Venting an SCT questions??????
Posted 28 April 2010 - 11:21 AM
Posted 28 April 2010 - 01:33 PM
My holes are 60mm. On the LX90, one hole is at the 7 o'clock position on the rear casting, viewed from the back, the other at the 1 o'clock position. On the CPC, one is at the 7 o'clock on the rear casting, the other on the top of the tube, above the secondary, this to hopefully encourage the air in the ota to exchange. Neither has shown any advantage over the other.
I used a hardware store hole saw and a drill to cut the 60mm holes. This creates a LOT of aluminum shavings, so total disassembly and masking of internal seams, and a thorough cleaning are necessary.
I use no filtering, other than bug screen to keep bugs out of the ota, to maximize airflow.
This is far more effective through the night than the Lymax (which I tried first, on both scopes), becasue it can be left running.
I also insulated the OTA of the CPC with 3/8" foam. This keeps the top of the ota from dropping below ambient, which helps reduce internal thermal currents. I've found this to make a huge difference, even without venting. I use insulation alone to great effect on my 127mm Orion Mak.
I've also tried letting the holes operate passively just to see what'd happen. While it's better than no venting, it's vastly inferior to the forced air.
For fans, I used Acousti brand low noise and vibration fans. Expensive (for cpu fans), but very quiet, and absolutely NO vibrations on the sct's with the 60mm version, nor my 16" dob with the 120mm version, all running at full 12v capacity.
If you're certain you don't want to go active, I'd be inclinded to drill a series of 5/16" holes around the periphery of the tube, just in front of the rear casting, and a series of holes around the periphery of the ota, just behind the front casting, being sure you allow room for the dew shield and/or heater. Personaly, I'd only use bug screen to maximize airflow-- a narrow ring the same diameter as the inside of the tube painted flat black and placed over the holes. The same could be done with coarse foam if you wanted better dust control.
Dust DOES enter my ota, but not enough to worry me. Plus, I'm now so adept at taking the ota's apart that I don't fear cleaning the mirrors much. It's been said here on CN that SCT mirrors are not overcoated the way Newt mirrors are, and can't be contact cleaned. I didn't find this to be the case on my SCT's, which required cleaning owing to user errors; the mirror cleaned up using standard practices with very very little in the way of detectable sleeking. In any case, I wouldn't clean unless there was no alternative.
Posted 29 April 2010 - 12:01 AM
However my C11 is a different story. I currently use the Lymax cooler and it helps a lot. I may eventualy make some vents, but as I just bought the OTA last year, I'm not in a hurry to try it.
Posted 29 April 2010 - 01:15 PM
Posted 29 April 2010 - 03:28 PM
Posted 29 April 2010 - 04:03 PM
If the amplitude, velocity or both of the temperature fall-off post Lymax are not huge, passive cooling may keep pace with ambient temperature changes.
If, however, ambient temperature changes post-Lymax are significant and/or rapid, a vented set-up might be the better solution.
I just received a Lymax two days ago. Perhaps I'll get a chance to use it this weekend, though the moon will make it a short, uneventful session for the most part.
Posted 29 April 2010 - 05:43 PM
First of all, you will need to remove the two screws that hole the rear of the mounting bar to the rear cell. There is no need to remove the front screws, just leave the mounting bar connected to the front corrector cell.
Remove the corrector making sure that the orientation is marked relative to the tube. This is very easy..there are no shims to replace, only two rubber strips that act as a grip, between the retainer ring and the corrector itself. Once removed, the three screws holding the corrector cell to the tube must be removed. "This was a ROYAL PAIN"! Celestron painted the inside of the tube with the nuts in place. You will need to apply some paint remover with a Q-Tip swab to the tops of the exposed nuts to loosen the bond,(I used Jasco varnish and stain remover from ACE hardware). This takes a bit of time and I did each of the three separately to keep the remover on the top of the nut and not running down the inside of the tube. Keep re-applying and removing the excess pain until the bond is broken. On on of the more stubborn nuts I also used some penetrating oil. As I said this part was not pleasant. I also want to stress that the inside of the tube..will not come away unmarked in this process, so I also made the initial decision to flock to tube and had the necessary Protostar material at hand.
Once the screws are removed the corrector cell is ready to be removed. The corrector cell fits very tight..must also be a paint bond, and found the only way to loosen the bon is to take a paint stick, place one end against the edge of the corrector cell and lightly tap the other. Working around the circumference this way will allow the cell to be removed. Needles to say that when I was done with all this, I made the decision to place the vents only at the front of the tube. I originally wanted to remove the entire tube, but I did not want to take the risk of damaging the mirror in trying to remove the rear cell nut attachments. (For those with 8SE's, there are four accessory attachment screws on the right side of the OTA at the mirror end. I removed these for added air flow at the mirror. Every little bit helps).
I had gotten this far..but now I had to deal with the fact that I'd be drilling hole in the OTA, with the mirror in place. I made a mask out of a Styrofoam plate, with just a little taken off the edges, and pushed it into the tube so that it rested on the baffle tube. I used masking tape to hold it in place and seal any openings. I did not want metal shavings anywhere near the mirror.
The vents I used were Heyco vent plugs, (http://www.heyco.com...05/pdf/5-07.pdf) Part # 2677. These have eight holes each with a 3/4 mounting hole diameter. A metal hole saw from Lowes does the trick for this. After determining where I wanted the vents I marke them, drilled pilot holes and proceeded to drill the 3/4 holes. We were now at the point of no return. The holes came out well but needed to be sanded smooth and the excess shavings removed. I used a vacuum hose to suck all the filings and paint residue out of the tube and wiped it down well with both a dry and damp cloth.
I then removed the Styofoam plate, vacuumed and wiped it down again. The mirror was spotless and I had to breath a sigh of relief.
Flocking the tube was fairly easy, and I did it in two sections instead of one big piece. I also flocked only 9 inches down into the tube not wanting to get too close to the mirror. I also decided to flock the first three inches of the inside of the baffle tube as well.
Holes needed to be cut into the flocking for insertion of the vent plugs. Once done the vent plugs popped in nicely. They have catches so they stay in.
In addition I also ordered 150 mesh (approx 100 micron screen similar to Celestron spec filter screen) and super-glued it to the inside of the vent plugs. Airflow is maintained but no foreign substances or critters can enter the OTA. I then reassemble the OTA and gave it a go last evening. I have also made a small cooling fan from a Radio Shak co-ax fan attached to a 2" ep holder. The fan blows out sucking air int through the vents. The scope cooled down very quickly and the views stabilized more quickly that I ever remember them doing before. Whats more..after all this dis-assembly, surgery and reassembly, the OTA was still in collimation. Hard to believe but there it was, picture perfect diffraction rings. I am attaching some pics. This was not an easy operation due to Celestron not taking into account that anyone might actually want to modify their OTA. Their lack of foresight in making the scopes user friendly to work on almost made this project a non-starter. I'd be happy to answer any questions.
Posted 29 April 2010 - 06:53 PM
Oops ..sorry I just reread..I saw the 100 micron material you got! Great idea!
Posted 29 April 2010 - 07:20 PM
Posted 30 April 2010 - 12:06 AM
How many people have set up passive cooling vents in their SCT? How big/shape of vent holes did you use? What did you use to actually cut the holes? Hole location do's and don'ts. What Filter types or covers for vents did you use? How did this work as compaired to forced fan ventilation like the Lymax thru the night?
Some time ago, I posted a writeup on installing fans in my C9.25:
Hope this helps.
Posted 30 April 2010 - 07:49 AM
Wow, you should post this on the nexstar forum
Posted 30 April 2010 - 09:06 AM
Nice work! Now all you have to do is put a permanent fan on the back, and you'll be able to leave the fan running while you observe.
Posted 30 April 2010 - 11:00 AM
Posted 30 April 2010 - 11:40 PM
My first serious scope was an LX10. So I've always lived with the idea of a closed tube design. But recently I've messed around with a couple of second hand Newtonians. Nabbed a gem of a 6" Orion XT for less than half the price. Cleaning and collimating it has demystified reflectors a lot for me.
Its OK to clean the mirror!
Modding a Newtonian doesn't seem as scary to their owners as removing the corrector plate and messing with the innards of an SCT is to an SCT owner. Admittedly the SCT is a different beast than the Newtonian, but if you are even moderately mechanically able, installing these vents and fans seems a very doable project.