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Post by mikey cee on Apr 8, 2011 18:13:11 GMT -5
When considering mounting a large refractor like anything 6" f/12 or above a real solid mount is a must. Anything less and it's "TWANG CITY" ala Duane Eddy style. I found from the very first over 30 years ago that a pillow block mounting cannot be beat. Sean here has made his own pillow block mount and has quickly found obvious weak points which needed to be addressed. Yes you can buy already built high priced mounts that are relatively jiggle and wiggle free. But they are nowhere near perfect. They still require several seconds to dampen down and they all offer that damnable twangy boy-yoing effect that I simply cannot tolerate. With my Brandt 8" f/13.3 and piggy backed Jaegers 6" f/8 a pillow block mount of sufficient mass was needed. My machinist who is an amateur astromomer recommended a mount with 3" bearings and solid shafts mounted on 3/4" plating. After locking down the Dec axis I can literally lay my head right into the winged eyecups of my binoviewer's eyepieces and cause little to no discernable movement. Yet the scope moves like it's floating on a bed of mercury. When you take a forefinger and rap the rack and pinion it makes a lurch forward and instantly in a split second returns to it's original position. The funny thing is this scope isn't precisely balanced as it probably should be, but it's close enough for visual. I've always preferred a stiff solid focus and I use large 2-3/8" knobs. The scope does move obviously when focusing finely but not like other scope I've viewed thru. Most of the time things are jiggling around so bad if it hasn't slid out of view it's next to impossible to get super precise. Enter a motorized focuser which I may someday install. Presently I'm going to change out mt straight bevel gears on my Dec slo mo unit with helical bevels. I've just never really saw the need for a motorized Dec althought it would be a relatively easy job. I've enclosed a few pics of my scope that some have seen numerous times over on CN. Later Mike  Attachments:
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Post by mikey cee on Apr 8, 2011 18:15:20 GMT -5
Here's another angle. Attachments:
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Post by mikey cee on Apr 8, 2011 18:16:36 GMT -5
Another shot. Attachments:
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Post by mikey cee on Apr 8, 2011 18:21:36 GMT -5
Finally from the previous shot I've added extensions to my Dec controls. Now I can effortlessly reach over from my binoviewer to make adjustments without looking and reaching way up underneath in the dark although like a blind man you do learn the art of braille rather proficiently. Hah. ;DMike Attachments:
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Post by mikey cee on Apr 8, 2011 18:24:25 GMT -5
And I might ad that if I had the room which I don't this mount would have no trouble hadling a 12" f/10-12 I'm sure. ;DMike
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Post by Watcher3 on Apr 8, 2011 20:26:25 GMT -5
Loved your mount from the first time a saw a pic of it Mike. I have to suffer the jiggles, unfortunately, even with my 4" F/13 Carton. Been thinking about somehow camouflaging a pier in my postage stamp sized front yard. Then, if I can find a used Losmandy G11 mount, I might have a good solution to get me by until I retire to roomier quarters under better skies.
It really is frustrating, that I have to wait til I retire before I can start living the big scope dream. I have the long Carton, that's pretty nice for all the viewing that I can do in my white zone, and I'll soon have the 6" F/5 that will be portable enough to get to some marginally darker skies, but I still live vicariously through you guys for now.
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Post by Mike on Apr 8, 2011 21:32:38 GMT -5
Mike, I have also admired your set up. I have never viewed through a large refractor. I have owned several but never larger than a 4 inch. A friend has a 6 inch APO but it's a short focal length. My new 6" F12 will be the first long focal length refractor I've looked through. I'm primarily a planetary guy. I can't imaging the view of Jupiter and Saturn through an 8" F12 or 15. I wish one of you guys was an imager so I could see what I'm missing. I will be mounting my new scope on a Celestron CGE. You know Steve Fisher from the refractor forum on CN? He has a D & G 6" F12 on a CGE and it's works quite well. Well, sorry I ran on and on.
Mike
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astrosean
Full Member
Jack and Jill
Posts: 50
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Post by astrosean on Apr 8, 2011 22:10:26 GMT -5
No fair! My mount is a continual work in progress. I rebuilt all of the weak spots over the winter and it is much improved. I need to get my camera charged up and write some updates! The reality of a large refractor is the need for a robust mount. I believe 6" is the threshold between portable and permanent mounts. My inspiration for my scope was an 8" f12 refractor brought to Prairie Skies. I camped out next to the owner who had it mounted on an AP900 and even that mount was a bit touchy at the eyepiece. I cannot believe Meade or Celestron would sell a 6" scope on an eq4 or even an eq5 mount and I am shocked more people don't complain about them! For all those interested in buying a 6" or 8" long refractor, I have a proven solution that can be purchased for about $100. The venerable pipe mount. 2" pipe can easily support and move my scope, I wouldn't hesitate to mount an 8" OTA on mine. The movements are shockingly smooth! Attachments:
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Post by Watcher3 on Apr 10, 2011 10:26:30 GMT -5
Sean. I can't wait to see the latest version of Jack, and hear about problems solved and improvements made.
Anybody have an idea of the shaft size I would need to be comfortable with my 20-25 lb. 4" F/13 carton? Think a 3/4" shaft would be good? I'd like to have something pretty rock solid, yet light enough to set up, and take down from a pier for each session.
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Post by Watcher3 on May 12, 2011 19:57:22 GMT -5
Well, my fortunes may be changing. There's a good chance I might be moving to the suburbs of St. Louis within the next year. I might not have room for an observatory, but I will have at least a permanent mount in the yard.
I definitely want a pillow block mount! I have decided on 1 1/2" solid shafts, as that seems to be the point where things are very sturdy, and parts are comfortably priced for me. I think that should be enough "over kill" for my 100/1300, and my 150/750 scopes. I've found that I can easily source the bearings, the shafts, and the steel plates that I would need.
I do have some preliminary questions, especially for Mikey Cee.
1. I like the proportions of your mount. What is the distance between the bearings?
2. Is there any benefit to longer or shorter shafts?
3. What diameter is the drive gear?
4. Is the gear better placed at the rear of the shaft, or between the RA and Dec assemblies?
5. Anybody have a source for a worm and gear set?
That's enough for now, but I will have more questions as I get my head wrapped around what I'll need to plan this project. I really hope I can pull this off. It will be practice for a bigger observatory mount when I finally retire.
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Post by mikey cee on May 13, 2011 19:10:05 GMT -5
Joe....I used Optic-Craft Machining's diagrams for the Model 7. You will have to scale yours down to the Model 3 or 4?? I went with 3/4" plates instead of 1/2". My gear is 5.5" or 6". All of their drives are on the end. Probably not enough clearance above the plates up higher. The scope is so well balanced and the bearings so smooth that it probably makes little difference. Later Mike Attachments:
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Post by boomboom on Dec 10, 2015 2:05:01 GMT -5
Four and a half years later I have to agree totally with Mikey...pillow blocks rule. I have two of these mounts...well really one is still in build mode but getting closer all the time. The one I use has 1 1/8th " shafts and just handles a 6" f12. The new one has 2" shafts and will take the 6" f12 and probably not know it is there. RA drive is 10" Byers gear and a Tangent on the Dec of my design. The beauty of mine is I designed it to be 'modular' It can be taken to bits...some of them still weigh 20kgs...which does make it 'transportable' I will weigh the thing when it is done. For now it is going on a tripod I made a few years back, eventually on a pier...refractors always look better on piers Matt  |
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Post by boomboom on Dec 10, 2015 2:19:35 GMT -5
I should add that I'm building this myself with nothing other than a drill press, bench vise, metal files a few power tools jigsaw, electric drill and lots of time No lathes and no Mills.....would have made a fair part of this easier if I did have those tools. Shafts are 2" precision stainless, 2" SFQD bushes with 50m straight bore, for the junction spots of RA DEC and Saddle plate, a couple of metres of 160 x12mm aluminium bar and some 6mm aluminium sheet. Add to that a few tubes of JB Weld and a whole bunch of bolts and washers.....hey presto....an unbelievable mess of aluminium shavings  When I get closer to finishing I'll post some more pics. |
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Post by istarmullet on Dec 10, 2015 16:27:51 GMT -5
That looks really great.
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Post by nobody on Dec 11, 2015 12:49:11 GMT -5
Nice work, Matt!
If I was going the [just portable] pillow block route I think I might prefer aluminium channel "girders" rather than flat plate. The girder webs provide extra stiffness without a massive increase in weight.
If weight is an issue then tapered or stepped axes make sense because the lightly loaded end of the shaft can be made smaller. Larger pillow blocks quickly become very heavy! For a permanently set up mount then Mike's use of thick steel plate and 3" shafts is arguably by far the best choice.
Increased focal length = longer moment arm = slower damping times. An extra foot in telescope length make a huge difference. My 6" f/8 was as solid as a rock on my vintage Fullerscopes MkIV. I used to take handheld extra-focal snaps of the Moon and planets without a single case of camera shake. The 7" f/12 finds the flexure modes but is still acceptable. The MkIV mounting enjoys 6" diameter disk type thrusts surfaces as well as widely spaced bush type bearings fitted in stiff, conical castings. The combination of disk and shaft means it can handle heavier and longer loads than its 1.25" shafts might suggest.
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