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7/22/2011

Thermals, Cooling and Dobsonian Telescopes; Reason for a Thinner Mirror in the XX14i

Well, sorry I haven't posted but I've been swamped this week. I had two finals on Monday and then had to to go to work on Monday and Tuesday morning. Tuesday afternoon I had the first of my last two wisdom teeth pulled and am looking to have my lower right 2nd molar, tooth 31 I believe, which has had a root canal and a re-treatment has to come out and soon as it is hurting. I hate all the dental work I have had to have because of my Celiac disease.

So on to the other news. I am going observing this weekend on Saturday and Sunday. Utah has this holiday called Pioneer Day, which is celebrated on the 24th of July so on Monday the 25th, we are off. I'll go in to work today to finish being ready to start teaching on Tuesday (well at least until a dentist calls and says they can pull my molar). The weather should be good all weekend and the moon won't rise to around or after 2:00a.m. on Saturday, later on Sunday.

I wanted to share a link to a wonderful article by Gary Seronik called Beat the Heat and is located on Gary's site at this link, while, that is Part 1 of the article. Here is a highlight from Part 1 discussing how to know if your mirror reaches withing 3 degrees C of the outside air, an place that Gary says will free you from thermal viewing problems.

"make a trip to your local hardware store to pick up an inexpensive indoor/outdoor digital thermometer. Tape the “outdoor” probe to the back of your scope’s primary mirror. Place the probe directly against the glass and insulate the other side of the probe with a piece of foam before taping it down. Next, attach the display (which houses the “indoor” temperature sensor) to the back end of the scope somewhere. This way you can compare the mirror temperature with the ambient air temperature. Your scope will mostly be free from thermal problems when the mirror is within 3° C (5.4°F) of ambient. Chances are, this is something you’ll rarely see."

In Part 2 Gary discusses several key items. First are the two reasons why your Newtonian has thermal problems. One is the difference in the air temperature between the outside air and the primary mirror. This is called delta T and is explained in depth in the article. Some may think in winter you have worse delta T, yet often summer is just as bad as winter. The next factor that determines how bad a thermal issue is, is the thickness of one's mirror because the mirror's heat capacity is determined by its thickness, not its diameter. Gary's article goes on and discusses thermal issues, where to store your mirror when your not observing and why, the need and location of fan(s) and how to control thermals next to the mirror.

I found this part of Gary's article very interesting since I own a XX14i. Specifically Gary makes it very clear that "if your mirror is thicker than 1½ inches, you should consider using fans to work both the rear and front surfaces of the mirror, which effectively doubles the cooling efficiency." If your using the XX14i the stock mirror is 1 13/16 inches thick (almost 2 full inches) and the XX14i has one mirror mounted in the back and no side mounted mirror with holes on the side so don't expect a ton of cooling and in terms of planetary performance, or having sharp images, this thermal issue will impact your viewing, especially on planetary objects. As Gary states in the article, "if optimum performance is your goal, avoiding thick mirrors is virtually a necessity."

This is one of several reasons why I have decided to put a Carl Zambuto 14 inch mirror in my XX14i. The money will go off in the next two weeks to Carl to pay for the mirror and it will take about 4 months for the mirror to get done. When it is done, my current 14 inch mirror will head off to Carl for review and then re-figuring and that will open the door for Carl's mirror program for Chinese Dobs to into the XX14i. I believe the mirror will be about 1 and 3/8 inches thick, and about 6 to 7lbs less than the current mirror. That means instead of 52.9lbs in the bottom mirror tube, it will be 46.9 or 47.9lbs in the bottom tube, less weight than in the XX12i. So if you have the extra money, and put in one of Carl's outstanding mirrors, there is no reason weight wise to not get the 14 inch instead of the 12 inch now (unless you can get Carl to do the 12 inch and take more weight off that as well). It will mean that I will have to add some counterweight to the back of the tube, but I have some cast iron dumbbells I can use for that so the center balance point doesn't come off and a new base is required. In addition, I may have to make some adjustments, about 7/16 or so to keep the mirror focused in the focuser but I intend to upgrade the focuser to a Moonlight and according to another XX14i owner, Ken, that adds about a 1/2 inch so I think I can compensate for that as well. So for around $3800 or so I'll have an excellent scope to use for some time.

Some may ask why I am doing this since I've said my 14 inch mirror is good. It is good, but I've seen what Carl's mirrors are like and what they deliver and for me as a visual observer, I really believe the views are just outstanding. I have the opportunity, I have the money and I'm going to do it and for me that is enough justification. There are other reasons as I believe the contrast is just tremendous, as is the amount of detail that contrast brings out and in the end, I know that I will have an exceptional mirror that provides exceptional views in the west desert of Utah or up in the Unitas Mountains or in the National Parks of southern Utah. That much more seeing and viewing is worth the cost to me. Bottom line is that it is my money and that means I get to choose how I spend it and for me, this is a worthwhile upgrade.

Some may say what about motions? You won't have premium motions. True to a point, but I like where I have my XX14i and unlike the Obsession, a gust of wind won't send the XX14i turning in the wind. In the end, the motions are adequate and I like the XX14i's motions and have no problem bumping it to keep an object in view and I have to do that often when I am sketching. It comes down to choice and sure, someday I intend to have either Dennis at Dobstuff or Rob at Teeter telescope make a system to hold the 14 inch mirror but that is down the road after my two teens are done with college (4 or 5 years). This will cap me off for sometime and I will be content. So if you own a XX14i or have considered it, and want a premium primary I'd contact Carl and get on the list to get a mirror. The ten inch mirror program is truly terrific. However, I think the 14 is a logical step up and you can get a premium mirror from Carl down the road to put into the 14 with some adjustment to counterweights, and enjoy observing for a very long time.

So check out the article by Gary, its quite interesting if you haven't seen it and I am very excited to be getting down the road a 14 inch mirror from Carl. I'll post up here of course the mods I make to the XX14i so those of you who are more adapt can make even better mods and I can learn from you.