Is this possible......

[Doc]

Over the last couple of months I've read two reports on I think CN and Iceinspace of secondary heaters on dobs melting the silicon and the secondary falling onto the primary.

What are your views on this. Is it possible to melt the silicon if the heater is set just above the dew point setting.

I can understand maybe if the heaters on full and left all night.

So what do you guys think.

[MikeP]

I suppose it is possible but I find it really hard to believe.

Mike

[adamsp123]

RTV Silicone rubber is used in moulds for molten wax and low melting point alloys for casting, so I reckon the answer is probably no, but if a glue was used to fix the mirror then some types melt at quite a low temperatures.

[Sam]

It would have to be overheating quite a bit.

[Astro_Baby]

There was someone on CN who had this happen. I think the dew heater developed a fault and overheated the secondary. The glue bonding the secondary to its holder failed and oo-errr - secondary impacted the primary.

I noticed on the Sky-Watcher 200 the fixing is in fact a kind of super double sided sticky pad and after seeing that post it put me right off ever doing the secondary heater lark.

[Doc]

There was someone on CN who had this happen. I think the dew heater developed a fault and overheated the secondary. The glue bonding the secondary to its holder failed and oo-errr - secondary impacted the primary.

I noticed on the Sky-Watcher 200 the fixing is in fact a kind of super double sided sticky pad and after seeing that post it put me right off ever doing the secondary heater lark.

I bet that was one of those two I read. The LB one is siliconed on.

Also thanks for all your input. I'm thinking of adding a secondary heater but coupled to a controller so I can adjust temperature.

[dobserver]

Cool - keep us posted on how you get on....

[steve_bham]

I think once the silicon is set the normal operation of the heating tape would'nt cause it to "melt" and fall off. If i had one that uses a double sided sticky pad id be tempted to replace with good quality silicon. Over time the pad might degrade and loose its tackyness.

[brianb]

If i had one that uses a double sided sticky pad id be tempted to replace with good quality silicon

When I upgraded the secondary on my Newtonian many years ago, I glued it onto the holder with Copydex. There are occasions when you want to be able to remove the secondary (for recoating) without undue violence (ruling out Evostick) .... and the adhesive really doesn't have to do any more than support the weight of the secondary. My solution was thoroughly satisfactory; rubber solution (cycle puncture repair outfit) would probably be just as good.

[FLO]

I don't know about the CN thread but we have never come across the glue/silicone/whatever failing.

I have heard of a situation where the glue holding a Maksutov's secondary softened causing the secondary to slide a few millimetres. I think it was an ETX left in a hot car.

HTH

[brianb]

I have heard of a situation where the glue holding a Maksutov's secondary softened causing the secondary to slide a few millimetres. I think it was an ETX

No it wasn't, the secondary in ETX Maks is a silvered spot on the corrector plate - there is no secondary mount at all! The issue with glue melting in ETXs is the glue which hold the baffle tube in place. This is a serious issue as, with most Maks & SCTs (including ETX Maks), the baffle tube also provides the bearing along which the primary mirror slides when focusing.

Secondaries or baffle tubes coming unstuck shouldn't be an issue but it's wise not to use reflectors for solar work, unless a full aperture solar filter is fitted over the objective, as heat from the Sun definitely would do damage if projection was attempted.

[FLO]

No it wasn't, the secondary in ETX Maks is a silvered spot on the corrector plate - there is no secondary mount at all!

Hope this helps (fifth post down)

etx 90 slipping secondary - Bad Astronomy and Universe Today Forum

[Doc]

Thanks guys my mind has been put at rest.