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Mak wrap - should really break the CoC

Stu

By Stu
in The Astro Lounge

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Stu

Stu

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Well, knock me down with a feather, it only blooming well works!

Unexpected clear skies tonight, so I made myself slightly unpopular with my dear lady who had been away all weekend, and put the scope out for a relatively quick look at the moon and a few test stars.

Immediately the out of focus diffraction rings looked much better, and the heat spike which normally disrupts them was not there. Any disturbance seemed to be seeing related.

Rigel showed much less flaring than normal, the secondary easily seen. More impressive was Sirius. Did I see the Pup? No, not a hint of it, but Sirius was again much better controlled than I'm used to and I felt like I would have a decent chance on a night of excellent seeing, and perhaps away from the surrounding houses.

The Trapezium stars were tighter than usual, and E was present quite clearly, with F coming and going with the seeing.

Images I took seemed to show round stars too, as opposed to the triangular ones previously which ties in with something Chris (I think) was saying.

The moon itself looked fab, lovely terminator detail and the earthshine showing in the scope clearly. I tried my binoviewers but found I could not reach focus. For a Mak, this scope seems to be a little limited in this regard. Anyway, I managed to just about get there with a x2.6 GPC and x2 barlow giving somewhere over x550, and I tell you, the image wasn't that bad! It was still holding up nicely! I've played around with some other kit now and should be able to reach focus at more sensible levels so am looking forward to trying that.

So, the Intes 'Hubble' Micro is a success, and the wrap will be staying, although perhaps I will try to tidy it up a little!

Two snaps attached just as a record.

20190210_191930-01.jpeg

PSX_20190210_222422.jpg

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Stu

Stu

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Just now, reezeh said:

I wonder if this hack will work with SCT's. If it does I'll be joining the Silly Telescope Club, or should that be Silly Looking Telescope Club? ?

 

No reason why not Reezeh. Maks are probably worse for cooling because of the thicker corrector plate but SCTs show the same sort of cooling issues too.

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reezeh

reezeh

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That's why I'm wondering, they're similar designs afterall. Now I'm seeing this and your preliminary results (gotta see over a few sessions to be sure) I'm almost regretting getting rid of that last bit of silver stuff that goes under laminate flooring.

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Chris

Chris

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7 hours ago, Gfamily said:

Hey, it's dark - who cares what it looks like?

 

 

7 hours ago, JamesF said:

That's pretty much my opinion :)

I didn't buy it to look at.  I bought it to look through.

James

so either you guys are bachelors? or you've got a garage or room in the shed to keep your scopes? If like me and you have to keep kit indoors in full view of SWMBO, it certainly helps if they don't find your hobby offensive to look at.  

For me, one of my scopes lives in the lounge, and other has to go in my wife's office where she spends 8 hours a day.   

Actually, I take back the first reason because thinking about it, if someone gave me a choice out of a bad or good looking scope and they both performed the same, I would pick the better looking one.....It's good to know you guys would grab the bad looking one leaving the pretty one for me;) (Just playing) 

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Stu

Stu

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3 minutes ago, Lockie said:

 

so either you guys are bachelors? or you've got a garage or room in the shed to keep your scopes? If like me and you have to keep kit indoors in full view of SWMBO, it certainly helps if they don't find your hobby offensive to look at.  

For me, one of my scopes lives in the lounge, and other has to go in my wife's office where she spends 8 hours a day.   

Actually, I take back the first reason because thinking about it, if someone gave me a choice out of a bad or good looking scope and they both performed the same, I would pick the better looking one.....It's good to know you guys would grab the bad looking one :) 

I do have a dedicated 'space' to keep my scopes out of sight which helps!

Believe me, I find the look offensive, particularly when the scope is actually a nice looking one. The performance improvement seems worth the ugliness though.

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Stu

Stu

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1 minute ago, bingevader said:

A nice initial review, but you mentioned heat differentials as the main reason for this.

You haven't mentioned the conditions outside.

Were the temps steady, did you get the rapid drop that this was to alleviate?

Further testing required. ;)

This was to alleviate the heat spike I have seen every time I have used this scope regardless of the conditions, and the poor stars shapes I have seen with every Mak I have used.

The improvement was significant (the reduction in flaring around Sirius was very noticeable) so despite your comments I'm happy that it is working. Obviously further testing  under more extreme conditions will prove how effective it is when temps are falling more rapidly.

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bingevader

bingevader

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Ok, a little confused, you originally said this was to alleviate a temperature differential caused by a rapid fall (or rise I suppose) in the outside temperature.

You now say it happens regardless of the outside conditions.

I am genuinely interested as I have a Maksutov-Cassegrain.

When my 'scope is at ambient temperature, I don't have a problem.

What is it that is causing the spike then?

I am genuinely curious and keen to understand what is going on with your 'scope. :)

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Stu

Stu

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23 minutes ago, bingevader said:

Ok, a little confused, you originally said this was to alleviate a temperature differential caused by a rapid fall (or rise I suppose) in the outside temperature.

You now say it happens regardless of the outside conditions.

I am genuinely interested as I have a Maksutov-Cassegrain.

When my 'scope is at ambient temperature, I don't have a problem.

What is it that is causing the spike then?

I am genuinely curious and keen to understand what is going on with your 'scope. :)

My apologies as my reply to you came across as a little blunt and that was not my intention. Sorry.

I don't have a full explanation for this, but I have been leaving the scope outside under a good cover and with dew heaters on gently to ensure it stays dry. Even then, I have found that I have been getting a strong heat spike in defocused star patterns which was reflected in the focused stars not looking great, flaring stars and poor diffraction patterns. This seemed independent of temperature conditions.

The insulation, based on this one experience, removed the heat spike and gave the stars much better diffraction patterns. I can't necessarily explain it, but it was a noticeable improvement which was very welcomed.

I will, of course, report back with future experiences.

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tonyowens_uk

tonyowens_uk

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I think the issue you are experiencing is due to sub-cooling of the OTA metalwork below ambient temperature under clear night sky. This is a recognised problem that has been investigated and solved elegantly some years ago by noted planetary observers with engineering inclinations such as Anthony Wesley of Canberra, AU: http://www.acquerra.com.au/astro/cooling/ballarat/  

The effect is identical to the one that results in frost on the roofs of cars but not on vegetation. Radiation exchange between the metal tube of a scope with enclosed optics and the sky, whose black body temperature is far lower than that of the earths surface results in subcooling not just of the tube exterior but also of the internal air adjacent the skyward surfaces, causing slow convection currents. The cure involves some or all of:

1. avoid use of enclosing tubes (truss scopes better than Newts better than SCT's better than Maks better than refractors)

2. minimise large thermal masses in the OTA (e.g. primary mirror)

3. protect the OTA exterior against subcooling below ambient temp (usually done by making the exterior surfaces reflective to longwave IR radiation using aluminised plastic film or by polishing a metallic tube exterior)

4. avoid the temperature of the OTA components deviating significantly from ambient air temperature (keep scopes in unheated buildings)

5. active temperature control of the primary (various methods)

6. stripping of boundary layers of warmer or colder air relaitve to ambient air (i.e. causing refraction) that might fall within the light path

7. making the scope tube from insulating material (or lagging it with cellular material supporting minimum convection and conduction through its thickness)

For general use 4. is often good enough. For high resolution use in so-so seeing (where Maks and large refractors justify their existence) the other factors come in. Remember the filtered vents and 'porous' internal tube baffles used by Intes-Micro on their instruments to try and minimise wall currents. They work to some extent. For larger apertures this isnt enough and the full suite of treatments  need to be considered if max resolution is an objective. Anthony Wesley took one direction with his 13" and 16" conical mirror planetary imaging Newts. Martin Lewis took a somewhat different direction with his 20" planetary imaging Newt: http://www.skyinspector.com/fossil-light--450mm-dobsonian

Both have delivered images of amazing quality.

I looked at the issue for my Intes-Micro MN86 around 18 months ago. I used a research-grade thermographic camera to assess the effect of removing the back of the mirror cell on cooldown of the 210mm dia x 30mm thick Sitall primary in my own use.

428047771_IMG_0718(1).thumb.JPG.f709f614b126d0644537072b67f6af02.JPG

I usually install one of various scopes kept in an unheated building with an insulated roof onto a permanent-sited outdoors mount. The thermographic video (taken on an evening in June) tells its own story.

EDIT: I should have added that the 50-frame timelapse TG videorecord below was shot at 2 minute intervals. Starting temperature of the primary was 14C . After an hour it was 10.5C. After 80 minutes it was 10.2 degrees while the upward-facing vegetation was just under 8C and the upper end of the tube appeared to have cooled to 0.4 deg below the 8.8C of the bottom end.

The last 8 frames of the video are corrupted by my having altered the temperature-to-colour mapping of the camera (very silly).

I concluded that:

1. the motors and counterweights of my mount were hot-running and thermally massive respectively

2. Not all vgetation is at the same temperature as cooldown proceeds, while cloud cover drastically affects sky temperature.

3. Even an exposed primary that is allowed to exchange radiation with surrounding objects still takes hours to equilibrate to within a degree of ambient air temperature

4. The sky-facing parts and upper ends of metal telescope tubes tend to cool faster than ground-facing parts.

5. Reading thermograms like these is mistake-prone due to the effects of reflection, and really accurate work takes a lot more prep and planning than I had time for!

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