Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Dew on Coma Corrector

  • Please log in to reply
19 replies to this topic

#1 Axys32

Axys32

    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 06 Jun 2020

Posted 29 September 2020 - 12:05 AM

Hey everyone,

 

I've lurked on the site ever since I started in the astrophotography hobby, but have never posted. This seemed like the type of niche issue that you all would have some great input on, and I would appreciate ideas.

 

I recently picked up an 8" Skywatcher Quattro with the Skywatcher coma corrector, and I also recently moved to New England. I quickly learned how big of a deal dew is once I got here. And when I say dew I mean heavy dew. Like dripping wet, "I hope my camera/computer works after tonight" dew. I picked up a Pocket Power Box and a couple dew heater straps - a 4" one for my refractor and an 8" one for my newtonian primary. Unfortunately, while the 8" band is doing a good job taking care of my primary, I'm finding that the secondary and even the coma corrector are collecting some really gnarly dew as well. I've seen some Kendrick Astro dew heaters that look like they'll do the trick for my secondary, but I'm not sure what exactly I need to do for the coma corrector.

 

I was thinking of picking up a small 2" heater band to go around the exposed optic of the corrector, but there are some very specific constraints on size. Check out the image I attached for my issue. The band needs fit around the outer diameter of the corrector (51mm), but within the inner diameter of the focuser opening (60mm) and not be wider than 15mm so as not to protrude past the end of the corrector. Is anyone aware of any solutions that might fit this envelope?

 

Thanks!

 

-Trey

Attached Thumbnails

  • PastedGraphic-4.jpg


#2 havasman

havasman

    Voyager 1

  • *****
  • Posts: 11,592
  • Joined: 04 Aug 2013
  • Loc: Dallas, Texas

Posted 29 September 2020 - 12:13 AM

If you do not have a fan behind your primary blowing onto the back of the mirror you may want to consider that solution. It will move air up the tube and out the front. That movement may very likely diminish the incidence of dew on your CC.

That's going to be a difficult place to fit a heating element.


Edited by havasman, 29 September 2020 - 12:36 AM.

  • Axys32 likes this

#3 DaveB

DaveB

    Surveyor 1

  • *****
  • Posts: 1,506
  • Joined: 21 Nov 2007
  • Loc: Maryland

Posted 30 September 2020 - 07:47 PM

I lived in central Mass until last year, so I know of the dew that you speak of. I went this route (mine is the 10" version): 

https://www.astronom...wo-notches.html

 

I originally purchased it for a 10" RC scope, but I sold that and now image with a 10" Newt, and it works just as well on that scope. In 2-3 years of use, I only had one night where dew reached the secondary, which is just as far in as the corrector. As Dick said above, a fan blowing air up the tube would also be beneficial if you can jury-rig something.

 

Dave


  • Axys32 likes this

#4 turtle86

turtle86

    Mr. Coffee

  • *****
  • Posts: 5,676
  • Joined: 09 Oct 2006

Posted 30 September 2020 - 08:23 PM

Hey everyone,

 

I've lurked on the site ever since I started in the astrophotography hobby, but have never posted. This seemed like the type of niche issue that you all would have some great input on, and I would appreciate ideas.

 

I recently picked up an 8" Skywatcher Quattro with the Skywatcher coma corrector, and I also recently moved to New England. I quickly learned how big of a deal dew is once I got here. And when I say dew I mean heavy dew. Like dripping wet, "I hope my camera/computer works after tonight" dew. I picked up a Pocket Power Box and a couple dew heater straps - a 4" one for my refractor and an 8" one for my newtonian primary. Unfortunately, while the 8" band is doing a good job taking care of my primary, I'm finding that the secondary and even the coma corrector are collecting some really gnarly dew as well. I've seen some Kendrick Astro dew heaters that look like they'll do the trick for my secondary, but I'm not sure what exactly I need to do for the coma corrector.

 

I was thinking of picking up a small 2" heater band to go around the exposed optic of the corrector, but there are some very specific constraints on size. Check out the image I attached for my issue. The band needs fit around the outer diameter of the corrector (51mm), but within the inner diameter of the focuser opening (60mm) and not be wider than 15mm so as not to protrude past the end of the corrector. Is anyone aware of any solutions that might fit this envelope?

 

Thanks!

 

-Trey

 

 

Maybe screw on a 2” barrel extender:

 

https://www.astronom...el-adapter.html

 

By itself it might solve your problem by acting as a dew shield.  If not it might still give you enough extra room for a heater band.


Edited by turtle86, 30 September 2020 - 08:24 PM.

  • Axys32 likes this

#5 DaveB

DaveB

    Surveyor 1

  • *****
  • Posts: 1,506
  • Joined: 21 Nov 2007
  • Loc: Maryland

Posted 30 September 2020 - 09:46 PM

Maybe screw on a 2” barrel extender:

 

https://www.astronom...el-adapter.html

 

By itself it might solve your problem by acting as a dew shield.  If not it might still give you enough extra room for a heater band.

The thing that you need to watch out for with an extender is whether it reaches into the light path of the reflector. If it does, then it may cause a flare on the side of brighter stars. It will definitely be an issue for imaging, maybe not so much for visual.



#6 turtle86

turtle86

    Mr. Coffee

  • *****
  • Posts: 5,676
  • Joined: 09 Oct 2006

Posted 30 September 2020 - 09:54 PM

The thing that you need to watch out for with an extender is whether it reaches into the light path of the reflector. If it does, then it may cause a flare on the side of brighter stars. It will definitely be an issue for imaging, maybe not so much for visual.

 

Good point.  Another possibility might be a heater band on the coma corrector, but on the other side of the tube.  Might be enough heat to keep the whole assembly dew free, including the exposed optic.


Edited by turtle86, 30 September 2020 - 09:55 PM.


#7 colinrm

colinrm

    Vostok 1

  • -----
  • Posts: 147
  • Joined: 16 Sep 2019
  • Loc: Akron, Ohio

Posted 01 October 2020 - 09:12 AM

If you do not have a fan behind your primary blowing onto the back of the mirror you may want to consider that solution. It will move air up the tube and out the front. That movement may very likely diminish the incidence of dew on your CC.

That's going to be a difficult place to fit a heating element.

This was my ticket.  I had horrible dew on my secondary and coma corrector, too.  I put a 120mm PC fan behind the primary, made a plastic baffle to keep the air going around the primary, and I added a little heater pad on a grill on the fan.  This blows warm air through the tube, but I don't seem to have any problems with tube currents with it.  Dew hasn't been a problem since.


  • havasman and Axys32 like this

#8 Axys32

Axys32

    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 06 Jun 2020

Posted 01 October 2020 - 07:32 PM

Thanks for all the feedback, everyone!

 

 

If you do not have a fan behind your primary blowing onto the back of the mirror you may want to consider that solution. It will move air up the tube and out the front. That movement may very likely diminish the incidence of dew on your CC.

That's going to be a difficult place to fit a heating element.

This was my ticket.  I had horrible dew on my secondary and coma corrector, too.  I put a 120mm PC fan behind the primary, made a plastic baffle to keep the air going around the primary, and I added a little heater pad on a grill on the fan.  This blows warm air through the tube, but I don't seem to have any problems with tube currents with it.  Dew hasn't been a problem since.

 

The rear-mounted fan sounds really promising. I think this will be my first line of defense. And it gives me an excuse to fire up the 3D printer to make a custom duct. Colin, what kind of heater pad did you use? I may start with unheated air and then move to a heated design if the ambient temp flow doesn't seem to be enough. 

 

 

Maybe screw on a 2” barrel extender:

 

https://www.astronom...el-adapter.html

 

By itself it might solve your problem by acting as a dew shield.  If not it might still give you enough extra room for a heater band.

 

This is an interesting thought as well. Like Dave mentioned, I'd be concerned with protrusion into the light path, but the bigger issue here is that the end you're seeing is not threaded, so I'm not sure how I could interface to an extender like this. Nonetheless, the thought of extending the tube just shy of the light path to allow more room for the dew heater is on my radar now. Thanks!

 

 

I lived in central Mass until last year, so I know of the dew that you speak of. I went this route (mine is the 10" version): 

https://www.astronom...wo-notches.html

 

I originally purchased it for a 10" RC scope, but I sold that and now image with a 10" Newt, and it works just as well on that scope. In 2-3 years of use, I only had one night where dew reached the secondary, which is just as far in as the corrector. As Dick said above, a fan blowing air up the tube would also be beneficial if you can jury-rig something.

 

Dave

 

I'm a little hesitant of dew shields for a couple reasons: 1) I had mixed luck with dew shields when I imaged with an SCT - granted, that SCT corrector plate is much more finicky than the secondary + coma corrector which don't have direct sight to the sky, so perhaps it's more effective on a newtonian? And 2) I'd like to keep the cross section of my telescope as small as possible due to wind catching my scope and ruining my guiding. I was actually seeing this issue last night when the wind picked up. I imagine it's even worse with an additional 12-20" of tube hanging off the front. Have you noticed any difference in wind resistance for your newtonian with/without the dew shield?

 

Thanks again, everyone!



#9 DaveB

DaveB

    Surveyor 1

  • *****
  • Posts: 1,506
  • Joined: 21 Nov 2007
  • Loc: Maryland

Posted 01 October 2020 - 09:51 PM

 

I'm a little hesitant of dew shields for a couple reasons: 1) I had mixed luck with dew shields when I imaged with an SCT - granted, that SCT corrector plate is much more finicky than the secondary + coma corrector which don't have direct sight to the sky, so perhaps it's more effective on a newtonian? And 2) I'd like to keep the cross section of my telescope as small as possible due to wind catching my scope and ruining my guiding. I was actually seeing this issue last night when the wind picked up. I imagine it's even worse with an additional 12-20" of tube hanging off the front. Have you noticed any difference in wind resistance for your newtonian with/without the dew shield?

It is true, the dew shield makes a nice sail frown.gif . It doesn't really affect me for two reasons: 1) my mount is overspec'ed for my scope, and 2) it is housed in an observatory with 7' walls. But I can definitely see that it could be problematic out in the open, especially if you're near the limits of your mount with the size, weight, and awkwardness of a newt. However, I would think that on a windy night, dew would be less of an issue. But if you are seeing bad guiding in the wind without the shield, it will only get worse with it.



#10 Axys32

Axys32

    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 06 Jun 2020

Posted 01 October 2020 - 11:10 PM

It is true, the dew shield makes a nice sail frown.gif . It doesn't really affect me for two reasons: 1) my mount is overspec'ed for my scope, and 2) it is housed in an observatory with 7' walls. But I can definitely see that it could be problematic out in the open, especially if you're near the limits of your mount with the size, weight, and awkwardness of a newt. However, I would think that on a windy night, dew would be less of an issue. But if you are seeing bad guiding in the wind without the shield, it will only get worse with it.

 

Purely weight-wise, my CEM60 is over spec'ed as well, less you apply the "1/2 the rated capacity" rule, in which case it's about dead on. In practice, I definitely see some serious guiding excursions when I hear the wind start to howl. And I wish I had an observatory! I travel for all my images...not even a backyard to image from :( 

 

You make a good point about windy nights being less dewy, though. That very well could be the case where nights that require the dew shield are mutually exclusive of windy nights. Last night, though, was quite variable. The first ~4 hours of the night were dead still, with the last 5-6 hours being really gusty. 

 

I think between the rear-mounted fan, the dew shield, and the potential draw tube extension I have a pretty solid solution set on my hands. Best of all, these options are all cheaper than a slew of dew heaters and since I have to travel, it saves power load on my batteries. 



#11 KLWalsh

KLWalsh

    Viking 1

  • -----
  • Posts: 847
  • Joined: 19 Feb 2014
  • Loc: North Georgia, USA

Posted 02 October 2020 - 07:52 AM

If those options don’t work for you, you could make your own dew heater from a bunch of resistors soldered in series.
Solder a number of them into a ring to fit around the drawtube when they’re covered with shrink sleeving.
The CC has a surface area of abt 3 sq inches. I think the ‘rule of thumb’ is 1 watt/sq inch.
Use Ohm’s Law, with the voltage you plan to use (9V? 12V?) to select the size and number of resistors you need to get to 3 watts.

#12 colinrm

colinrm

    Vostok 1

  • -----
  • Posts: 147
  • Joined: 16 Sep 2019
  • Loc: Akron, Ohio

Posted 02 October 2020 - 11:39 AM


The rear-mounted fan sounds really promising. I think this will be my first line of defense. And it gives me an excuse to fire up the 3D printer to make a custom duct. Colin, what kind of heater pad did you use? I may start with unheated air and then move to a heated design if the ambient temp flow doesn't seem to be enough. 

 

Here is a picture of what I was trying to describe:

 

1002201230_HDR.jpg

 

I used a piece of plastic sheet to cover the back of the mirror cell so it would keep the air going around the mirror.  I cut a square out of the middle for the fan.  Just a regular 120mm PC case fan.  I also got that little metal grill for it.  The heater is a generic 12V 12W polyimide heater pad I got from Amazon:

 

https://www.amazon.c...uct/B07P1H8N8H/

 

It was all hacked together quickly one day after dew covered all my optics the night before.  It works, so I haven't bothered making it nicer yet.

 

It creates a good bit of heat.  Enough to keep the primary, secondary, and coma corrector dew free.  I've had problems with tube currents before with heaters on the secondary, but this setup hasn't been a problem.

 

The ultimate test was one morning when I woke up around daybreak to check stuff out after an imaging sequence.  It was so foggy out that I could barely see 100ft away in my yard.  All optics were perfectly dew free within the scope, even though everything else was soaking wet.


  • Alterf and PederP like this

#13 Axys32

Axys32

    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 06 Jun 2020

Posted 02 October 2020 - 11:02 PM

If those options don’t work for you, you could make your own dew heater from a bunch of resistors soldered in series.
Solder a number of them into a ring to fit around the drawtube when they’re covered with shrink sleeving.
The CC has a surface area of abt 3 sq inches. I think the ‘rule of thumb’ is 1 watt/sq inch.
Use Ohm’s Law, with the voltage you plan to use (9V? 12V?) to select the size and number of resistors you need to get to 3 watts.

 

I've considered this route - I'm really trying to avoid letting my scope get too "hacky," though. But you're 100% right - if I comes to it, I wouldn't take this option off the table. 

 

 

Here is a picture of what I was trying to describe:

 

attachicon.gif1002201230_HDR.jpg

 

I used a piece of plastic sheet to cover the back of the mirror cell so it would keep the air going around the mirror.  I cut a square out of the middle for the fan.  Just a regular 120mm PC case fan.  I also got that little metal grill for it.  The heater is a generic 12V 12W polyimide heater pad I got from Amazon:

 

https://www.amazon.c...uct/B07P1H8N8H/

 

It was all hacked together quickly one day after dew covered all my optics the night before.  It works, so I haven't bothered making it nicer yet.

 

It creates a good bit of heat.  Enough to keep the primary, secondary, and coma corrector dew free.  I've had problems with tube currents before with heaters on the secondary, but this setup hasn't been a problem.

 

The ultimate test was one morning when I woke up around daybreak to check stuff out after an imaging sequence.  It was so foggy out that I could barely see 100ft away in my yard.  All optics were perfectly dew free within the scope, even though everything else was soaking wet.

 

Colin, this is great! I've found a little USB-powered 80mm fan with a built-in speed switch that'll be perfect for my smaller 8" scope. Your plastic sheet cover was exactly what I was thinking of printing. I'll probably velcro the printed duct to the rim where my collimation knobs are and I'll print in some bolt holes so I can screw the fan directly to the printed piece. Two questions: 1) do you ever have issues with vibration? And 2) does the heater pad actually provide noticeable heat in the air itself? I'm wondering whether my 8" dew heater strap will have the same effect or if I need to pick up something like this... Again it kind of comes back to not wanting my scope to turn into a Frankenstein of my mediocre soldering skills lol. 

 

-Trey



#14 PederP

PederP

    Sputnik

  • -----
  • Posts: 30
  • Joined: 23 May 2020
  • Loc: Denmark

Posted 03 October 2020 - 02:19 AM

Here is a picture of what I was trying to describe:

 

attachicon.gif1002201230_HDR.jpg

 

I used a piece of plastic sheet to cover the back of the mirror cell so it would keep the air going around the mirror.  I cut a square out of the middle for the fan.  Just a regular 120mm PC case fan.  I also got that little metal grill for it.  The heater is a generic 12V 12W polyimide heater pad I got from Amazon:

 

https://www.amazon.c...uct/B07P1H8N8H/

 

It was all hacked together quickly one day after dew covered all my optics the night before.  It works, so I haven't bothered making it nicer yet.

 

It creates a good bit of heat.  Enough to keep the primary, secondary, and coma corrector dew free.  I've had problems with tube currents before with heaters on the secondary, but this setup hasn't been a problem.

 

The ultimate test was one morning when I woke up around daybreak to check stuff out after an imaging sequence.  It was so foggy out that I could barely see 100ft away in my yard.  All optics were perfectly dew free within the scope, even though everything else was soaking wet.

That is inspiring.smile.gif

Wonder if vibrations from the vent will be too much for astrophotography?

Tried copying your idea with the stuff I had. An alu-duct with a heater around it.

IMG 6044

I will make a plastic seal around the fan.


Edited by PederP, 03 October 2020 - 02:20 AM.


#15 colinrm

colinrm

    Vostok 1

  • -----
  • Posts: 147
  • Joined: 16 Sep 2019
  • Loc: Akron, Ohio

Posted 05 October 2020 - 09:03 AM


Colin, this is great! I've found a little USB-powered 80mm fan with a built-in speed switch that'll be perfect for my smaller 8" scope. Your plastic sheet cover was exactly what I was thinking of printing. I'll probably velcro the printed duct to the rim where my collimation knobs are and I'll print in some bolt holes so I can screw the fan directly to the printed piece. Two questions: 1) do you ever have issues with vibration? And 2) does the heater pad actually provide noticeable heat in the air itself? I'm wondering whether my 8" dew heater strap will have the same effect or if I need to pick up something like this... Again it kind of comes back to not wanting my scope to turn into a Frankenstein of my mediocre soldering skills lol. 

 

-Trey

Yeah, it's a bit ugly, but it at least works!

 

I tried just the fan previously and had dew form on the primary one bad night anyways.  With the heater pad, like I said, it never dews, even during intense fog.  Just the one or two degree difference from the heating pad seems to be enough.  The heating pad is quite warm to the touch when it gets full power.

 

And I have not noticed any vibration problems from the fan.  My camera has a fan mounted within it, so I think fans might not be too bad overall.  Yes, that's just some 3/16" plastic sheeting that I horribly miscut the collimation bolt holes out of, and I velcroed it to the mirror cell.  Now that I've got an observatory, maybe I'll work on making it all a bit prettier.  smile.gif

 

 

That is inspiring.smile.gif

Wonder if vibrations from the vent will be too much for astrophotography?

Tried copying your idea with the stuff I had. An alu-duct with a heater around it.

 

I will make a plastic seal around the fan.

I bet that'll work!  Not you might need something to cover the outside back of the primary, so the warm air can only go around the mirror and up the tube.  It might just "bounce" off of the primary now and not affect the secondary at all.
 


Edited by colinrm, 05 October 2020 - 09:05 AM.


#16 Axys32

Axys32

    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 06 Jun 2020

Posted 05 October 2020 - 10:16 PM

Yeah, it's a bit ugly, but it at least works!
 
I tried just the fan previously and had dew form on the primary one bad night anyways.  With the heater pad, like I said, it never dews, even during intense fog.  Just the one or two degree difference from the heating pad seems to be enough.  The heating pad is quite warm to the touch when it gets full power.
 
And I have not noticed any vibration problems from the fan.  My camera has a fan mounted within it, so I think fans might not be too bad overall.  Yes, that's just some 3/16" plastic sheeting that I horribly miscut the collimation bolt holes out of, and I velcroed it to the mirror cell.  Now that I've got an observatory, maybe I'll work on making it all a bit prettier.  smile.gif
 
 
I bet that'll work!  Not you might need something to cover the outside back of the primary, so the warm air can only go around the mirror and up the tube.  It might just "bounce" off of the primary now and not affect the secondary at all.

 

Colin, thanks again for the tips! I finished the print of my fan enclosure last night and got it installed today! Check it out! I can feel a nice, slight breeze out of the top of my scope with the fan running. Pretty cool!

And based on you saying you still got dew without the heater, I went ahead and put in an order for a heater similar to yours, just a bit smaller since my scope and fan are smaller. I plan to attach it to the metal fan grill you see in the second photo since it looks like these Nichrome heaters can get hot enough to melt PLA pretty easily.

 

Do you know if I can just solder an RCA connector to the leads of the heater and plug it into my dew heater controller like any other dew heater? It looks like all these types of heaters are in the 12V, <1A range, which seems reasonable for a dew heater controller to drive. 

 

-Trey

Attached Thumbnails

  • IMG_3399.jpg
  • IMG_3400.jpg


#17 colinrm

colinrm

    Vostok 1

  • -----
  • Posts: 147
  • Joined: 16 Sep 2019
  • Loc: Akron, Ohio

Posted 06 October 2020 - 09:26 AM

Dang, that's nice!  Now I need to redo my hack job.  :p

 

Good luck!  Hopefully you figured out the dew problem quicker than I did.


  • Axys32 likes this

#18 Axys32

Axys32

    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 06 Jun 2020

Posted 06 October 2020 - 04:36 PM

Dang, that's nice! Now I need to redo my hack job. :p

Good luck! Hopefully you figured out the dew problem quicker than I did.


Haha, thanks! Let’s hope it works as well as it looks! I’ll update you all whenever I get the heater installed and have a chance to go out and image. I feel pretty optimistic about this solution, though!

#19 junomike

junomike

    Hubble

  • *****
  • Moderators
  • Posts: 19,799
  • Joined: 07 Sep 2009
  • Loc: Ontario

Posted 06 October 2020 - 05:25 PM

Colin, thanks again for the tips! I finished the print of my fan enclosure last night and got it installed today! Check it out! I can feel a nice, slight breeze out of the top of my scope with the fan running. Pretty cool!

And based on you saying you still got dew without the heater, I went ahead and put in an order for a heater similar to yours, just a bit smaller since my scope and fan are smaller. I plan to attach it to the metal fan grill you see in the second photo since it looks like these Nichrome heaters can get hot enough to melt PLA pretty easily.

 

Do you know if I can just solder an RCA connector to the leads of the heater and plug it into my dew heater controller like any other dew heater? It looks like all these types of heaters are in the 12V, <1A range, which seems reasonable for a dew heater controller to drive. 

 

-Trey

 

 

I'll take Three! One for my 10", 12" and 16"!!!!!post-333219-0-62617000-1601953636_thumb.


  • Axys32 likes this

#20 Axys32

Axys32

    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 06 Jun 2020

Posted 06 October 2020 - 11:05 PM

I'll take Three! One for my 10", 12" and 16"!!!!!post-333219-0-62617000-1601953636_thumb.

Haha, thanks, Mike! I'll take that as a compliment! If I had a printer with a larger print bed I would totally entertain the offer, but this is at the very extreme of my printer's bed size (210mm square). Perhaps in multiple pieces that are bolted/glued together, though... 




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics