The effects of dew?

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Larry 1969 United States of America
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The effects of dew?

#1

Post by Larry 1969 »


Good day all!
Yet another unsuccessful imaging session... I think this makes 5 in a row... (VERY FRUSTRATING)!!!
I checked all of my apps and conditions should have been good. No clouds or smoke and the dew point was 42F and the projected low was 46F.
Well, it turns out the low reached 40F and the outside of my rig was covered in dew in the morning...
I started my imaging plan after astronomical darkness and finished well before astronomical twilight and here's what I noticed...

My first bunch of frames looked pretty good (at least to me) so I went to bed. I awoke just as the plan finished and the last image displayed appeared much darker and I couldn't see the galaxy (I was imaging M33... Again...). M33 was easily identifiable in my first bunch of subs but I couldn't make it out at all toward the end of the session... Could that be dew formation? My last post turned out to be dew related (at least I think so) but the effect was different... There were several bright stars in that image and they appeared to have halos around them... My view of M33 doesn't have any bright stars near by so maybe dew is darkening the images???

I have the first 5 and the last 5 of my light frames along with a master dark, master flat and master dark flat that I will link to via OneDrive just in case anyone want's to take a look...

https://1drv.ms/u/s!ArYaleH4xdi5gysgiPX ... s?e=DuHqJ4

Thanks!

Larry
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Re: The effects of dew?

#2

Post by STEVE333 »


Hi Larry - Sorry to hear about your frustrating journey.

Have you considered using dew heater straps to keep your optical surfaces free of dew? I use one heater strap around the outside of my refractor at the location of the objective lens. I use a second one on my guide scope at the location of the objective. I never have any problems with dew.

You can check with Orion to find out what they recommend for your telescope.

Just FYI.

Steve
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Re: The effects of dew?

#3

Post by Larry 1969 »


Thanks Steve.
I'm looking into shields now. I'm not sure how I would heat my secondary mirror and I'd like to avoid the ridiculous cost of dew controllers.

Larry
For visual:
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Orion 8" astrograph 800mm @ F3.9
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Re: The effects of dew?

#4

Post by bobharmony »


Larry - the thing I notice on dewy nights is that the background gets brighter as the night goes on and dew builds up on the OTA. I have tried using a fan to keep air moving around the scope and discourage dew but haven't done enough with that to say if it actually works for me. I used that fan on warmer nights and oscillated it between the scope (for dew) and me (for insect control). I can report that in that setup it wasn't effective at keeping the mosquitoes away!

Like you, I am not thrilled about the idea of dew heaters, nor have I figured out a way to protect the secondary, camera sensor, or coma corrector lenses from the ravages of dew. I have put a cardboard "dew shield" around the top of the OTA and that may have some positive effect, not to mention it helps keep some of the ambient neighborhood lights out of the final product. I am looking for something sturdier and rounder to fit the 6" OTA.

Bob
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Re: The effects of dew?

#5

Post by Juno16 »


Sorry to hear of your woes Larry.

Yeah, dew is a real pia. Bob’ s fan is a really good idea. If you want to go cheap and are a do it yourselfer like me, I made a 8 watt and a 3 watt dew heater with nichrome wire and duct tape. I control it with a single $3 led controller I bought on Amazon. Been using it for over a year and cost under $20 for all (including the tape). Pre made versions aren’t very expensive either. If you are interested, let me know
Jim

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Re: The effects of dew?

#6

Post by Star Dad »


I currently use one of those cheap plastic binders on sale at WallyWorld... the $1 dollar type - some come with two pockets. I use a set of 8" hose clamps to clamp it to my OTA. On the other end I use a wooden expandable hoop on the inside to keep the binders in the shape of a tube. The hoop I got from JoAnn's on sale for $2 and some change. I used black duct tape to keep the hoop and plastic binder in shape. Seems to be working well.

Another type I'm trying is, again the hoops from JoAnn but this time a couple of wooden strips connecting the two hoops and then wrapping and gluing fabric inside the wooden framing strips and around the hoops.

Both shields are about 12" long.
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Re: The effects of dew?

#7

Post by Graeme1858 »


It's so disheartening when things don't go right. But if your first five and last five are anything to go by, I would be quite happy with that!

Image

There's a gradient diagonally and some colour tweaking and sharpening wouldn't go amiss but I just had a quick go. What do you get from your final stack?

As far as dew goes and to quote Jim, it's a pia! I have a heated dew shield but you would never know! I have never felt any change in temperature but it does draw current so something is happening. Having said that, it does work to a degree because the problem is worse when I don't use it. I have a hair drier, so I pause the sequence and use that every 10 minutes on the SCT and the guide scope. I got a 12V reptile tank heat mat to wrap around the guide scope dew shield but I've not done anything with it yet!

Regards

Graeme
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Re: The effects of dew?

#8

Post by Larry 1969 »


bobharmony wrote: Fri Oct 16, 2020 1:32 pm Larry - the thing I notice on dewy nights is that the background gets brighter as the night goes on and dew builds up on the OTA.
Bob
That's interesting Bob.
I can't really explain what's going on here... Possibly a few different problems?
I've had several fairly successful sessions in the past with the same capture and processing techniques so I'd like to think it's not operator error...
I'm going to have another go at it this weekend. I'm making a make-shift dew shield right now so we'll see...

Thanks!

Larry
For visual:
10" Skywatcher collapsible goto dob, various EP's and a Celestron StarSense auto align.

For imaging:
Orion 8" astrograph 800mm @ F3.9
Eq6-R Pro controlled by APT via EQmod with an OTA mounted mini PC
Tele Vue Paracorr Type 2 coma corrector
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Re: The effects of dew?

#9

Post by Larry 1969 »


Juno16 wrote: Fri Oct 16, 2020 1:41 pm Sorry to hear of your woes Larry.

Yeah, dew is a real pia. Bob’ s fan is a really good idea. If you want to go cheap and are a do it yourselfer like me, I made a 8 watt and a 3 watt dew heater with nichrome wire and duct tape. I control it with a single $3 led controller I bought on Amazon. Been using it for over a year and cost under $20 for all (including the tape). Pre made versions aren’t very expensive either. If you are interested, let me know
Thanks Jim!
I'm ok with do-it-yourself projects but have you used this solution on a newtonian scope? How do you heat the secondary?

Larry
For visual:
10" Skywatcher collapsible goto dob, various EP's and a Celestron StarSense auto align.

For imaging:
Orion 8" astrograph 800mm @ F3.9
Eq6-R Pro controlled by APT via EQmod with an OTA mounted mini PC
Tele Vue Paracorr Type 2 coma corrector
Altair Hypercam 26C
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Re: The effects of dew?

#10

Post by Larry 1969 »


Graeme1858 wrote: Fri Oct 16, 2020 4:54 pm It's so disheartening when things don't go right. But if your first five and last five are anything to go by, I would be quite happy with that!

Image
Regards

Graeme
That's far better than my attempts with that data! Did you incorporate my calibration frames?
What software did you use? Mine were so poor I discarded them...

Larry
For visual:
10" Skywatcher collapsible goto dob, various EP's and a Celestron StarSense auto align.

For imaging:
Orion 8" astrograph 800mm @ F3.9
Eq6-R Pro controlled by APT via EQmod with an OTA mounted mini PC
Tele Vue Paracorr Type 2 coma corrector
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Re: The effects of dew?

#11

Post by UlteriorModem »


Tom

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Re: The effects of dew?

#12

Post by Larry 1969 »


I'm ok with spending a bit on the heater itself if necessary but there has to be a reasonable controller (even a DIY controller)...

Larry
For visual:
10" Skywatcher collapsible goto dob, various EP's and a Celestron StarSense auto align.

For imaging:
Orion 8" astrograph 800mm @ F3.9
Eq6-R Pro controlled by APT via EQmod with an OTA mounted mini PC
Tele Vue Paracorr Type 2 coma corrector
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Re: The effects of dew?

#13

Post by UlteriorModem »


Tom

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Scope: 130mm f7 APO
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Re: The effects of dew?

#14

Post by Graeme1858 »


Larry 1969 wrote: Fri Oct 16, 2020 5:24 pm Did you incorporate my calibration frames?
What software did you use?

Larry

All three calibration frames dropped into DSS with the light frames and left to get on with it. Then Curves and levels stretched in Gimp.

Regards

Graeme
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Re: The effects of dew?

#15

Post by Larry 1969 »


Graeme1858 wrote: Sat Oct 17, 2020 7:36 am
All three calibration frames dropped into DSS with the light frames and left to get on with it. Then Curves and levels stretched in Gimp.

Regards

Graeme
Thanks. That's interesting. I couldn't get much out of it in StarTools and I'm not very proficient with GIMP. I do use it sometimes to make minor adjustments though..

Larry
For visual:
10" Skywatcher collapsible goto dob, various EP's and a Celestron StarSense auto align.

For imaging:
Orion 8" astrograph 800mm @ F3.9
Eq6-R Pro controlled by APT via EQmod with an OTA mounted mini PC
Tele Vue Paracorr Type 2 coma corrector
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Re: The effects of dew?

#16

Post by Star Dad »


UlteriorModem wrote: Sat Oct 17, 2020 2:52 am
I am liking this... but the only electrical resistance heater wire I've found required buying a spool of 100 feet. Any idea where one might get a smaller amount.

Edit: I guess I hadn't really explore Amazon - but I went to electronic supply places. I wonder if something like this: https://www.amazon.com/Kanthal-A1-Gauge ... B07CHSG169 might work? I could make a snake like layout of wire on the secondary and use a temperature resistive glue to hold it in place.
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Re: The effects of dew?

#17

Post by Larry 1969 »


This is getting interesting.
I've never dealt with dew before. Maybe because my old dob was collapsible and I never used a shroud so I was open all the time? It was never an issue.
I'm having some trouble understanding the purpose of the "controller"... Is it just a variable resistor used to adjust the temp of the heater?
If so, why not just run it on "high" all the time with no controller?
I like the resistance wire idea but how do you run it along the spider vane without it affecting your image? (diffraction spikes)

Larry
For visual:
10" Skywatcher collapsible goto dob, various EP's and a Celestron StarSense auto align.

For imaging:
Orion 8" astrograph 800mm @ F3.9
Eq6-R Pro controlled by APT via EQmod with an OTA mounted mini PC
Tele Vue Paracorr Type 2 coma corrector
Altair Hypercam 26C
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Re: The effects of dew?

#18

Post by Lady Fraktor »


Larry 1969 wrote: Sun Oct 18, 2020 6:07 pm This is getting interesting.
I've never dealt with dew before. Maybe because my old dob was collapsible and I never used a shroud so I was open all the time? It was never an issue.
I'm having some trouble understanding the purpose of the "controller"... Is it just a variable resistor used to adjust the temp of the heater?
If so, why not just run it on "high" all the time with no controller?
I like the resistance wire idea but how do you run it along the spider vane without it affecting your image? (diffraction spikes)

Larry
The controller is for power regulation and adjusts the temperature.
You only want 1-2° above the dewpoint to keep the optics dry without causing heat waves or heating the glass to much.

UlteriorModem (Tom) had a tutorial on how to make your own dew heaters if you do not want to buy ready made ones.
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Re: The effects of dew?

#19

Post by Star Dad »


The resistance wire I've seen is capable of something like 1200C so you don't want to use anywhere near full voltage. I've read something like 10-20% is more than sufficient. My concern is how much wire do I really need in contact with the mirror to get the desired heating? I suspect the controller is nothing more than a variable resistor. As Lady Fraktor says, we just need a very small amount of heat to keep the mirror clear. I'm not sure I would even heat the primary as I've only seen that get dew once or maybe twice in 11 years. But if I have to buy a 100 foot roll, I'll probably just go ahead and wire it as well.

AFA as the spider vanes is concerned: I intend to put a connector at the back of the mirror to the middle of the vanes. Then I can connect power to it only when needed. But thin electrical (insulated) wire well taped to the vanes should not really do too much to enhance the spikes.
"To be good is not enough when you dream of being great"

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Re: The effects of dew?

#20

Post by UlteriorModem »


Lady Fraktor wrote: Sun Oct 18, 2020 7:41 pm UlteriorModem (Tom) had a tutorial on how to make your own dew heaters if you do not want to buy ready made ones.
LOL I tired to follow the link to that article I did on that 'other' forum and of course the forum is pretty much dead 'database error'. I guess Guss just holds the URL for the hell of it. Wish I had saved that article somewhere.

Anyhow the original article is still available here. He even outlines how to make one for the diagonal.

https://www.dewbuster.com/heater-resistors.html

Also I just got a new diagonal holder with a built in dew heater from Protostar. He has a very elegant solution. Uses two of the vanes as conductors! No internal wiring visible! It does however require some 'special' attention to the mounting to insulate the vanes from the OTA.

https://www.fpi-protostar.com/s4vmnts.htm
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