22 replies to this topic

[jproehl]

I just picked up an older Celestron 11" SCT OTA, its the model with the flat back (photo attached) and I'm wondering if the Celestron Dew Heater ring could be made to work on this OTA. I do not know when my OTA was made, but Celestron tech support says based on the photo that its not compatible but did not supply a reason (FYI that was the same story I was told about the Celestron Focus motor and all that I needed for a retrofit was to 3-D print a 15mm long tube with an ID of 19mm and an OD of 21.5mm to insert around the focuser and inside the focus motor).  Both the dew heater ring and the OTA have 8 evenly spaced screw holes. Do the newer and older 11" OTA's have the same diameter, or did that change in 2006?  

 

Has anyone tried to fit one of the new dew rings to a Celestron SCT pre-dating 2006? Thanks for any information anyone has on this.

 

Jeff

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[Traveler]

"....but Celestron tech support says based on the photo that its not compatible but did not supply a reason"

 

Funny reaction from them.I think Celestron is missing an open marketopportunity. I wonder if anyone has come up with something ingenious. I'm following this topic with great interest.

[BillHarris]

Celestron is bypassing a great marketing opportunity. I've gotten the same "won't fit" line for an older C8. Therefore, I'm backing off and installing the same anti-dew-heater I used on my 1982 8" Wright-Newtonian: each gold rectangle is an aluminum-encased wirewound resistor heatsunk to the aluminum lens retainer. The sum power of those 16 resistors is 20 watts at 12v. The photo shows this 1982 arrangement

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[Chris Johnson]

I watched the Celestron youtube installation video and the only potential problem that I can see is that the id of the rim in the older scopes may be shorter than the newer scopes. Perhaps someone with the newer version C-11 could measure theirs and we could compare to my olderC-11.

[jproehl]

Celestron is bypassing a great marketing opportunity. I've gotten the same "won't fit" line for an older C8. Therefore, I'm backing off and installing the same anti-dew-heater I used on my 1982 8" Wright-Newtonian: each gold rectangle is an aluminum-encased wirewound resistor heatsunk to the aluminum lens retainer. The sum power of those 16 resistors is 20 watts at 12v. The photo shows this 1982 arrangement

Interesting approach - do you have the part number for the resistors you used (or at least their resistance) or is it just an Ohm's Law calculation?  I assume you just wire them in series and connect the string up to a dew controller to vary the heating?  

 

Since the edge of your image a bit fuzzy, what type of heatsink paste did you use?  Did you remove the ring to adhere the resistor ring to it or can it be done with the retaining ring in place on the scope?

 

Thanks!  

[jproehl]

I watched the Celestron youtube installation video and the only potential problem that I can see is that the id of the rim in the older scopes may be shorter than the newer scopes. Perhaps someone with the newer version C-11 could measure theirs and we could compare to my olderC-11.

That is possible. I've dug around on the web and cannot find the OD measurement for the Celestron Dew Heater ring anywhere. I'll have to send a message to their tech support to see if I can get that value to compare to my particular OTA. What would be nice would be to get an engineering drawing showing the actual size of the ring and placement of the holes, so that a direct comparison to the hole pattern on an older scope can be done.

 

I have seen some posts on CN, that suggest that the new dew heater ring may be causing spikes in star images on new OTAs. Those posts suggest that this behavior may be connected to how much heat is put into the corrector plate (i.e. they appear at higher heating levels). But this issue doesn't relate to whether the ring can be retrofitted.

[Chris Johnson]

I think I know the problem with the older models. Reading the instruction manual the gaskets are not replaced. The ring bears directly on the corrector. The new models have 4 plastic screws that are used to center and hold the corrector in place. The older model’s, like mine, rely on on having sufficient torque on the gaskets to keep the corrector plate centered. I think that if you use the dew heater on the older model’s you will have to overtighten the screws to keep the corrector from moving around.

[mclewis1]

.... I think that if you use the dew heater on the older model’s you will have to overtighten the screws to keep the corrector from moving around.

or just upgrade the spacers to ensure they solidly keep the corrector in the appropriate position.

[Chris Johnson]

or just upgrade the spacers to ensure they solidly keep the corrector in the appropriate position.

There are no locating screws in the rim of the older models. They rely on the friction of the gasket under the ring.

[jproehl]

There are no locating screws in the rim of the older models. They rely on the friction of the gasket under the ring.

Could spacers placed between the edge of the plate and the tube wall (either rubber or 3-d printed) be made to fix the position of the corrector plate under the heater ring?  Any number could be created, but if you made rigid spacers, 3 should suffice. You would obviously need to account for thermal expansion and contraction of both the corrector plate and the tube, so maybe rubber would be preferable.

Edited by jproehl, 24 May 2022 - 07:23 PM.

[jproehl]

There are no locating screws in the rim of the older models. They rely on the friction of the gasket under the ring.

Doesn't the new Dew Heater Ring provide just as much clamping force as the older ring?

[BillHarris]

My Smartphone isn't doing a quote, so this is a reply to Jproel's msg #5: Plans are to use the same approach I used on my home-made 8" and my C8. Both have an aluminum retaining ring for the corrector. Remove the retaining ring, scour the surface under the power resistor and attach the resistor with thermal epoxy. Resistors are 7.5 watt, 500 mohms (0.5 ohm) 16 resistors for 8 ohms total. Ohm's Law. I made a boo-boo and got 1% tolerance, which cost me more. 5% or 10% would be adequate. I'm planning to install the heater in the C8 this week and will do a write-up with better photos (that old photo was pitiful!). Here is a cut-and-paste from the recent Mouser invoice: MFG Part No: RH005R5000FE02 Vishay / Dale 7.5watt .5ohm 1% MKD / Wirewound Resistors - Chassis Mount MFG Part No: 8329TCM-6ML MG Chemicals Adhesive - Thermal C / Thermal Interface Products

[mclewis1]

There are no locating screws in the rim of the older models. They rely on the friction of the gasket under the ring.

Cork and paper spacers (depending on the thickness needed) have been added around the edge of the corrector to Celestron SCTs to set the final position of the corrector in the factory since they were first produced. The small synthetic bolts are a recent addition (and they do make more sense than the spacers). The gasket friction only prevents the corrector from rotating.

[Chris Johnson]

Cork and paper spacers (depending on the thickness needed) have been added around the edge of the corrector to Celestron SCTs to set the final position of the corrector in the factory since they were first produced. The small synthetic bolts are a recent addition (and they do make more sense than the spacers). The gasket friction only prevents the corrector from rotating.

To my surprise My GPS nexstar 11 had no spacers the first time I removed the corrector for cleaning. I considered making some spacers from Evergreen Teflon strips but did not. I use a caliper to center the corrector whenever I have to clean the corrector plate. If I had a serious dew problem, I would make some spacers and try the Celestron dew heater ring.

[BillHarris]

On that cut&paste from the invoice:
Two lines, each starting with "MFG part No". One for the resistors, the other for the thermal epoxy. Posting the message ran that pasted material together.

--Bill

[jproehl]

I received this answer back from Celestron Tech support:

 

"The front cells on the older castings had a smaller inner diameter and different screw positioning. There is no way to fit new dew heater ring to an older C11 scope."

 

Since I fully expect somebody on this site will figure out how to do it, I'm tempted to reply back: Wanna bet?

Edited by jproehl, 25 May 2022 - 12:16 PM.

[Chris Johnson]

I am tempted to use some dimensional Teflon strip parts to locate my corrector accurately and try the Celestron dew heater. I do believe the comments re the id of the older model re the C-8. When I got my new C-8 I had a Celestron hard -pastiche dew shield for the older flat back model that hooked to the inside rim. It was definitely too small. If someone with a new C-11 can measure the spacing between between the screws. We can find out. I suspect that Celestron is right because you need some thickness in the rim ro allow drilling and tapping the 4 centering screws. My C-11 is not thick enough to allow drilling and tapping for screws.

[BillHarris]

Yeah, "wanna bet?"

[luxo II]

I’d do a different solution, noting that the glass should be not touching the metal - and that glass and spacers are poor conductors of heat anyway.

Instead I would remove the corrector and glue 1W resistors around the corrector cell on the underside, inside the OTA. Join them in series, put a socket on the side of the OTA. Replace corrector. This way the heat from the resistors will be distributed through the corrector cell and also back into the OTA and slightly warm the air inside; this in turn will warm the corrector more effectively than resistors on the outside wher most of the heat will be lost.

On my 10” mak in conditions with 90% humidity - without insulation - 6W was sufficient, and with insulation 3W. Applying heat outside the OTA is wasteful - most of it is going straight up and out without doing much for the corrector. Worse, the air rising off the resistors creates turbulence and degrades your seeing.

Edited by luxo II, 26 May 2022 - 06:03 AM.

[davidgmd]

Wouldn’t heating the inside of the tube cause distortion from thermal tube currents?

[mclewis1]

To my surprise My GPS nexstar 11 had no spacers the first time I removed the corrector for cleaning. I considered making some spacers from Evergreen Teflon strips but did not. I use a caliper to center the corrector whenever I have to clean the corrector plate. If I had a serious dew problem, I would make some spacers and try the Celestron dew heater ring.

There have been reports of seeing just the outline of a spacer remaining on the edge of the corrector. The assumption is that the original spacers simply broke down over time. Synthetic materials like Teflon would make a lot of sense, thin enough to add multiple layers to get the exact spacing, and not going to break down from age or cleaning fluids.

 

A C11 I had (from a 2003 NS GPS scope) had cork and paper spacers, 10 years ago I replaced them with a magazine cover material (paper with paint layers), this seems to have lasted well. My C6 from 2006 appears to have some very thin spacers but I haven't removed the corrector to verify exactly what they're made of.

[luxo II]

1mm cork or card (like a business card) were what was used ling ago. The point was to allow the metal cell to contract without applying stress to the corrector.

[BillHarris]

Luxo--
Resistors on the retaining ring was how I did it years ago. I considered mounting the resistors inside and on the bottom of the lens cell, but on the retaining ring was chosen. The ring is 1/8" thick and the 1.25w each resistors were thermally well-coupled to the ring, with a total wattage of 20w. The corrector sits on thin cork on the cell, and has a black construction paper gasket between the retainer and the lens. I typically ran it at 1/4 to 1/3 power. More in frosty-cold and drippy-humid. I had ring and ambient thermistors and ran the heater at 3-4* temperature difference. Even at 4/4 setting I never detected heat turbulence. The proposed control box is going to be a PWM (pulse width modulation) based on the venerable 555 Timer.

--Bill