Dewing and ODK?

[DaveS]

Has anyone in these damp climes had experience with dewing on an ODK telescope (Of any aperture)?

I've just had a quote for dew heaters to be fitted to an ODK12, and as expected with OOUK, not cheap!

I'm now trying to decide whether to have them fitted during the assembly, but if dew isn't a big problem I may forgo them, possibly adding Kendrick tapes later if I find dew occuring. I know Sara @swag72 had an ODK10 at one point, but she is in sunny, dry Spain, not damp grey UK .

[Jkulin]

Hi Dave, I know my mate who has an ODK version designed by Peter Shah has dew heaters on the primary and secondary, he hasn't used them on the primary as he uses the extraction fans to draw air over the mirror and out of the rear, he does use a dew heater wrapped around the secondary.

For my secondary I fitted a dew heater to the back of the secondary and then ran two copper strip across the vain and soldered the contacts, will save you quite a bit as well, I have no heaters on my primary: -

 

[DaveS]

Thanks John. Yes, I remember reading that thread when I was still looking at GSO RC telescopes.

I will need to give it some thought, as I doubt the dew heater is a retrofit. May be able to get away with Kendrick / Astrozap dew tapes.

[Jkulin]

From what I know Dave the primary on the ODK is not an easy retrofit without stripping the whole lot down, the secondary was just a wrap around affair but it still meant running the power feeds across the vanes, which wasn't what I wanted, so that was why I ran the copper strips..

My RC is running perfectly fine with just the fans on the rear and the secondary dew heater.

[swag72]

On 18/02/2019 at 17:04, DaveS said:

I'm now trying to decide whether to have them fitted during the assembly, but if dew isn't a big problem I may forgo them, possibly adding Kendrick tapes later if I find dew occuring. I know Sara @swag72 had an ODK10 at one point, but she is in sunny, dry Spain, not damp grey UK .

I do get some really bad dew out here at certain times of the year. I never found that the ODK dewed up and I just ran with the fans.......

[DaveS]

Thanks Sara. If I don't need extra dew control I can use the money to get a G3 16200 rather than the Atik.

[Oddsocks]

Hello Dave.

I have the ODK 10, just along the coast, a bit east of you.

A long reply, took some time to write with a lot to cover, hope it proves useful.

I have the OO heater kit for the 10” ODK here ready to fit, had it for five years but never got around to fitting it.

When I ordered the kit from OO they were having problems obtaining the secondary heater pad and were only able to supply the primary heater, they suggested I bought the secondary heater directly from Kendrik in Canada. OO recommended that the full-width heater pad was best as it provides direct contact between heater pad and the mirror while the ‘C’ shaped external heater pad that wraps around the outside of the mirror support did not work so well because there is no direct contact between the mirror and the mirror support plate, the two are separated by thick blobs of silicone rubber and the screw-on baffle shroud.

Partly, the reason for not fitting the kit so far is that even though I’m an experienced engineer the prospect of having to unglue the secondary from the secondary support plate to install the full-width heater pad across the back of the mirror is not something to be taken lightly even though OO told me it’s just a matter of cutting the silicone rubber at the join between the secondary support plate and the mirror with a hobbyists scalpel, levering off the mirror from the back plate and refixing with acid free silicone on both sides of the heater pad. My hands (and nerves) are just not steady enough for that these days!

I might abandon the idea of fitting the full width pad and buy and fit the external ‘C’ type heater ring, for now I’m using a quadruplet refractor in the observatory and have no plans to begin using the ODK again any time soon.

The vanes of the ODK 10 OTA are thick cast/machined aluminium, much wider than the secondary heater wires, so it’s relatively easy to attach the wires to the top or bottom of the vanes with silicone, or little blobs of hot glue and hide them within the width of the vane, they would not intrude into the light path. 

Once the secondary heater pad is attached it will be much more difficult to carry out occasional mirror cleaning as the heater wires are supplied as a one piece kit, one length of continual wire that runs from the heater pad, down inside the OTA tube and back to a pre-wired audio type jack socket that you have to mount on the OTA backplate.

As it stands, to remove the secondary for cleaning once the heater is installed requires that the heater wires are unglued from the vanes and the wires and socket removed from the backplate. If you hot-glue or tape the wires to the inside if the OTA to prevent them straying into the light path on the run from the secondary vanes down to the backplate then obviously you can’t remove the wires and can’t detach the secondary from the vanes/OTA so the wires need to have an inline joiner of some sort at the juction of the secondary support and the vanes so that you can detach the wires and remove the secondary. Mini spade terminals and shrouded receptacles would probably do.

There is a tight, close-tolerance fit between the circular secondary mirror, the threaded secondary baffle shroud and the threaded secondary support plate. In order to bring the secondary heater wires of a full-width heater pad around the edge of the secondary support and refit the baffle-shroud it would be necessary to file or grind a notch into the edge of the secondary support plate to take the wires. Of course, this would not apply to the ‘C’ type, external, secondary heater as the wires are routed around the outside of the secondary support and baffle-shroud.

The OO supplied primary heater is not attached directly to the mirror, this is a semi rigid wrap-around collar that sits against the OTA wall and radiates heat towards the mirror, there is no direct contact betwen primary heater and mirror, which makes regular cleaning a lot easier. The heater collar has to be siliconed or hot-glued against the OTA wall to hold it in place.

Even with careful handling, removing the primary heater from it’s packaging causes large patches of the anti-reflective black paint to flake off the GRP base, leaving the yellow-green (shiny) GRP base showing through, it will need repainting after fixing in place next to the mirror (with the mirror removed).

Unless OO have changed the design of the OTA backplate since mine was supplied, two extra holes have to be drilled in the backplate to accept the pre-wired audio-jack sockets that are attached to the ends of the supplied heater wires.

My ODK 10 was last used inside a domed observatory and there was very little dewing. When dew did appear it occurred on the primary first and I found that keeping the OTA fans running largely prevented dewing except for a handfull of occasions.

Before having a domed observatory I had a Skyshed Pod that fully exposed the OTA to the sky. I had continual problems with dewing and icing on both mirrors at that time which was why I bought the heater kit but before getting around to fitting it I had issues with condensation forming inside, between the lenses, of the ODK’s built-in corrector assembly.

I was able to use a heater tape wrapped around the corrector where it projects beyond the rear of the OTA back plate to evaporate the moisture but it would reform within a day if the heater was switched off. In the end I brought the OTA inside the house to dry out but even after a month indoors the condensation reappeared inside the corrector after a few days back in the Pod and eventually I had to remove the corrector, unscrew the lens retainer ring at one end, remove and clean the lens groups of water marks, reassemble the corrector and leave it in a sealed plastic box with silica gel for a few weeks before refitting. For the remainder of that winter season I had a heater tape around the corrector that was switched on permanently.

As you are probably aware, collimating an ODK is not an easy task. The distance between the primary, secondary and corrector are critical to achieving specified spot sizes and optimum optical correction. I believe OO now provide a collimation tool which comprises a calibrated distance tube and Ronchi eyepiece which is used on a bright star to set the distance between primary and secondary, the tolerance is small, a fraction of a mm away from the correct distance results in star bloat and under/over correction.

When I bought my ODK 10 the distance tube and Ronchi eyepiece were not supplied and had to be obtained separately. You need these tools for accurate re-collimation whenever you take the mirrors out for cleaning. If you have to remove the corrector, as I did for drying out, there are no optical tools provided to ensure it is refitted at the correct distance from the secondary and I used a vernier to measure the distance between the centre of the rear corrector lens and backplate before removing the corrector assembly for drying, setting the same distance after cleaning and refitting.

If you lost this corrector distance I think it might be really difficult to set it back again empirically given that ODK correction seems to be wavelength dependent, i.e, best compromise of spot size for the full optical bandwidth, and most likely this might need an optical bench to set up, this is just a guess though and it would be best to talk to OO regarding cleaning/drying of the corrector if the same internal corrector condensation issue that I experienced happened to you. 

So, if you want to want to fit the full size, full-width secondary heater be aware that collimating afterwards requires the special distance tube and Ronchi eyepiece to get the distance between secondary and primary correct since you now have a thick heater pad between mirror and mirror support thus changing the distance between the two mirrors.

Using the Ronchi on a bright star, making small adjustments to the secondary distance, refocussing, recentering on the star, evaluating the Ronchi pattern and repeating takes a long time. Don’t be suprised to have to spend a few nights on this after mirror cleaning or fitting a full width secondary heater.

For many reasons I have not used the ODK for the last couple of years. Partly, the observatory is used remotely and unattended for much of the observing season and an open tube OTA required too much maintenance. The observatory is adjacent a mixed arrable and dairy farm with ploughing/tilling dust, flies, spiders eating the flies and pollen a constant problem. A smaller refractor with an electric remote opening dew shield flap to keep the dust, flies and spiders away was needed and there just is not the space between a  10” OTA and the roof of the dome to fit one. A long dew shield extending from the OTA would probably be enough, together with the OTA fans to prevent, or reduce, dewing but again in my observatory there is no room.

To sum up:

If you are in a domed observatory most likely you woudn’t need heaters at all.

If the OO kit uses the ‘C’ type secondary heater it is relatively easy to fit yourself but not as effective as the full width heater pad that requires major surgery to install.

Both primary and secondary heaters are wired to sockets on the back plate. You may need to drill holes in the back plate to mount the sockets if OO do not now provide the back plate pre-drilled.

After fitting the primary heater, you most likely will need to touch up the black paint on the heater (unless OO now prime and paint the GRP base with a suitable flexible paint and not with a single coat of some water? based paint that doesn’t stick to GRP as was the case with the one they sent me.

If you change the distance between the primary and secondary of an ODK, even by a fraction of a mm, you need to collimate using the calibrated distance tube and Ronchi eyepiece that I think OO now supply as standard with the telescope, when mine was supplied you had to obtain these separately.

If your ODK 10 is to be kept outside permanently watch out for condensation forming between the lenses of the back plate mounted corrector, consider fitting a wrap-around heater tape where the corrector protrudes from the back plate and apply a liitle heat right through the winter to mitigate against this. Unless you have some experience of dissasembilng lens assemblies, and the tools to do so, then preventing condensation forming inside the corrector in the first place is a whole lot better than dealing with the problem and cleaning up residual water marks afterwards.

If OO are supplying the full width secondary heater pad and have not ‘notched’ the secondary backplate to accept the wires that allow the baffle-shroud to be refitted once the heater is glued in place then it might be best to pay to have them fit the kit. If OO are supplying the ‘C’ type external heater pad then you can fit this relatively easily yourself, provided that the OTA backplate is pre-drilled for the heater wiring sockets otherwise you will need to attack this with a drill yourself.

If not supplied, obtain a couple of mini in-line spade terminals and shrouded receptacles, cut the wires for the secondary heater close to the secondary stalk and fit the terminals there so that you can easily remove the secondary for routine cleaning without having to unglue the wires from the vanes. 

William.

 

[DaveS]

Thanks William.

I'm reading your post several times to get the full gist.

I'll be emailing OOUK to let them know which camera I'll be using, and making further enquiries regarding the heaters, specifically as to fitting. The 'scope will eventually be going in a Pulsar dome, but I'm thinking that it might be used in the open air until I get the obsy constructed. I'm wondering if standard Kendrick dew tapes around the tube might be enough, though CF might not have the thermal conductivity to pass the heat through.

My location is pretty much all sheep farming, so no plowing dust (I hope).

[bob-c]

Hi Dave, i am in this same position as regards to heaters for OOUK  ODK. I have got to let them know pretty quickly. Have you now decided what is the best way going forward for your scope.

I have ordered the 10in..but also thinking of going for the 12in instead.. I hope everything is going to plan for you and on track.

Clear skies,

              Bob.

[andrew s]

I got mine fitted by OO on my ODK 16 at the time of order. 

Regards Andrew 

[bob-c]

23 minutes ago, andrew s said:

I got mine fitted by OO on my ODK 16 at the time of order. 

Regards Andrew 

Hi Andrew,

Is that for the primary and secondary, or just one fitted. Also which one is the most needed do you think.

Thanks Bob.

[DaveS]

I ordered the primary heater to be fitted during construction after a short email exchange with OOUK, it makes it much simpler. They don't supply a secondary heater, but apparently the holder and spider have notches to make fitting a Kendrrick easier. They said the dew shield could be useful but can be ordered later as it just fits onto the end of the OTA

HTH

 

Edit: when I was looking at the ODK versions at Astrofest it looked like the 12 was the sweet spot in terms of optical performance. Good thing, as no way could I have afforded anything bigger, even the 12 was pushing things.

Edited March 28, 2019 by DaveS

[andrew s]

1 hour ago, bob-c said:

Hi Andrew,

Is that for the primary and secondary, or just one fitted. Also which one is the most needed do you think.

Thanks Bob.

Not sure. I just leave them on as I don't  image local seeing is not a big issue for me.

Sorry I can't help more.

Regards Andrew 

[bob-c]

1 hour ago, DaveS said:

I ordered the primary heater to be fitted during construction after a short email exchange with OOUK, it makes it much simpler. They don't supply a secondary heater, but apparently the holder and spider have notches to make fitting a Kendrrick easier. They said the dew shield could be useful but can be ordered later as it just fits onto the end of the OTA

HTH

 

Edit: when I was looking at the ODK versions at Astrofest it looked like the 12 was the sweet spot in terms of optical performance. Good thing, as no way could I have afforded anything bigger, even the 12 was pushing things.

Thanks Dave for your help, I think that I will do as you have and tell them to fit it to the primary.

Yes, the 12 is pushing me also ,but I think it's what I want so what the eck....go for it..

Best regards,

Bob.

[bob-c]

40 minutes ago, andrew s said:

Not sure. I just leave them on as I don't  image local seeing is not a big issue for me.

Sorry I can't help more.

Regards Andrew 

Ok Andrew, thanks for your help.

Regards Bob.

[ollypenrice]

Like Sara we do get damp seasons here but the 14 inch didn't dew up. The camera did because it had no chip window heater but the scope was OK.

Olly

[DaveS]

Thanks Olly. I doubt I'll get much dewing, but there is a stream at the bottom of my garden and it can get a bit humid. Having the dew heater there ready, "just in case" will be better than trying to fit it later, see oddsock's long post above.

[bob-c]

15 hours ago, ollypenrice said:

Like Sara we do get damp seasons here but the 14 inch didn't dew up. The camera did because it had no chip window heater but the scope was OK.

Olly

That is good to know Olly, thanks.

Bob.

[bob-c]

On ‎28‎/‎03‎/‎2019 at 16:09, DaveS said:

I ordered the primary heater to be fitted during construction after a short email exchange with OOUK, it makes it much simpler. They don't supply a secondary heater, but apparently the holder and spider have notches to make fitting a Kendrrick easier. They said the dew shield could be useful but can be ordered later as it just fits onto the end of the OTA

HTH

 

Edit: when I was looking at the ODK versions at Astrofest it looked like the 12 was the sweet spot in terms of optical performance. Good thing, as no way could I have afforded anything bigger, even the 12 was pushing things.

Hi Dave,, it has been a while since last we talked about the OOUK  ODK. 

I have also placed an order for the 12 ODK with the primary heater fitted. I am wondering if you have received it now, and if so are you pleased with everything.

Bob..

[DaveS]

Hi Bob

I haven't got it yet, but I'm in no great hurry, as we've lost astro dark for a while, and, although I can afford it, I want to wait 'till I've finally sold my bungalow and have some secure cash in my bank account.

Will report back when it's all up and running.

[bob-c]

1 hour ago, DaveS said:

Hi Bob

I haven't got it yet, but I'm in no great hurry, as we've lost astro dark for a while, and, although I can afford it, I want to wait 'till I've finally sold my bungalow and have some secure cash in my bank account.

Will report back when it's all up and running.

Ok Dave thanks for the reply .

Bob.