Cooling newtonian

[Peter Drew]

46 minutes ago, NGC 1502 said:

Indeed, that’s one of several things I don’t like about OO UK Newtonians, the tube diameter is not much more than the primary diameter, and the upper tube trim has an opening virtually the same diameter as the primary.

Having said that, my OO UK 10” Dob is a great scope that I enjoy using,  but would be better without those issues.  I lessened those problems by increasing the primary cell ventilation (earlier OO mirror cells had a solid back with just a single 10mm diameter hole) and removing the lip from the upper tube trim.

Ed.

 

And yet, the Mewlon has a rolled over front edge to their tubes that closely defines the primary mirror aperture.  ???      🙂

[JOC]

Just an idle thought - if the OP is imaging I imagine this calls for a very calm and stable environment.  Is there any potential problem arising from possible vibration from the proposed fans?

[Peter Drew]

7 minutes ago, JOC said:

Just an idle thought - if the OP is imaging I imagine this calls for a very calm and stable environment.  Is there any potential problem arising from possible vibration from the proposed fans?

There is always a potential problem but it is usually avoided by using very low vibration fans and attaching them in a manner that isolates vibration.      🙂

[John]

2 hours ago, NGC 1502 said:

Having said that, my OO UK 10” Dob is a great scope that I enjoy using,  but would be better without those issues.  I lessened those problems by increasing the primary cell ventilation (earlier OO mirror cells had a solid back with just a single 10mm diameter hole) and removing the lip from the upper tube trim.

Ed.

 

My OO primary cell could not be more ventilated really. There is a small fan there but I don't use it.

 

[andrew s]

11 hours ago, Peter Drew said:

One good start to mitigate tube currents is to have a decent clearance between the tube and the lightpath to the mirror.  The OP mentions millimetres clearance, I would prefer centimetres.     🙂 

I tend to agree and my home made Newtonians always had an inch! However there is the school of thought that adds baffles down tubes to stop stray light. These must push tube currents into the light path.

1 hour ago, Peter Drew said:

And yet, the Mewlon has a rolled over front edge to their tubes that closely defines the primary mirror aperture.  ???      🙂

Yes my 180 has a front opening of exactly 180mm. So anything off axis will suffer vignetting  but hay it's a narrow field instrument.  I suspect this is in part to avoid sky flooding. However,  I also suspect it is to add rigidity without spoiling the looks of the tube.

Regards Andrew 

[markse68]

45 minutes ago, John said:

My OO primary cell could not be more ventilated really. There is a small fan there but I don't use it.

 

I use the one I built for my dob religiously 😂 I noticed last year that I was having terrible tube currents even after an hour of use leading to awful star tests like this. Probably not helped by the insulating properties of the pvc drain pipe used for the OTA. I get much better more stable star tests now after only a few minutes with the fan on full blast. Vibration doesn’t seem to be an issue though when I was trying to split Sirius I did switch them off for best stability. In my mind it’s been a very worthwhile upgrade. 
 

I did plan do do something similar to op when I designed my 12” dob (that I didn’t build ) using an aluminium baffle plate that’d serve to mask the mirror bevel and deflect the air flow- seemed a good idea at the time. But I didn’t build it yet lol. 

[Robindonne]

2 hours ago, markse68 said:

I use the one I built for my dob religiously 😂 I noticed last year that I was having terrible tube currents even after an hour of use leading to awful star tests like this. Probably not helped by the insulating properties of the pvc drain pipe used for the OTA. I get much better more stable star tests now after only a few minutes with the fan on full blast. Vibration doesn’t seem to be an issue though when I was trying to split Sirius I did switch them off for best stability. In my mind it’s been a very worthwhile upgrade. 
 

I did plan do do something similar to op when I designed my 12” dob (that I didn’t build ) using an aluminium baffle plate that’d serve to mask the mirror bevel and deflect the air flow- seemed a good idea at the time. But I didn’t build it yet lol. 

Wow nice.  I like the camo print on the alu ring😉.   But nice to see its almost sealed when the fans are running, and when they dont run they offer maybe enough airflow between the fanblades.  Im glad is suffer from collimation bolts sticking out of the bottom.  Not very handy when trying to let the tube rest on its back, but handy if i choose to close the back like you did with the red plexiglass.  

No vibration issues with 3 big fans?

but again.  I dont know how often i will suffer from a mirror thats not on temperature.   Maybe its minor.  The description tells us it made from a low thermal expansion glass, but maybe all newtonian mirror are from that type of glass.  i live in the middel of the forest.  Dew is a standard here.  Almost daily.  But its darker then dark, and very quiet.   
 

i like the fact that this problem doesnt have just one solution.  Its not use this when you suffer from this or use that when you suffer from that.  
 

im curious how many think their setup is great while it has the possibility to give even greater views due to some unsolved an unknown mysteries, but comparing is so difficult.  You should Have at least 2 or more identical scopes to check if one gives better results then the other.  
 

The “mine gives razorsharp views” comments cant really be read as useful because of this, not being able to compare, issue.   
 

I do think i bring myself in trouble with this 3d printed ring😬 where this thread was all about.  I didn’t check the inside of the tube recently.  And when i did today, to see my margin around the mirror, i noticed I might run out of room to even reach the area im planning to mount the ring.  Only if another quattro owner can tell me the interior of the tube is easily being removed.  The baffle rings.  
maybe the lowest baffle ring can support 2 half 3d printed rings   But that lowest ring is still some cm’s above the mirror

Edited June 20, 2020 by Robindonne

[Robindonne]

4 hours ago, JOC said:

Just an idle thought - if the OP is imaging I imagine this calls for a very calm and stable environment.  Is there any potential problem arising from possible vibration from the proposed fans?

That is the plan indeed.  I hope to get an as good as i can setup for my journey into imaging.  It must be indeed a good balanced fan and mounting. 
I can do some visual with a spare setup.   But i know i wont (until now) feel the need to do long nights of visual enjoying the sky.  (Have to admit that hours can pass by before you know it while “stargazing”😬)

[NGC 1502]

4 hours ago, Peter Drew said:

And yet, the Mewlon has a rolled over front edge to their tubes that closely defines the primary mirror aperture.  ???      🙂

Well of course the Tak Mewlon is a premium instrument and well designed........but when I read the Kreige & Berry book on making Dobs I think I recall the upper tube assembly opening always being larger than the primary.  I’m certainly no optical expert but possibly as the 2 types of scope are very different alters the necessary design of the upper tube opening ?

I’d be interested if anyone has views on this 👍

[markse68]

You bring up a very valid point- John’s fan looks like it’s only going to have any effect cooling the back of the mirror and like has been mentioned- is that really the issue? Mine makes a semi seal at that bottom of the tube and shifts quite some air though that tube- if I leave the top cap on the tube and remove the eyepiece it’s like a hair dryer air flow out of the focuser on full blast 😉 I’ve found that very beneficial and I can see it from star testing but maybe my ota material made this more of an issue that needed solving than a metal ota tube would 🤷‍♂️
 

The fans are noctua quiet pc fans that run at a lower speed than average and I fitted a noctua speed controller. I usually just leave it running full blast but if I’m out a long time I’ll drop the speed to where I can barely hear them at all. With hand on ota I can detect they’re running but haven’t been able to see anything through ep. The plexiglass disk is somewhat isolated by Velcro attachment pads which likely helps too. The ota is no lightweight either at ~16kg which likely helps damp out vibration too

The other nice thing about the fans is when I bring the scope home from a night outside. I store it indoors and coming in from the cold the mirror instantly dews up and everything gets dripping wet. I run the fans for half an hour or so to quickly warm it all up again and clear the moist air from the tube to help prevent corrosion 👍

I have no idea if your air diverter will help- seems logical that it would but maybe it’ll impede airflow and that will outweigh the benefits? I think air flowing up past the mirror edge will tend to pull the air from the front of the mirror anyway by Venturi effect? Like you say the outcomes of these sort of experiments are very hard to measure 

Edited June 20, 2020 by markse68

[Robindonne]

5 hours ago, alacant said:

Hi

I've never come across this before.

How does a non-cooled mirror -without a fan- manifest itself? 

Cheers

I think, and its really only that, for what i have read, is that even when it doesnt show much suffering from dew, it still suffers from a warmer mirror and therefore maybe a warmer surface area.   
I’m not really sure, but starting to change my mind while as a rookie i thought the dreamsetup has a nice big apo in it to thinking perfect newtonians can do also be part of that dreamsetup.   And have to start with the tools i have.  Only own this 8”. 

[NGC 1502]

3 hours ago, John said:

My OO primary cell could not be more ventilated really. There is a small fan there but I don't use it.

 

That’s an excellent example of a properly ventilated cell.  It’s very different from older OO UK cells that were poorly ventilated, a solid closed back with just one central 10mm hole.

Ed.

[Robindonne]

3 hours ago, John said:

My OO primary cell could not be more ventilated really. There is a small fan there but I don't use it.

Even if that fan axle has the shape of a banana it still wont cause vibrations i guess

[alacant]

24 minutes ago, Robindonne said:

it still suffers from a warmer mirror

Ah, OK. I've never noticed it. I was wondering what I'd see if the mirror was warmer.

 

[jetstream]

17 hours ago, John said:

Is the cooling about removing warmer air from within the tube and, if possible, disrupting the boundary layer or is it about actually cooling the glass temperature ?

I've always considered the former to be the key thing rather than the latter ?

 

 

Not sure if youve seen this John but I found it interesting

 

https://www.fpi-protostar.com/bgreer/sep2000st.htm

[Robindonne]

5 minutes ago, alacant said:

Ah, OK. I've never noticed it. I was wondering what I'd see if the mirror was warmer.

 

😅

[jetstream]

5 hours ago, JOC said:

Just an idle thought - if the OP is imaging I imagine this calls for a very calm and stable environment.  Is there any potential problem arising from possible vibration from the proposed fans?

Intuitive thought JOC

"Fan Vibration

Detecting vibration

Microvibration in the image frequently goes undetected. The best way to detect it is by examining the Airy disk of a bright star at high magnification, and switching the fans on or off. The size of the Airy disk should not change. This test requires magnifications of at least 30X your aperture in inches (the May 2004 Sky & Telescope article describes this test in more detail).

Sources of vibration

All fans vibrate, and even the smoothest fans create a microvibration problem if improperly mounted. The primary sources of fan vibration are a) impeller blade imbalance, b) slight variations in the airfoil shape of each blade (called "reaction imbalance"), and c) bearing resonance"

 

 

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

[Robindonne]

1 hour ago, Robindonne said:

Only if another quattro owner can tell me the interior of the tube is easily being removed.

Excuses me.  I can just remove the back to reach the area🤪

[Robindonne]

44 minutes ago, markse68 said:

maybe it’ll impede airflow and that will outweigh the benefits? I think air flowing up past the mirror edge will tend to pull the air from the front of the mirror anyway by Venturi effect?

 

6 minutes ago, jetstream said:

The primary sources of fan vibration are a) impeller blade imbalance, b) slight variations in the airfoil shape of each blade (called "reaction imbalance"), and c) bearing resonance"

Do you guys mean that even if the vibration from the fan is eliminated, the view can still be affected by to much airmovement? Or do you only refer to fan vibrations?   

[jetstream]

1 minute ago, Robindonne said:

 

Do you guys mean that even if the vibration from the fan is eliminated, the view can still be affected by to much airmovement? Or do you only refer to fan vibrations?   

There should be no air movement... if the primary mirror is equalized along with the rest of the scope there will be no convection- and no need for induced air movement.

As always we observe with whatever is the least of ills- if my view is better with the fan I use it.

My very best views are with no fan- the 15" dob goes 600x on Jupiter and over 760x on lunar when equalized - very very sharp. Obviously seeing has to support this. Testing has shown me that even 2c-3c temp diff is enough to hinder views. I use a Fluke IR heat gun to test.

Suiters wobbly stack definitely at play here.

[John]

1 hour ago, NGC 1502 said:

That’s an excellent example of a properly ventilated cell.  It’s very different from older OO UK cells that were poorly ventilated, a solid closed back with just one central 10mm hole.

Ed.

The Orion Europa range had the "mirror cell" that you describe. They dropped that range when they moved to the VX series a few years ago. My scope was what used to be known as the SPX series.

 

[bottletopburly]

Came across this this week interesting read 4parts http://www.acquerra.com.au/astro/cooling/ don’t think anyone posted this link .

[Robindonne]

 

 I have been having a play this evening, but I don't possess a Newt so can't prove or otherwise the attached. Producing the sheet I have assumed standard temperatures and pressures for the specific heat capacity of air. Obviously if you live on top of a mountain the figure is going to be different. I have also not allowed for the figure of the mirror when calculating the surface area(s) as on amateur set ups the increase in surface area of the mirror due to curvature is miniscule. The calculations also assume dry air, as a consequence the calculated cool down times are probably in excess of what would occur in reality. I would however  be greatly interested in any feed back to determine if the calculated theoretical cool down time ties up with a real world situation. I have further left the sheet "unprotected" so if you want to have a play you can.

Mirror Cooling.xlsx 14.59 kB · 2 downloads

K

Here comes that big thank you for putting time and effort into this list.   I can’t really test it on my phone but does it have some calculators build in that work with different mirror sizes?

[Robindonne]

45 minutes ago, bottletopburly said:

Came across this this week interesting read 4parts http://www.acquerra.com.au/astro/cooling/ don’t think anyone posted this link .

Really interesting indeed.  It basically says nobody will ever fully benefits its newton without a very active cooling?   Wow and almost 2 decades later we still think about solutions😬

[John]

1 minute ago, Robindonne said:

Really interesting indeed.  It basically says nobody will ever fully benefits its newton without a very active cooling?   Wow and almost 2 decades later we still think about solutions😬

Making the best of the imperfect is what we are about I think