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Dew shield and 100* eyepieces

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#1 philc

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Posted 21 August 2021 - 12:24 AM

I bought a standard rolled tube dew shield to go with my Evo EdgeHD.

 

Now that I’m seeing the tube protrude so far from the corrector plate, I’m wondering if that restricts the FOV of 100 degree eyepieces?



#2 Astrojensen

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Posted 21 August 2021 - 01:00 AM

No. 

 

 

Clear skies!

Thomas, Denmark


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#3 Tom Masterson

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Posted 27 August 2021 - 03:00 PM

Nope, you're good to go.



#4 Echolight

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Posted 27 August 2021 - 03:13 PM

My dew shield seems way too long also. I’ve recently read somewhere, or maybe I dreamed it, that the dew shield just needs to protrude a little farther out than the aperture is wide.

I think I’ll cut mine down a bit because it seems to sag a bit because of the length. And it throws the balance off more than I’d like.


Edited by Echolight, 27 August 2021 - 03:14 PM.


#5 maroubra_boy

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Posted 27 August 2021 - 06:44 PM

Echolight,

 

The idea that a dewshield protrude a little longer than the aperture is a good starting point.  Unless it is so long that it starts to impinge upon the TFOV your scope can provide then it isn't too long.  Even at twice the length of your scope's aperture it won't be a problem.

 

What is more significant though is the material that you made it from and/or how you made it that it cannot support its own weight.

 

As an example I have been making the dewshields of my Maks and SCTs of late from Coreflute.  It is a two in one situation where the dewshield is an extension of the insulating wrap of the scope.  Easy to attach and structurally very strong.  The neat thing about Coreflute is you can attach something like Reflectix or thing foam to its inside surface that touches the scope to provide extra insulation if you need.  It all comes off the scope in one piece and rolls up nice and tight for storage.

 

I also made a dewshield out of an old yoga mat.  It too doubled as an insulating wrap it was so long, and when in place around the scope the dewshield part was longer than the aperture.  With all my dewshields, this section is around 1.5X the aperture, and no sag.

 

If your dewshield is throwing off the balance of your scope, I need to ask why haven't your changed the position of your scope in its mount to accommodate the weight of the dewshield?  Now if your C8 (I see in your signature) is fork mounted, I can see why you have a problem.  I had the same issue with my orange tube C8 and its yoga  mat dewsheild.  You will always have this issue with your fork-mounted scope, and the only way around it is to add ballast to the rear end if you want to maintain balance in alt/dec.  A 2" diagonal and big heavy eyepieces also helps, but only to a point.  Because of the geometry of such a short tube, just swapping between a small to a large eyepiece can dramatically alter balance in alt/dec.  On the most part you can only control this by tightening the dec clamp and being careful.  You can also look at creative ways to make a wrap/dewshield unit that adds a bit more ballast over the whole of the OTA, not just limited to the front 2/3's of the tube like my yoga mat did.  This will add a bit more weight towards the rear of the scope and help with balance.

 

See the pics I've attached.  The first pic is of the yoga mat wrapped C8.  The second of my 9" Santel Mak and third of the wrap made for a 127mm Mak.  You can see that with all of these the dewshield part extends more than the diameter of the aperture and no issues.

 

With the Santel's wrap you will also see there is a small fan in the dewshield end.  This is because a dewshield alone won't prevent dew forming if conditions are heavy, and the fan is the heat-less dew elimination method I have developed.  Simple, effective, consumes next to no power and no heat being introduced to my rig.  I have also done this to the dewshield of the 127 Mak though not seen in this pic.

 

Alex.

Attached Thumbnails

  • Alex sketching Moon - lo res - Copy.JPG
  • SCT & Mak dew control (2) - Copy.jpg
  • 127 Mak wrap (1) - Copy.jpg


#6 carolinaskies

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Posted 28 August 2021 - 04:44 PM

My dew shield seems way too long also. I’ve recently read somewhere, or maybe I dreamed it, that the dew shield just needs to protrude a little farther out than the aperture is wide.

I think I’ll cut mine down a bit because it seems to sag a bit because of the length. And it throws the balance off more than I’d like.

The shield rule is 1.5x OTA diameter.  This rule is in place to limit off-axis light sources both terrestrial and astronomical including bright stars or the Moon.  

Rather than cutting your shield down consider an experiment with a temporary dew shield to see if that even is a worthwhile alteration.  Make a temporary shield out of cardboard and attach it to the telescope and look at various star fields with your widest eyepiece.  Cut this shield down by 1" increments to see if any difference results.  If you never see a change then you know cutting is not necessary, and if you see an improvement you'll know the length to cut the shield to. 


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#7 rmollise

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Posted 28 August 2021 - 05:50 PM

I bought a standard rolled tube dew shield to go with my Evo EdgeHD.

 

Now that I’m seeing the tube protrude so far from the corrector plate, I’m wondering if that restricts the FOV of 100 degree eyepieces?

 

That's a big negatory, good buddy. :)



#8 RalphMeisterTigerMan

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Posted 09 September 2021 - 05:52 AM

From what I remember about a wise, old telescope maker from many years back. He had a simple formula; the Light/Dew shield should protude from the end of the tube length the full diameter of the main tube. In other words, for those of you who do not understand "old-speak", if you have a 10" SCT telescope, the light/dew shield should eaily wrap around entire tube with more than enough over-lap (depending on the material being used) the entire circumference of the tube, have about 3 - 4" actually "on" the scope and at least 10" protruding from the from of the scope.

 

Hope this was helpful and the above will in no way cause any Vignettting at all, even with the widest super "apparent" field eyepieces.

 

Clear skies and keep looking up!

RalphMeisterTigerMan




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