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Celestron SLT 102 refractor dew heaters or shield

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#1 sieade245

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Posted 13 July 2020 - 05:26 AM

Hi guys. After a bit of help with this. Had a look around the forums first but the SLT has kind of a unique design when it comes to its built in detachable dew shield.

 

Last night I was observing via EEA nice and warm inside! However after about 2-3 hours it became harder to see things. Can't believe I didn't think about actually going outside and having a look at the scope but when I did at 3AM, it was dripping and fogged up front lens. Normally I would spot this if I was outside!

 

I want to fix this and I was looking at a heater ring to put around the front lens. However, the factory dew shield presents a bit of a problem. Here are some pics. There is a gap between the shield and the tube where the lens is. If I fit a heater ring to the outside, I was thinking that it will heat the dew shield, but then would the air gap be a problem for transferring this heat to the lens area?

 

20200713_101712649_iOS (Small).jpg

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20200713_101736424_iOS (Small).jpg

 

Options I've thought of so far:

1. Fit to outside of dew shield and try it

2. Fit to inside of dew shield and drill small hole in shield for cable (will it fit though?)

3. Remove factory dew shield and replace with a heated dew shield

4. Fit a heated dew shield over the top of the existing one

5. Fit unheated dew shield over the top of the existing one and also a heater ring to existing near lens...but may have same issue with the air gap.

 

Does anybody have any recommendations or experience with dew prevention on this scope?

 

Also I'd was looking at 5V USB type. I know these don't kick out as much power as the 12V, but I'm out of 12V sockets on my power setup ATM. Anybody try these? If they are rubbish I'll stick to 12V and sort something out. I don't really want to spend a load on dew controller for just one heater ring though if anybody has any cheaper suggestions?

 



#2 doublevodka

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Posted 13 July 2020 - 06:20 AM

5V one should be fine I think, the dew heater only needs to keep it a degree or two above the dew point.

 

I'd stick with the outside for now, just to save drilling any holes, although if it's a thin enough cable you may be able to snuggle it in and put the shield back on with a small gap?



#3 sieade245

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Posted 13 July 2020 - 06:37 AM

Thanks, I came to the same conclusion plus will be easier for me to connect up than 12V.

 

I'm think I'm might try one of these:

 

https://www.amazon.c...c/dp/B07MHGW4DG

 

They're £20 ish and they do a 45cm x 4.5cm version. The circumference of the dewshield is 43cm. I'll give it a go on the outside. If its no good, I'll try and find a smaller one for the inside.

 

It will be here on Wednesday so I'll see how I get one and get some pics up.


Edited by sieade245, 13 July 2020 - 06:57 AM.


#4 sieade245

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Posted 14 July 2020 - 11:46 AM

My first heater to test turned up early so here are some pics and some info.

 

This one I bought from Amazon. It's meant for camera lens less than 4.3" diameter. The inner lens area of the scope under the dew shield is about 4.5" diameter so I thought it would be close enough.

 

For Lens External Dia. 8CM-11CM/ 3.1''-4.3"

https://www.amazon.c...U_gdEdFbP6A09G8

 

Got if from here. Importantly, there are 2 versions of this. One comes with a heat control and a timer which cannot be switched off. I noticed that the other one has no timer, just the heat control. I assume this means its on all the time. This is the one I bought, it was £3 more than the other one.

 

It is a tight fit, but it DOES fit in the gap under the dew shield. If I use this one, I'll use a dremel to cut a little notch in the back of the dew shield for the USB cable so its not sticking out the front. It was very tight though, its not something you'd want to keep putting on and removing as I reckon eventually it would get damaged, probably the cable more than the strap. 

 

I tested it on low and medium settings for 15 mins each. Medium had the whole dew shield noticeably warm to the touch. My IR temp gun is at work right now so can't give you any accurate temps. The lens felt slightly warm which I guess is what I need, not super hot. Low was obviously not as warm. I didn't want to risk high just yet as medium seems like it will be good enough. Plugged into the USB on my powertank. I need to try it outside now but weathers looking pants for the next few days with cloud cover so not sure it would be accurate test, I might give it a go though I could stick it outside for a few house pointing up and see what happens. 

 

USB power is deffo enough though I think. I won't bother with 12V, this is much better than I thought it would be for £17.

 

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#5 doublevodka

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Posted 15 July 2020 - 05:38 AM

Looks great waytogo.gif

 

You'll probably find that "low" would be fine in most conditions unless it's very cold / heavy dew :)



#6 sieade245

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Posted 20 July 2020 - 09:14 AM

More info. Tested them outside a few times now and they will be fine. I did run into one particular issue though:

 

I'm using a Celestron 7Ah power tank (the lead acid battery version). The two USB ports can kickout 1.5A between them. There is also a red flashlight on the top of the tank. I normally have the flashlight on all the time when observing, mainly so I don't walk into the scope. With the flashlight on, after a few secs/ mins, the flash light would "flash" once, then the dew heaters would switch off.

 

I tried this with a battery charger connected too to make sure it wasn't low battery voltage, same thing.

 

What I've discovered is that the red flashlight also used the 5V power rail, and the stock bulb fitted is rated at 0.5A. This means if you have the red flashlight on, you don't actually have 1.5A of USB power available, you only have 1A. If you exceed this, there is some kind of overcurrent protection which switches the rail off. This causes my dew heater to switch off and must then be manually switched back on....and then the cycle repeats red flash and dew heaters off.

 

With the red lamp off, this doesn't happen and the dew heaters continue to work. This also doesn't happen with the main large white LED flashlamp on, only the red one.

 

The bulb is a standard torch size bayonet 4.8V 0.5A 2.5W. I'm going to try and replace this bulb with an LED version which can be found on Ebay. I'll post back with the results.


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