The joys of owning an old banger of a scope

[markse68]

2 hours ago, CraigT82 said:

I think the plastic will be better than aluminium for thermal management, as the alu would radiate heat quicker?  I'm not sure. 

Thanks Craig

There seem to be different schools of thought on this but to my mind having something that settles to ambient temperature as quickly as possible without offering an insulation barrier to slow that equalisation down makes some sense ? 🤔 Parallax seem to think so but then they would as that’s their product lol... http://www.parallaxinstruments.com/tubing.html

 

I haven’t had opportunity to test the fans yet- I’m really itching to though- if only the weather would allow... They’re “quiet” pc fans though so quite low noise and vibration. Running through the speeds with the controller there is a resonance about mid way which I’ll avoid. If vibration is an issue I’ll just run them full blast for 15mins or so for rapid cooling then drop back to minimum speed for a gentle breeze to maintain equilibrium or just switch them off. 

[CraigT82]

That's very interesting... In that case, taking it to the extreme, I wonder if a truss frame OTA constructed from ally box section or angle and then enveloped in fabric would be best from a thermal management perspective?!  Box section/angle is much more readily available and cheaper than 10" OD tube!  Would probably need some sort of thermal shield to prevent warm air from the observers body from entering the tube.

Quite a few posts about long focus planetary newts over on cloudy nights: I've found the general consensus there is to have the tube ID oversized (a good 50mm larger than the primary) that way the warm air that gathers up against the sky facing tube wall is out of the light path and doesn't affect views.  

You've probably seen these videos on the FPI protostar website shwing Schlieren tests with the mirror in an inclined tube, they demonstrate the need for the oversized tube quite well. https://www.fpi-protostar.com/bgreer/sep2000st.htm

Saying all that though my fullerscope only has a 12mm gap between primary and tube wall, so I've considered drilling some holes around the tube just in front of the primary, to let this warm air out however I've not been able to bring myself to butcher the tube to that extent just yet!

 

 

[CraigT82]

Have you seen Anthony Wesley's segmented aluminium 16" newt?  Now there's an idea... no more banging the long tube on doorways!

http://www.acquerra.com.au/astro/equipment/Nemesis-16/

[markse68]

3 hours ago, CraigT82 said:

You've probably seen these videos on the FPI protostar website shwing Schlieren tests with the mirror in an inclined tube, they demonstrate the need for the oversized tube quite well. https://www.fpi-protostar.com/bgreer/sep2000st.htm

I hadn’t seen those despite being on that site to buy my new secondary 🤦‍♂️ Will check them out after work and after looking up what Schlieren means 😉 Thanks. 

Having an oversized tube makes sense but then none of my new bits would fit very well... so if I do anything it’ll likely stay the same 10” dia as the current tube.

When I first got the scope I did think about cutting the tube in half and using something like one of the clamp rings used to seal barrels like this-

should hold the tube halves accurately enough but tbh having it one piece is a godsend really as it’s ready to use in minutes with no messing- well apart from the cooling down. I did also think about making a truss tube assembly for it - It might be my increasing age but I’m really appreciating the simplicity of this plastic tube that I can just sling in the car- even if it is a bit awkward to carry up fire escapes. 

 

Edited December 16, 2019 by markse68

[markse68]

1 hour ago, CraigT82 said:

Have you seen Anthony Wesley's segmented aluminium 16" newt?  Now there's an idea... no more banging the long tube on doorways!

http://www.acquerra.com.au/astro/equipment/Nemesis-16/

Thanks, I hadn’t- it’s a beautiful piece of work and that mirror blank looks amazing. 

[markse68]

7 hours ago, CraigT82 said:

Have you seen Anthony Wesley's segmented aluminium 16" newt?  Now there's an idea... no more banging the long tube on doorways!

http://www.acquerra.com.au/astro/equipment/Nemesis-16/

Actually taking a closer look I don't really get his cooling system 🤔 He's using a sealed system by the looks of it with peltier elements on a cold plate at the end of the tube but it isn't in contact with the mirror. The outside is already cold though- I guess the peltiers would increase the temperature differential and speed up radiative cooling somewhat but there's no air movement in the tube or around the mirror- surely you need air movement to quickly move the heat away? Something like that would probably work great for a sealed tube like a Mak maybe. Perhaps he wanted to avoid blowing dust on the mirrors- perhaps it's very dusty where he lives? Be interested to learn if it worked well

Edit- this is really interesting stuff and he has documented his experiments thoroughly here http://www.acquerra.com.au/astro/cooling/

He is using fans to circulate the cold air from the Peltier cooled cold plate and is able to computer control the cooling to track the rapid ambient temp cooling where he is in Australia. Not sure I want to be lugging huge batteries around with me to power Peltiers but in an observatory this would be awesome. I might have to do some temp logging with the fan running and not to see what’s going on in there too.

Edited December 17, 2019 by markse68

[Paul M]

 

Don't often see "pimped" Fullerscopes. Nice to see this work in progress. It's looking good!

Anyway, I just blow on my mirror to cool it; I'm a Fullerscopes fan...

 

[markse68]

30mm Delrin arrived so I’ve made the cupped Velcro standoffs originally planned- should make location and attachment of the fan unit a bit easier in the dark at the expense of possibly transmitting more vibration- if the clouds ever clear I’ll find out 🤷‍♂️

[markse68]

The cupped standoffs were meant to make attachment in the dark a bit easier- self-aligning, but they didn’t work out. They directly coupled the fan vibration to the tube in a bad way. So I’m back to plain flat standoffs but larger diameter. This seems to work great- can barely feel any vibration in the tube and can’t see any difference even at high mag with fan on or off. I made a mistake measuring the id of the tube with a tape measure and ended up making the Perspex disk about 3mm smaller but this has turned out to be a blessing as it can be mounted without direct contact with the walls and the Velcro does seem to be working well decoupling the fans.

And the fans do really help- star testing I’m getting pretty stable diffraction rings in the Airy disk. I do seem to have some spherical aberration though- the rings are clear on one side of focus but a blur concentrated in the outer ring on the other with barely discernible rings. Could be the glass still not in perfect equilibrium  or the cell not supporting the mirror correctly or an imperfectly figured mirror I guess. But the image is much clearer than I’ve had before so I’m not going to fuss too much about it.

The new secondary was a very worthwhile investment too- no sign of the astigmatism I was getting star testing with the old mirror. Feels like all the improvements really are improvements and I’m getting the best from the primary now 😊

[CraigT82]

How are you getting on with your Fullerscope Mark? 

I've discovered an issue with mine in that the plastic tube flexes beneath the focuser when the imaging train is heavy (Adc, barlow, cam, and filter drawer). If I collimate the scope with the focuser pointing straight up and the turn the tube in it's rings so the focuser points to the side the collimation changes. This is an issue with my EQ mounted scope but probably not for your dob mounted one. 

I'm gonna try to get hold of a couple more tube rings to clamp around the OTA either side of the focuser to stiffen it up. I'm hoping it's not actually my focuser that's flexing as that's a more expensive fix! 

In the pics I've used the heavy body of my APM barlow, without the Barlow cell, to simulate the weight of the imaging train. 

[markse68]

Hi Craig, good thanks I think- well actually it’s been such a long time what with the weather that I don’t really recognise the stars up there anymore... 😬

I haven’t noticed an issue like that with mine but I made the focuser base quite broad and to fit the tube closely, and I’m only using it visually so probably nothing like your load. Are you sure it’s not the secondary shifting? I check collimation with a collimating cap each time I set it up  in normal use position with focuser pointing out sideways and it’s seems to be holding collimation pretty well- past couple of times it didn’t need tweaking at all which is nice