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Mak, dewcap and tube currents

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#1 luxo II

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Posted 04 August 2018 - 07:51 PM

Finally made a decent dewcap yesterday and tried it out last night. At one stage I was considering wrapping the whole OTA in EVA foam permanently applied with adhesive, but decided that would be a bit of a shame and in addition, once done, if ever I wanted to undo this it will mean complete disassembly and repainting the OTA. So rather than do that I tried this solution to see if it was sufficient to stop the tube current - and indeed it is.

 

The aims were:

 

1. Improve the baffling against stray skylight entering the OTA to increase contrast, and

2. Kill the tube current.

3. Prevent dewing of the corrector without needing a heater strap.

 

Materials used:

 

1 sheet of 5mm Coreflute, lightweight hollow plastic sheet like corrugated cardboard much beloved by sign makers, this cost $8;

1 piece of black 3mm EVA foam rubber to line it, or you could use flocking or even black paint.

Velcro strap 1.2m long.

 

Method:

 

1. Aligh the Coreflute so that the flutes run parallel to the OTA optical axis. Cut the length of the coreflute to cover as much of the OTA plus 1.2X the aperture of the scope (for the dewcap).

 

2. Using a craft knife and a long straightedge, slit the topside only of every second flute so that the sheet will flex easily in one dimension to wrap around the OTA.

 

3. Trim to fit the OTA and line the part sticking out the front with black EVA foam. Spray contact adhesive worked fine.

 

4. To fix to the OTA a velcro strap wrapped around the outside is sufficient.

 

Results:

 

Set up and ready to observe  takes me about 30 minutes. From the start there was NO tube current - zip, nada, none.

The contrast was indeed improved, both on DSOs and on the planets.

And best of all, no issue with dew all night.

 

This little experiment confirmed that its not really necessary to insulate the entire OTA and backend as others have done here on CN - at least not on this scope.

 

Will try again next week with an observing site that is typically -5 degrees C with the scope starting at a balmy 21 degrees C.


Edited by luxo II, 04 August 2018 - 08:00 PM.


#2 GUS.K

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Posted 04 August 2018 - 10:08 PM

Will be interesting to see how it works next Saturday, I’ve been looking at options for insulating my SW180 Mak.

Ivan.



#3 FlankerOneTwo

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Posted 05 August 2018 - 02:10 PM

Glad it worked for you. Would be interesting to know the starting temperature differential, as one would expect the degree of insulation required to eliminate visually apparent tube currents to depend on the gradient across the tube wall.



#4 luxo II

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Posted 05 August 2018 - 07:32 PM

The starting temp was 21C (stored indoors + aircon) and when setting up, outside was 13C. When I packed up at 11pm, 10C.


By having the bottom of the OTA exposed (non insulated) this end cools faster than the top. The central baffle and primary mirror is attached to the backend so by this means the baffle is cooled by conduction to the back and I suspect this is a helpful condition, compared to completely insulating the backend as some others have done.

There’s room for more experimentation IMHO to figure out the optimal solution.

FWIW the scope doesn’t have a fan and at the moment I see no reason to fit one.

Edited by luxo II, 05 August 2018 - 07:46 PM.

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#5 MortonH

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Posted 05 August 2018 - 08:51 PM

Pictures?



#6 FlankerOneTwo

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Posted 06 August 2018 - 02:12 AM

Thanks for the info! I took same approach, simply wrapping the tube itself with Reflectix and leaving the backend exposed. It's worked well so far in the summertime and fortunately not really any dew issues here in Vegas, but I'm wondering about the winter when I will be going from from 21 C or so to more like 0-5 C as you are planning.

 

Some of the conversation that I've heard regarding more rapid radiative loss from the skyward-pointing corrector plate has me thinking that leaving the back end uninsulated is the way to go. Because of the difference in cooling rates between the secondary cell and the corrector, some have reported that pre-cooling the secondary cell with cold packs for 30 min was even better than simply bringing the insulated OTA outside into the cold. I'll be curious to hear the results of your next session; I will be dealing with OTA heating rather than cooling for a while yet as we will be above 40C for a good bit, even with the clouds and smoke.

 

Clear skies!



#7 SteveFour86

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Posted 06 August 2018 - 02:25 PM

Pics of your handy work would be great.  



#8 luxo II

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Posted 06 August 2018 - 08:39 PM

Apologies for the delay - I've been sick with the 'flu and not feeling like pulling out the OTA.

 

1. The customer awaiting fitting. The CoreFlute is lying flat on the floor, with the inside facing up. The OD of the corrector cell is 1-2mm more than the OTA, so to make a neat fit, two strips of 3mm EVA foam are stuck to the CoreFlute sheet, where it fits on the OTA - one fits just behind the corrector cell, one towards the back end.

 

The broad black area has 3mm EVA applied to flok the front of the dewcap.

 

2. Closeup of the end of the Coreflute showing how I slit it. Doing this makes the sheet very, very flexible and it will wrap the OTA very easily.

 

3. Wrap it around the OTA, thus;

 

4. Secure with a velcro strap so it doesn't fall off,

 

5. The front view. The small gap is irrelevant. You can close it, if you like.

 

PS I've since discovered CoreFlute is also available in a BLACK sheet 2.5mm thick which would be even better as this would eliminate the need to blacken the interior.

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Edited by luxo II, 06 August 2018 - 08:44 PM.

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#9 SteveFour86

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Posted 06 August 2018 - 10:30 PM

That is awesome.  Thank you for sharing.




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