer (it doesnt exist when equalized) and much more.

https://www.fpi-protostar.com/bgreer/fanselect.htm

[Ships and Stars]

9 minutes ago, jetstream said:

Lots of ideas out there but Mr Greer has done a lot of testing- this site has much good information. Microblur, boundary layer (it doesnt exist when equalized) and much more.

https://www.fpi-protostar.com/bgreer/fanselect.htm

Thank you jetstream! Microblur - that sounds nasty! 😁 Just want to squeeze as much out of my setup, as most of us do. Nice 24" dob by the way!

[jetstream]

Dew heaters can cause secondary astig if not installed right - just a heads up and I have zero experience with them.

You can see the boundary layer "crawling" across a mirror if you defocus a bit- boundary fans can cause issues and might blow dust across the face of the mirror. I would try those 3 rear fans first and collimate well.

What do you use to collimate? what f ratio is it?

[Ships and Stars]

Hi Jetstream, I use a Cheshire eyepiece for the secondary, and a cheapo laser for the primary, seems ok but the endcap with on/off switch has movement affecting the laser. I'll probably get something better, hotech or similar. The scope is F4. Don't have much time on it yet! Waiting for mid-August to start DSO hunting again. Thanks for the information!!

[Ricochet]

Google for barlowed laser collimation, much more accurate than a simple laser and your switch issues will have no impact (unless you mean sometimes it won't turn on). 

[jetstream]

22 minutes ago, Ships and Stars said:

Hi Jetstream, I use a Cheshire eyepiece for the secondary, and a cheapo laser for the primary, seems ok but the endcap with on/off switch has movement affecting the laser. I'll probably get something better, hotech or similar. The scope is F4. Don't have much time on it yet! Waiting for mid-August to start DSO hunting again. Thanks for the information!!

F4 likes (needs) good collimation.

I am not an expert, Khadder and Menard are...but...

We could discuss the attributes and flaws of different methods- up to you. Or, I can relate my experience with different methods. A sight tube is needed to center the secondary, it automatically sets the offset sec towards the primary and ensures it looks "round" under the focuser. Your cheshire will do this. I do this.

An accurate laser is better than the cheshire/sightube crosshairs IMHO to line up the sec to primary. I use this.

As Ricochet rightly states the primary needs a barlowed laser to collimate, when using the laser method. I use Glatters Tublug so I can watch collimation from the back of the telescope, very nice.

Thing is your Cheshire itself is great for it all... an accurate laser offers an advantage over the crosshairs on the sec IMHO. The Tublug/laser is a great, accurate way to conveniently line up the primary.

HTH.

 

 

 

 

[Ships and Stars]

Thanks yes, the Howie Glatter laser one the one I was trying to think of. I don't fully trust the cheap laser I have. Why spend all the money on a big scope and fancy eyepieces, then have collimation out because of a £15 laser, haha. So that shall be on my list as well. For the first time in my life, I'm waiting for summer to end (in a way) so dark skies return!

[Ships and Stars]

2 hours ago, Ricochet said:

Google for barlowed laser collimation, much more accurate than a simple laser and your switch issues will have no impact (unless you mean sometimes it won't turn on). 

Thanks for the tips on that Ricochet, I've heard of barlowed laser collimation somewhere along the line, but had forgotten about it. Shall look that up now, cheers.