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Dew heaters and focusing

Astrophotography Equipment Imaging Refractor
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#1 MSFFT

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Posted 28 July 2021 - 06:22 PM

Hi all,

I’ve been really happy (and more productive) ever since getting dew heaters set up on the 80mm refractor and guidescope. I have a Pegasus Powerbox Advance to do the automatic power adjustment.

Despite the differences of opinion in some previous threads:

https://www.cloudyni...paced-elements/

https://www.cloudyni...-dew-heater-go/

I’ve tried to place my heater on my dew shield as close to the objective lens as possible. Mounting on the OTA itself is tough with my ring setup.

Here’s my question: I notice my HFR starts to grow (from 1.5-2 to upwards of 4, units unclear) once the heaters kick on near 65% relative humidity. Is this typical for others? Do you just refocus and keep going? It takes about 30 minutes for the HFR growth to settle for me - do you just wait before continuing to image?

#2 Creedence

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Posted 28 July 2021 - 07:49 PM

I'm sure others will be along with a finer point on the cause, and you may already know this as well since it wasn't really in your question, but I imagine the heaters are creating turbulent air within the dew shield.  (que Captain Obvious comment here if you already had that assumption in hand).  It's worth mentioning though, because it informs the thinking behind my response:

 

Assuming this is the case, I'd say you're limited to two options- adjust placement of the heating element so most of its energy is absorbed by the objective (in my opinion, the most effective strategy).  If you can't get it on the OTA itself, it may still be worth experimenting over a few nights with placement to any extent possible.  Failing that, you could lower the heat and perhaps just let it get an earlier start (less effective of the two).  This could cut down on the turbulent air.

 

Failing both of those options, it sounds like waiting it out would be all there is left, and though you're seeing an improvement over time, it will always be there to some degree as long as it's heating the air along with the objective.  This isn't the easy solution that you'll want to hear, but modifying the ring setup to allow for more traditional placement is where you may want to invest your energy.


Edited by Creedence, 28 July 2021 - 07:51 PM.


#3 rgsalinger

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Posted 28 July 2021 - 08:23 PM

On nights when I expect dew, I run the heaters on a low setting from the beginning of the run. That way they don't really heat the objective on my 5" scope very much but they do keep the dew off. 

 

I'm kind of surprised that you need to start them up full blast(?) when the humidity hits 65. I don't even bother to look at mine until the humidity gets to around 90 or so. Comment?

 

Rgrds-Ross



#4 MSFFT

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Posted 28 July 2021 - 09:16 PM

I'm kind of surprised that you need to start them up full blast(?) when the humidity hits 65. I don't even bother to look at mine until the humidity gets to around 90 or so. Comment?

Rgrds-Ross


The way the auto-dew function works in the PPBA is that after about 65% relative humidity, it activates the heaters at 65% power. At higher RH levels, it seems to follow RH%=Heater power %.

While I’ll have to watch it more closely a few more nights, the effect I recall is that for the 30 (or more, potentially) minutes after I cross that threshold and the heaters kick on, the HFR grows. It will eventually level off at some steady state value and usually a manual or automatic refocus (depending on whether I’m awake or not) at that point will more-or-less fix the out-of-focus state.

#5 MSFFT

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Posted 28 July 2021 - 09:40 PM

I'm sure others will be along with a finer point on the cause, and you may already know this as well since it wasn't really in your question, but I imagine the heaters are creating turbulent air within the dew shield. (que Captain Obvious comment here if you already had that assumption in hand). It's worth mentioning though, because it informs the thinking behind my response:

Assuming this is the case, I'd say you're limited to two options- adjust placement of the heating element so most of its energy is absorbed by the objective (in my opinion, the most effective strategy). If you can't get it on the OTA itself, it may still be worth experimenting over a few nights with placement to any extent possible. Failing that, you could lower the heat and perhaps just let it get an earlier start (less effective of the two). This could cut down on the turbulent air.

Failing both of those options, it sounds like waiting it out would be all there is left, and though you're seeing an improvement over time, it will always be there to some degree as long as it's heating the air along with the objective. This isn't the easy solution that you'll want to hear, but modifying the ring setup to allow for more traditional placement is where you may want to invest your energy.

All good points. Because a re-focus after the heater/objective seems to get to some steady state temperature. I don’t think it’s air currents that are driving the HFR. But I could definitely be wrong.

I’ll definitely try placing the strap on the OTA side and seeing if it solves the issue - and still keeps the dew off.

Edited by MSFFT, 28 July 2021 - 09:40 PM.



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