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Equipment Discussions >> Mounts

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sternenhimmel
super member


Reged: 06/06/13

Re: Beautiful Homebuilt GEM new [Re: Starhawk]
      #6017465 - 08/10/13 11:40 AM

Observing? A fair amount over the years, mostly with an 8" Newtonian. But my real goal is an astrophotography platform for relatively light scopes. I see the merit in buying an introductory setup like the CG-5, and going from there as I become more advanced and my appetite expands. However, I also get the feeling it would be better to invest in something more substantial right off the bat, so that it will last me years to come. If that's my philosophy, then to me it makes sense to build something myself.

Again, I'll only have access to this machine shop for another year, and I would like to become somewhat proficient in my machining skills. This seems like a good project to achieve both my goals while the resources are still at my disposal.


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Chucke
member


Reged: 03/12/10

Re: Beautiful Homebuilt GEM new [Re: sternenhimmel]
      #6023519 - 08/13/13 05:20 PM Attachment (40 downloads)

Here is an example. 0.4" overhang. I could have made it less but assembly in the field would have been more difficult.

Chuck


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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: Beautiful Homebuilt GEM new [Re: Chucke]
      #6024354 - 08/14/13 01:24 AM

That's a handsome mount... home-built?

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Starhawk
Space Ranger
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Reged: 09/16/08

Loc: Tucson, Arizona
Re: Beautiful Homebuilt GEM new [Re: orlyandico]
      #6024791 - 08/14/13 10:15 AM

Sternenhimmel, the light weight astrophotography mount is nothing less than the holy grail in this hobby. The real problem comes down to tracking accuracy, which can be broken down a little unconventionally this way:

Mount:
Drive mechanical accuracy
Drive movement accuracy

Mount and telescope:
Telescope focal axis flexure (SCTs are famous for this issue)
Telescope to guidescope flexure
Telescope/ mount rigidity and natural frequency
Telescope and mount environmental disturbance response (wind, jarring).

Telescope:
Focal length
F/#

So, mount mass ends up being positively correlated with dealing with about a third of that list. What isn't obvious is picking a telescope with a longer focal length means any tracking error is easier to see in the image- and this is linear. However, f/# drives what you are required to do. So, an f/10 system can get to minutes of exposure time very easily, so even if the error is fairly low, the opportunity to damage the image is large. But image brightness is proportional to the inverse square of the f/#. And this is why very fast systems like the f/2 hyperstar exist. I invented this dimensionless brightness number to predict performance:

BR= 1000/(f/#)^2

BR (f/10)= 1000/100= 10

BR (f/2)= 1000/4= 250

So, the f/2 image is 25 times brighter than an f/10 one, meaning the magnitude it reaches in 30 seconds is equivalent to what the f/10 system needs 12 minutes and thirty seconds to do. So, the mount tracking burden is far less than it is for the slower telescope.

So, for small and light, you're talking about a light weight telescope, short focal length, and low f/#. This is a specific solution set more in the direction of camera lenses or things like the C6 hyperstar. Things like the Vixen Polarie, Takahashi SkyPatrol, or Celestron NexStar SE mounts fit in this zone depending on size.

Hopefully this will help for seeing what you would be trying to get with larger mounts.

-Rich


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windjammer
member


Reged: 06/02/13

Re: Beautiful Homebuilt GEM new [Re: sternenhimmel]
      #6025696 - 08/14/13 06:02 PM

Hi -

>>astrophotography platform for relatively light scopes

yes please!

You might be interested in my upgrade projects on an EQ2 to try and achieve this (I know, quixotic). One of the projects is electronic setting circles using home made encoders which might be useful for the goto aspects of your mount - you can find all the mechanicals, electronics and software on the site:

www.astrobling.weebly.com

I get totally the pleasure of machining beautiful precision parts, and some of the designs mentioned look fantastic and lovely. At the risk of sounding heretical, a lot of this is overkill - the trick surely is to combine cheap simple mechanics with smart software and electronics to get over the mechanical shortcomings. For example, any piece of junk can be made to track accurately for as long as you like if you have closed loop guiding - autoguiding.

Simon


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Chucke
member


Reged: 03/12/10

Re: Beautiful Homebuilt GEM new [Re: orlyandico]
      #6025798 - 08/14/13 07:04 PM

Hi orlyandico,

Yes, I made it in my garage. I have a lathe and a mill. It took about 6 months working about 6 or 7 hours per day. I was unemployed at the time.

Chuck


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Beautiful Homebuilt GEM new [Re: Chucke]
      #6025816 - 08/14/13 07:14 PM

Chuck, it is a very good-looking mount. But at 6 months x 20 days x 6 hours a day, a very expensive mount at 700+ hours of time. I get the unemployed part, but this illustrates well why AP, SB mounts are actually a bargain - if you were charging for your time, your mount would cost maybe double of an equivalent AP.

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Chucke
member


Reged: 03/12/10

Re: Beautiful Homebuilt GEM new [Re: orlyandico]
      #6025952 - 08/14/13 08:34 PM

orlyandico,

It would have been a lot quicker if I had a CNC mill like SB and AP do. Face milling those big plates to thickness takes a long time.

It is really sturdy. I had a 250lb OTA and 250lb of counterweights on it at one time and it didn't blink.

Chuck


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sternenhimmel
super member


Reged: 06/06/13

Re: Beautiful Homebuilt GEM new [Re: Chucke]
      #6026268 - 08/14/13 11:59 PM

Heaps of good information is accumulating right here.

I'll start by picking my jaw up of off the floor after seeing Chuck's mount as well as reading about how long it took. I'll get back to this in a minute. Nice scope, as well

Starhawk, your point is well received, and fast scopes certainly do change the game. Honestly the fact that most scopes in the astrophotography community tend to be refractors and SCTs is surprising. Newtonians are cheaper and generally faster (save for additions like the HyperStar), yet seemingly scarcely utilized. I see the advantages in using refractors and SCTs, but these don't seem to be performance related.

I think a good target scope for the mount I am building will be the 8" f/4 Newtonian I already own. I have also considered some 80mm apochroamtic refractors, but I suspect I'll get better performance out of the Newtonian.

Windjammer I like your philosophy and the website is full of helpful information. The mount I am using now is a barndoor tracker, and I've even considered guiding with it, which would be ludicrous. Yet, it could be done with a few changes. I have also considered buying a cheap mount second hand, like the CG-4, and modifying it to do accurate guiding.

Anyway, I digress, and here is what is important. I talked with one of the machinists today about this project. I showed him the photos from the original link, and discussed techniques to make the various parts. This was incredibly helpful, and I now believe that this project could be reasonably completed in 100 hours. He seemed to think material costs would be cheap, but I am suspecting at least $200 in aluminum. One question I have: should any of the parts be steel? Perhaps the shafts? It also would seem wise to purchase the worm gears, and possibly even the spur gears (at least the large one for the dec axis). Thoughts?

One aspect I had not carefully considered is the need to create tools/rigs to accurately make/position certain parts. This will consume more materials and more time, and this is something I am learning is where real machining skills come into play.

Edited by sternenhimmel (08/15/13 12:04 AM)


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TxStars
Pooh-Bah
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Reged: 10/01/05

Loc: Lost In Space
Re: Beautiful Homebuilt GEM new [Re: sternenhimmel]
      #6026286 - 08/15/13 12:13 AM

You can get some gears from Ed Byers and not have any tracking issues.
Make the CW shaft and weights from stainless and you should be good to go.


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EFT
Vendor - Deep Space Products
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Reged: 05/07/07

Loc: Phoenix, AZ
Re: Beautiful Homebuilt GEM new [Re: sternenhimmel]
      #6026307 - 08/15/13 12:29 AM

Axis shafts and counterweight bar should be stainless steel. Worms and wheels are an art form and for the best result should be purchased rather than trying to make them. Aeroquest Machining is one source, Byers is another (but they are harder get). They should be good sized and will not be cheap.

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Starhawk
Space Ranger
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Reged: 09/16/08

Loc: Tucson, Arizona
Re: Beautiful Homebuilt GEM new [Re: EFT]
      #6026331 - 08/15/13 12:48 AM

From another thread, this is probably be closer to your mental image of what a beginning machinist can have done in a year:

http://www.cloudynights.com/ubbthreads/attachments/6025789-Optic-Craft%20COPY...

-Rich


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TxStars
Pooh-Bah
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Reged: 10/01/05

Loc: Lost In Space
Re: Beautiful Homebuilt GEM new [Re: EFT]
      #6026340 - 08/15/13 12:58 AM

Ed Byers can be reached at 760-221-2127 and has several gears on hand so they are easy enough to get (for now).
You can get them with or without a slip clutch just give him a call and talk to him.


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sternenhimmel
super member


Reged: 06/06/13

Re: Beautiful Homebuilt GEM new [Re: TxStars]
      #6026350 - 08/15/13 01:09 AM

Starhawk, a pillow block mount may still be the route I go, but I think it's a little disingenuous to say that a beginning machinist can't come up with something a little more sophisticated in that time frame. We'll see what things look like when I draw them out, then I'll access feasibility.

I checked out Byers' ebay store and some classifieds for drive systems. Expensive, yes, but more reasonable than I thought. I would probably invest in just the wheel and worm, and make my own slip clutch (if necessary) and and buy spur gears to link to the steppers, or direct drive the worm.


Edited by sternenhimmel (08/15/13 01:14 AM)


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Beautiful Homebuilt GEM new [Re: sternenhimmel]
      #6026451 - 08/15/13 03:44 AM

100 hours

I wish I had that kind of time to throw around.


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Starhawk
Space Ranger
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Reged: 09/16/08

Loc: Tucson, Arizona
Re: Beautiful Homebuilt GEM new [Re: orlyandico]
      #6026822 - 08/15/13 10:14 AM

I've been machining for 9 years for my own stuff, and the only "Better" I could add to that project would be some details like a finishing pass with a fly cutter to make parts pretty, making sure the support for the upper section was statically determinate, and cleaning up finishes with a sanding pad. It's very easy to imagine building something extremely elaborate, but every setup you haven't done before takes quite a bit of time to get right, and complex parts with a stack of operations are hazardous- I have a nice dovetail replacement block I made for my AT66ED with the 15 degree dovetail sides, radiused interior corners and the works, and was doing the last operation- tapping the last of 6 tapped holes and broke off a solid carbide tap in the part. So, I'm remaking it. I do have a jig I made just for doing vixen dovetails, but this will take time. I've had a project I'm doing to convert a NexStar8GPS to a trapeze, so all I had to to was make two end plates to take the place of the original one, a trapeze to go between them, a dovetail locking mechanism for the dovetail clamp, and a modification to a second dovetail I made earlier to produce an over and under arrangement. I've been working on that off and on for two years, now.

So, use as many bought components as you can. A year is a lot less time than it sounds like for this.

-Rich


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Beautiful Homebuilt GEM new [Re: Starhawk]
      #6026845 - 08/15/13 10:27 AM

I think that is the challenge of every would-be mount maker out there.

They have to develop the jigs, the techniques to manufacture the pieces efficiently and cost-effectively...

This is why nobody can challenge the incumbents, who already have the in-house skills, designs, techniques, and have amortized those over hundreds or thousanfs of mounts. The last guys to try that - Morning Calm - quit. I also remember the William-Optics GT-1, which proved too expensive to manufacture.


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sternenhimmel
super member


Reged: 06/06/13

Re: Beautiful Homebuilt GEM new [Re: orlyandico]
      #6026948 - 08/15/13 11:23 AM

Once optimistic, now I feel I should heed these warnings and go more conservative on the project.

I am a graduate student, but not in a field that normally utilizes the machine shop. That said, my time is probably worth less than most. I don't dare speculate on how much I actually get paid/hr I work, because the hours tend to be long. Thus, it's more difficult for me to justify spending $1,500-2,500 on a decent mount than it is for me to justify spending 4-5 hours in the machine shop a week and learning some new techniques.

I still naively believe that this project could cost less than $800.

Edited by sternenhimmel (08/15/13 11:26 AM)


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Sean Cunneen
Let Me Think
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Reged: 08/01/07

Loc: Blue Island Illinois
Re: Beautiful Homebuilt GEM new [Re: Starhawk]
      #6026977 - 08/15/13 11:40 AM

I have something like $1500 in mine, half materials cost and the other half machining costs. Had I not gone through so many revisions, I suspect my cost would be more like $700-800 but I don't believe anyone can build a mount for the first time without any revisions.

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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Beautiful Homebuilt GEM new [Re: Sean Cunneen]
      #6027080 - 08/15/13 12:30 PM

OP: I would suggest to look at some of the classic mount designs in ATM documents etc.

http://www.astrosurf.com/re/unusual_telescopes_russell_porter.pdf

I don't know where to get a hold of these but German equatorial mounts are a multi-hundred year old design. I'm sure all of the design problems have been ironed out, unless you want some holy combination (e.g. super high precision and light weight).


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