St120 dewshield, why so chunky?

[LondonNeil]

It's cloudy,  I'm bored so I'm musing.  Why is the due shield on my st120  (and it seems most or all  refractors) such a solid lump?  I get the impression it's heavy. Would a flexible dewshield like astrozap not be much much lighter, more convenient for packing the scope,  and otherwise as good? It's supposed to just pull off although mine seems incredibly tight. If I could get mine off I'd weigh it and try replacing it with a flexible plastic shield,  I'd guess it could save the weight of a decent eyepiece.  Has anyone tried it or can anyone tell me why it might be a bad Idea?

[mikeDnight]

From what I remember, the dew shield on the ST just pulls off, although it may be a bit tight to begin with. It would be easy to replace. Having said that, because the ST is a short tube refractor the dew shield can help balance the tube on the mount so it isn't too back-end heavy.

[LondonNeil]

Yes from what I read it pulls off the st120 but is always tight and sometimes just can't be budged.  Mine so far has resisted any tugs, and I've had a good try. 

I get the impression that it's chunky as a balance weight too.  However even with a mahoosive 2" diagonal and a moderate heavy ep like a Morpheus or Hyperion the scope balances in a position that would allow me to move rearward 30+ mm, maybe more.  Its why i reckon the dew shield must be so heavy.   if I were to use a longer dovetail that overhung the rearmost ring a little  I would hope I could reduce the weight of the ota by 2 or 300g by Swapping to a plastic dew shield and still balance the tube fine,  even with a Barlow. Maybe an Ethos would cause trouble but well..... an ethos in an st120 would be interesting in many ways!  

[John]

Meade used plastic dew shields and objective counter cells on some of their refractors so I guess balance plays a part in the choice of materials used.

Meade made their refractor dew shields generally a bit on the short side though. Maybe that was also a weight saving move ?

All the Skywatcher fracs I've owned have used metal dew shields. With my ED120 it screws off but I need to add an Astrozap dew shield extension in dewy conditions and fortunately those are quite light.

Interesting what we start mulling over when it's cloudy 🤔

 

[LondonNeil]

I'll have another tug at it.  If I can get it off I'll weigh it, and test how much the balance shifts.

Not sure I'll get it off though!

[LondonNeil]

Aha!  Got it off.  Firm pulling or pushing back and forth and noticed movement,  after I'd got it 4 or 5mm off it got easier and soon slid off.

Kitchen digital scales have it at 459g for comparison the astrozap on my mak127,  which is easily big enough for the st120, is 170g.  So a swap would save 289 grams if the scope still balances, less if, as I might,  I have to extend the dovetail rearward a little.   

My rough estimate of 'about one Morpheus/Hyperion ' weight saving was pretty close!

[Paz]

I have wondered about this for a long time, why so heavy and so over engineered.

...but it does come in handy when balancing heavy loads. I have the dovetail bar set well back past the rear scope ring to be able to balance heavy eyepiece or binoviewer set ups without having to add weight back to the front end of the scope.

[LondonNeil]

I guess a bino viewer would be pretty heavy, as would that set up pictured. There was me thinking an ethos would be silly in an st120 🤣  or is it a panoptic or delos? A heavy ep either way,  plus a 2" diagonal and a decent finder so you've probably got 1.5kg added to the ota. You could still move the mounting point rearward though.  Move the rear ring 30 or 40mm closer the front ring and the dovetail would extend that much dither further rearward towards the focuser allowing balance I reckon,  even with your set up and the heavy dew shield removed.  That porta would thank you for it. 

[LondonNeil]

Well I've got a spare 8 inch dove tail so I've ordered a 1100x650x1.1mm sheet of sand textured black polypropylene..... enough to make 4 or 5 dew shields it was £11.38 + £10 pp, and 15m of black velcro cable tie strap, £5.

I'm going to try making myself a lovely long, light weight and removable dew shield for my st120.   I'll set the rings such that the scope should be close to balanced and .... if the clouds allow,  I'll see how it goes.   It should be longer,  lighter,  removable and well.... better hopefully.