Skywatcher Startravel 150/750 refractor Dew shield

[BranocsSky]

Hi all.

I want to know if anyone has ever tried to remove the Dew shield on a Skywatcher Startravel 150/750 refractor. I thought that they were screwed on but when I tried to remove mine so that I could clean the front lens it would not move. Not wanting to damage anything I thought I would ask if anyone has been successful removing one.

[Cosmic Geoff]

On the smaller 102mm Startravel the dew shield just pulls off.

[LondonNeil]

I've had the same on my 120,  doesn't seem to want to pull off, twisting I started to unscrew the whole front,  lens and all, so.... it's staying put.

[LondonNeil]

Update @BranocsSky it definitely pulls off the st120. I just did it as I wanted to see how heavy it is.   I read on CN to wiggle/tug it back and forth and sure enough I saw a little movement and persevered. Once off 4 or 5mm it got easier.  It's just a snug fit with some felt in there to  hold it. 

Once off I noticed how dusty the primary was so unscrewed the retaining ring (it has a couple of small holes that a large paper clip or similar will fit in and allow you to turn the ring).  Beneath that is a plastic spacer which drops out,  then a rubber seal which lifts out, then the 2 lens elements will drop out along with a paper thin plastic (?) Spacer between them.   It was simple to give everything a wipe with a soft/glasses cloth,  blow a bit of dust from the tube and then replace it all.......I think I got the lenses back the right way around....

If I can't get focus next session I'll know why!

BTW the dew shield weighed in at 460g, so I'm thinking of replacing it with a flexible plastic removable astrozap like item.   It would save 300g (my mak127 astrozap which is larger weighs 170g), and make storage easier!  I need to check I can still balance the scope though..... might need to use a longer dovetail extended rearward past the last tube ring.

[BranocsSky]

On 03/05/2024 at 17:07, LondonNeil said:

Update @BranocsSky it definitely pulls off the st120. I just did it as I wanted to see how heavy it is.   I read on CN to wiggle/tug it back and forth and sure enough I saw a little movement and persevered. Once off 4 or 5mm it got easier.  It's just a snug fit with some felt in there to  hold it. 

Once off I noticed how dusty the primary was so unscrewed the retaining ring (it has a couple of small holes that a large paper clip or similar will fit in and allow you to turn the ring).  Beneath that is a plastic spacer which drops out,  then a rubber seal which lifts out, then the 2 lens elements will drop out along with a paper thin plastic (?) Spacer between them.   It was simple to give everything a wipe with a soft/glasses cloth,  blow a bit of dust from the tube and then replace it all.......I think I got the lenses back the right way around....

If I can't get focus next session I'll know why!

BTW the dew shield weighed in at 460g, so I'm thinking of replacing it with a flexible plastic removable astrozap like item.   It would save 300g (my mak127 astrozap which is larger weighs 170g), and make storage easier!  I need to check I can still balance the scope though..... might need to use a longer dovetail extended rearward past the last tube ring.

LondonNeil this is good news, I will give the tug and twist method a go, and see if it moves

[LondonNeil]

Best of luck. 

[LondonNeil]

I thought I'd update this to provide a record of some stuff that isn't easy to find about the st120 but my own Google efforts found a few questions that are asked frequently!

St120 dew shield removal

Yes it is just a friction fit, a very tight one though.  The technique I found works for me is to sit in a chair (one without arms) and rest the ota, across my thighs. Have the primary lens cell at the point of resting on my left thigh,  hold the focuser end of the tube down with my right hand,  grasp the end of the dew shield in my left hand and push downward, quite hard,  as if trying to bend the ota over your leg. Watch carefully and you'll see the faintest movement of the shield vs the primary lens cell. Then turn/rotate the ota 180 degrees and repeat so the push would be 'trying to straighten' or bend the ota other way. Keep doing this and for the first 5 or 10 pushes it'll seem the shield isn't going anywhere even though you'll see the faintest of movements each push.  Eventually you'll see it's moved a mm then 2.  Once it's moved about 10mm it does get easier and is soon off. Don't worry about getting close to the end and push/ pulling it right off when pushing hard and damaging the lens or yourself,  the collar is 30mm deep and the lens is slightly recessed. 

 

Primary lens cell removal.

  As described above the lens cell is easily removed to clean it or access the tube,  it's the same way you access the cell om a Mak127 and probably many sky watcher telescopes .  The 2 glass elements are held in by a plastic collar that has 2 small holes in it.  Something like a large paper clip will fit in these and with something in the 2 holes and the tube gripped between your knees the collar can be unscrewed allowing a rubber seal and the glass to simply fall out. 

Weights

Exact weights of the skywatcher star travel st120 are hard to find, so with it like this I weighed it on my new kitchen scales.

Ota minus dew shield but with my skywatcher electric focuser fitted  3185g 

So without the electric focuser it's going to be 3kg ish

Dew shield 460g

Rings bolted to a 9.5" dovetail 578g 

Red dot finder 40g

My Bresser 2" star diagonal 490g

A typical cheap 1.25" diagonal is more like 180g

2 to 1.25" adapter 80g

Then a typical decent EP such as a Baader Hyperion  I weighed at 370g and a Morpheus a bit less at 310g (just the ones I happened to pick up, I would think different FLs would be a bit different weight)

My2" barlow  (errr...stellalyra?) 240g

So taking the whole set up with a heavy ep, Barlow, 2" diagonal,  adapter, red dot and the chunky steel dew shield, rings and dovetail,  the total weight is

5443g

But,  if you use a 1.25" diagonal and no barlow you reduce that by 550g to 4893g and a lighter EP could reduce that anther 200g

If you ditch the chunky dew shield and make up an astrozap like flexible one (I'm using 1mm polypropylene sheet) you can save 300-350g.  I've made up a 200mm long dew shield, so longer than the 160mm steel one,  and it's about 150g (about,  as I've not quite finished,  I'm making a guess at the weight of some press studs I intend to add).

So tube, finder, electricfocuser, rings, 2" diagonal,  (no adapter as using a 2" ep), Hyperion ep, and new flexible dewshield, rings and dovetail  totals 4813g

Ie, changing the dew shield...... could be worth it as it brings the weight below the spec max for an az gti mount (5kg) and well below the max for one side of the gtix (6kg). It's also weight right at the front that is saved,  so reducing the moment quite a bit. 

Doing the maths, the CofG moves backwards about 20mm so the scope should still balance very easily.  Again I say 'should ' cloud and 3 kids keeping me occupied,  plus summer,  I've yet to test the new ds!

Without the dew shield also makes the ota 130mm shorter, so easier to pack.

I bought a large sheet of polypropylene for not very much... about 11 quid I think it was.   I've used about a fifth of it.  I'll use some more to make up a solar filter with the film I've just bought (should be more durable than cardboard), I might also make a bahtinov mask.... and I'll still have ⅔ left. 

 I'll likely be making flexible and light weight dewshields for more scopes having bought an Altair starwave 102ed.... ever so slightly heavier and longer at f7 so 2-300g saved from the end could be the difference between joy and frustration on my gtiX!

I hope those tips and weights may be of use to someone!