HELP! -Dew danger for fluorite refractor

[Profiler]

I would very much appreciate any and all advice anyone could offer with a concern I have.

I recently took possession of a lovely but quite old fluorite refractor. I have read some literature on the internet and it is of such an old vintage that the fluorite element mounted in the rear of the lens cell is not multi-coated - only the front mating element is said to have been multicoated.

The lens cell looks fine to me but with this information about the coatings I am worried about dew/condensation forming on the rear fluorite element.

We have had some pretty cool nights here in Sydney and for the past couple of weeks whenever I came inside to my warm house after using one of my regular ED/FPL glass refractors the objective lens element would haze up presumably from the cold glass contacting the warm air inside my house. In this situation I would simply place the telescope at a safe distance near the heater and allow the condensation on the lens element to gently and gradually evaporate away from the heat.

My concern is that whilst I am comfortable with this process for ED/FPL glass I am very worried about potentially damaging the fluorite element should it likewise develop condensation after being brought inside the house from the cold air. My knowledge is limited but from what I understand fluorite crystal lens and water in any capacity are a major no - no.

I have been told that the use of a dew heater is one possible approach/solution which will stop the formation of condensation on the objective as it is kept warm(ish) whilst outside in the night air and thereafter doesn't really cool to the extent where condensation will form once brought into a warmer environment such as the house.

Consequently, if anyone can offer some adivce I would be extremely grateful.

Is fluorite really as fragile as I fear or am I being overly cautious?

Does this fluorite lens need any special care beyond a regular ed glass refractor?

Will the dew heater idea work and prevent condensation once inside a house?

Can anyone offer any other ideas?

:thanx:

[gregbradley]

What model is it?

Roland Christen has commented before about being careful about thermal shock and fluorite elements. But I think he is talking about a large temperature shift. It can break the lens.

Use a dew heater and avoid the risk. One of the reasons I wanted a dew heater on my RHA scope is I don't want dew staining on the inside elements that would be hard to clean. So for no other reason that's a good reason to prevent dew.

Also you could leave the scope outside when you want to use it so it cools off without going from warm inside to cold outside.

Also I used to use this wrap around insulation blanket with a Velcro strip to prevent dew on refractors. It worked really well. You wrap it around the outside of the scope where the objective is and it keeps it a bit warmer than the surrounding air and no dew. Its like that car windscreen insulation material.

Greg.

[LewisM]

Fluorite is not the shattering, exploding bomb many paint it's picture to be. Roland talked about thermal shock in the context of him pushing the glass during figuring where it can accumulate heat caused by friction. If then something like a drop of sweat falls on it, it can contract just enough at that point to fracture.

I will tell you a story. My old Takahashi FC-100 arrived to me in SHOCKING condition. SHOCKING! It was scratched, filthy, bent pinion of the focuser, collimation of the cell was off, the objective was absolutely sordid - dew stain, dirt, other issues. It had seen a hard life with at least 2 owners before me. I cleaned it up (repaint, objective clean etc). The objective came up to be in super condition. The rear fluorite element was just fine.

I have had a LOT of fluorite over the years, most of them OLD telescopes. NEVER an issue, whether they be front fluorite or rear fluorite.

Yes, the danger IS there, but if you take the SAME precautions you do with ANY telescope, all should be just fine. It is not going to spontaneously shatter! I wish the urban myths would stop about fluorite.

Regarding Vixens, ALL their fluorite models were REAR element fluorite. Takahahi varied this - the FC models are all rear fluorite, the FS models are all front fluorite (Takahashi developed a technique to MC the fluorite, so the FS series are all MC fluorite).

I am willing to bet this is the FC-76 recently offered in Sydney (I will repair and repaint it for you Richard if it bothers you - it would me!). FC models have a REAR fluorite element as already stated, so are NOT coated (only the front mating KZFN2 glass element is). This keeps them safe from scratching and dew. A dew heater is a WISE idea, placed just behind the objective ring on the OTA.

[PlanetMan]

Ditto - good advice

[Profiler]

Thank you Lewis and Greg - your respective advice is very much appreciated!

[Profiler]

I should clarify that my concern isn't so much 'dew' per se whilst the refractor is outside. Instead, my worry is actually after letting the refractor cool down and reach thermal equilibrium with the cold night air to maximise the optical performance but then - after 1 or 2 hours when I bring the refractor inside my warm house the condensation (ie moisture) that might then form on the objective lens.

From my own experiences this rarely occurs during Summer/Spring (ie warm months) but once it starts to get cool this condensation starts to become a regular circumstance and whereas with regular ED/FPL glass objectives I am comfortable with the scope very slowly and safely drying out in a warm room I am concerned whether any condensation formation on a fluorite may actually damage the fluorite element