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Equipment Discussions >> ATM, Optics and DIY Forum

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starman345
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Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: starman345]
      #5214007 - 05/09/12 03:09 PM Attachment (64 downloads)

My jig for cutting four struts all the same length

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starman345
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Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: starman345]
      #5214050 - 05/09/12 03:29 PM

What is the best way to install tube inserts? I ran a 1/4" bolt through a piece of 3/4" oak and fastened an insert up tight then backed it off about 1/4" and tapped it into the tube.A couple of them didn't go in straight, they seem to be tipped to one side. Any way to straighten them out?

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starman345
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Re: 14" Hubble Optics Project new [Re: starman345]
      #5220171 - 05/13/12 12:33 PM

I"ve been playing with Plop and am wondering what others are using as a thickness value for their Hubble Optics mirror. The selections in Plop are for solid mirrors not the sandwich type that Hubble makes. At least that is what I think, I see "fused silica" is that a sandwich type?

Edited by starman345 (05/13/12 12:35 PM)


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davidpitre
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Reged: 05/10/05

Loc: Central Texas
Re: 14" Hubble Optics Project new [Re: Mirzam]
      #5220445 - 05/13/12 03:32 PM

Quote:

Astrosystems has excellent spiders and secondary holders, but for the secondary itself I like to have one that comes with test data. That pretty much limits the choice to Protostar or Antares Optics.



I believe Ostahowski's secondaries come with a test report:
http://www.ostahowskioptics.com/


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starman345
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Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: davidpitre]
      #5220486 - 05/13/12 03:53 PM

Thanks David, I've already ordered a Protostar 3.1", good to know about Ostahowski, I"d never visited that site before.

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thinairart
professor emeritus


Reged: 11/18/09

Loc: Denver, Co
Re: 14" Hubble Optics Project new [Re: starman345]
      #5254160 - 06/04/12 01:14 AM

Hi Brian, I just noticed this thread or I would have replied sooner. I also built a truss dob for a 14" Hubble Optics sandwich mirror, completed last October.

The thread for my build is here: 14" ATM DOB

In regards to mirror thickness for PLOP, I found this quote on the datasheet from HO:

Quote:

To be conservative, you can treat the mirror as a solid mirror with equivalent thickness of 1.2"




I ended up with an 18pt cell, which was recommended by HO, but was probably a bit overkill. If you end up going that route and want more details on all the cell dimensions I used, let me know.

One issue I ran into with the 14" HO mirror is that it only weighs 18 lbs (compared to a full thickness mirror at around ~25lbs). So keep that in mind when you are deciding on the height of your mirror box, or you could get into trouble with balance issues.

Another potential gotcha is when calculating the tailgate dimensions, measure the actual diameter of the mirror to account for the ~3/16" bevel around the circumference.

When it comes to edge support, I ended up going with a 2" nylon sling. The channel between layers rules out using a simple cable sling system. I would like to rebuild my mirror cell so that I can use 45 degree edge supports.

For the tailgate I used box channel aluminum and that worked out fine. An advantage of a lighter mirror is that welded steel is nice to have but not a requirement.

That is all I can think of right now, but I'll be watching your progress!


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starman345
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Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: thinairart]
      #5254364 - 06/04/12 08:44 AM

Quote:

Hi Brian, I just noticed this thread or I would have replied sooner. I also built a truss dob for a 14" Hubble Optics sandwich mirror, completed last October.
The thread for my build is here:




Hi Aaron, thanks for chiming in. I have your ATM build thread bookmarked and have read it over and over many times. Lots of good advice in there. I have to admit, I stole a few of your ideas from that thread.

Quote:

I ended up with an 18pt cell



That is what I"m building as well, I'm waiting on some aluminum to be delivered to get started. I asked the Hubble Optics people about an 18 pt cell for the 14" mirror and they said that would work fine, also a 6 point would work ok, so it seems like the 18 is a bit of overkill as you say but I would rather go that route. I have seen that they use a 6 point with their UL 16" telescope also.

Quote:

One issue I ran into with the 14" HO mirror is that it only weighs 18 lbs (compared to a full thickness mirror at around ~25lbs). So keep that in mind when you are deciding on the height of your mirror box, or you could get into trouble with balance issues.



Thanks for that bit of information, I hadn't even considered the lighter mirror weight. I haven't gotten to the torque calculations yet but will definately keep that in mind--don't know if I mentioned in this thread that I am using the Kreige&Berry book for direction....otherwise I wouldn't even know what torque calculations were for

Quote:

Another potential gotcha is when calculating the tailgate dimensions, measure the actual diameter of the mirror to account for the ~3/16" bevel around the circumference.




This is something I noticed when using PLOP, I had been using the 14" dimension for diameter and after actually measuring I discoverd its really 14 5/16" edge to edge through the center. It made a difference in PLOP.
I have a full size drawing of the cell with the mirror resting in it to help me determine the cell size, hopefully I won't have any disasters there but thanks for the warning, I'll definitly pay attention when figuring the cell dimensions.

Quote:

I would like to rebuild my mirror cell so that I can use 45 degree edge supports.



This is what I've decided on too, Starman1 and Pinbout gave me the idea in some of their posts. Without this forum and the Kreige & Berry book I would not have the confidence to try building this telescope.
I've ordered an aluminum plate that will hold the triangles and bars and also the 45 degree support posts. The plate will be connected to the mirror cell frame by the collimation bolts, that way, when collimating, everything moves together, there should be no binding as I've read happens with a sling..
My Protostar spider and secondary mirror arrived recently and a Stellarvue finder should arrive next week so I can get the weight of the secondary cage and continue on with the build. Thanks for your help.


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careysub
Carpal Tunnel
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Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: 14" Hubble Optics Project new [Re: starman345]
      #5254998 - 06/04/12 05:02 PM

In using PLOP you should use the actual thickness of the sandwich mirror for the mirror thickness -- the advantage of any type of hollow core construction is that you get the stiffness of the full thickness at a reduced weight.

For the glass type, the best one to pick may be fused quartz. Why? Well, the glass actually used is soda-lime which is stiffer than pyrex, but about the same as fused quartz; and the fused quartz is lighter (by about 10%) than soda-lime itself so it simulates to a certain extent the lightness of the mirror. However, this selection does not have a very large effect.

I have an HO 18" mirror that is 1.93" thick. I am planning on using a 6-pt cell, which gives me an RMS error of 2 nm, effectively perfect.


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thinairart
professor emeritus


Reged: 11/18/09

Loc: Denver, Co
Re: 14" Hubble Optics Project new [Re: starman345]
      #5255067 - 06/04/12 05:52 PM

Quote:

I have to admit, I stole a few of your ideas from that thread.




Steal as much as you like! A lot of my ideas came from other ATM threads as well

Using a two-layer approach to the mirror cell is definitely the way to go if you want to use posts for edge support. I added edge posts to my single layer K&B style cell, and the mirror would bind up on the posts frequently, throwing the collimation completely off.

If you have not looked through a Stellarvue product before, you are in for a treat with the finder you ordered. The image is very crisp, with pinpoint stars and easy focus controls. The model I ordered also supported interchanging the 1.25" eyepiece, which I've found helpful in some situations for star hopping with a bit more magnification.


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starman345
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Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: careysub]
      #5255123 - 06/04/12 06:24 PM

Quote:

In using PLOP you should use the actual thickness of the sandwich mirror for the mirror thickness



Full thickness is what I ended up using. Its kind of a *BLEEP* shoot with these Hubble mirrors, PLOP really can't accomodate them. Hopefully I'll get acceptable views with the 18 point but if not I can always re-build the cell...it will be made from aluminum with no welding so should be easy to modify.

EDIT: hehe, *rap got bleeped

Edited by starman345 (06/04/12 06:26 PM)


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starman345
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Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: starman345]
      #5255136 - 06/04/12 06:30 PM

That makes me feel good about the Stellarvue choice, I"ve never looked through one but they seem to be highly rated on the forums and I am tired of going through the contortions of looking through the straight through finder on my Skywatcher. I ordered the F050M2 model. Hopefully I can figure a way to connect it to the secondary cage.

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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: 14" Hubble Optics Project new [Re: starman345]
      #5255324 - 06/04/12 09:16 PM

Quote:

A couple of them didn't go in straight, they seem to be tipped to one side. Any way to straighten them out?




I tighten the insert up against the wood. I like using softcore ply wood to hit the inserts in, the soft ply compresses a little more allowing for that last bit of adjustment. as I'm hitting them in I can adjust the insert by hitting on either side, but they don't have to be perfect either.

to straighten them out they will have to go deeper as you straighten them.


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starman345
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Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: Pinbout]
      #5255747 - 06/05/12 07:24 AM

Quote:

to straighten them out they will have to go deeper as you straighten them.




Yes, that is exactly what happened, I tapped the high side with a screwdriver and hammer until they were pretty much level all around. I was afraid of getting them too deep and in the way of the spider mounting screws but it turned out ok.


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thinairart
professor emeritus


Reged: 11/18/09

Loc: Denver, Co
Re: 14" Hubble Optics Project new [Re: starman345]
      #5256327 - 06/05/12 02:54 PM

I used the Stellarvue finderscope mounting ring kit (#R50A) they recommended for dobs: Mounting rings

I attached the base to a 1"x 1/2" strip of cherry sandwiched between the two secondary cage rings, and that has worked out pretty well. Learning how to align the finder with two rings has been interesting, but I'm slowly getting the hang of it.


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starman345
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Re: 14" Hubble Optics Project new [Re: thinairart]
      #5256419 - 06/05/12 03:54 PM

Quote:

I used the Stellarvue finderscope mounting ring kit (#R50A) they recommended for dobs




Great, that's the one I ordered but was unable to see from the picture on their site exactly what the mounting looked like.
I also ordered their tripod mount, my better half loves birdwatching so I may lose my finderscope to her before I get it installed on the telescope


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starman345
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Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: starman345]
      #5263015 - 06/09/12 08:33 AM

I'm going to use 3/8" bolts for collimation and am thinking of going with the Belleville springs. On McMaster Carr there are many different springs for this size with different load ratings..from 110 lbs to 597 lbs. Which would be ok for an 18lb mirror? I don't want to buy something that a tank could drive over and not depress...or something too mushy either

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piaras
scholastic sledgehammer


Reged: 01/26/09

Loc: Niagara Region
Re: 14" Hubble Optics Project new [Re: starman345]
      #5264805 - 06/10/12 02:09 PM

The way that would work best with Belleville springs is the use a stack of the lighter weigh rated. Depending on which way you stack them you can vary the rating. Mounted with opposing faces they are the normal rating but stack two in the same direction and the rating goes up. The best thing about bellevilles is the way the you can combine the washers to get a rating that you require. We use them in the shop all the time. just remember that you want the washers to be under compression all the time. Start at 90% compression and collimate from there. Also use fine pitch instead of regular course thread, it will give you better control.
Pierre


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careysub
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Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: 14" Hubble Optics Project new [Re: starman345]
      #5265068 - 06/10/12 05:22 PM

Quote:

I'm going to use 3/8" bolts for collimation and am thinking of going with the Belleville springs. On McMaster Carr there are many different springs for this size with different load ratings..from 110 lbs to 597 lbs. Which would be ok for an 18lb mirror? I don't want to buy something that a tank could drive over and not depress...or something too mushy either




What cell support/collimation arrangement are you planning?

You can use 3 springs supporting the cell, and then a threaded knob travelling on a bolt to push the cell down against the springs for top collimation (I think this what you are proposing). The springs and the bolt/knob arrangements do not need to be collocated (though that works also) - the springs could be under the mirror, with the adjustment push at the mirror edge.

Think about the spring support this way, at vertical the mirror plus cell puts 7 lb of weight on each spring (if the cell weighs 3 lb). The basic adjustment of the mirror should push it down a distance against the spring to add, say, another 7 lb of force. This means that the mirror is being held in place against the collimation adjustment with the equivalent of 1 G of force (rising to 2 G at horizontal); less force could work, but lets make sure the adjustment is firm.

So a spring that takes a load of more than 14 lb is called for, the remaining consideration is how much additional adjustment travel you want/need. If the spring constant) rate is 14 lb/in then you could back off the initial adjustment by 1/4" and still have 1/2 G holding the mirror, and if it had a max load of at least 17.5 lb you would have 1/4" of further travel (>1/2" total).

Consider for example, McMaster-Carr compression spring 9657K323, which is 1" long, compresses to 0.32", has a max load of 29.27 lb, and a rate of 42.80 lb/in. The 14 load would compress it 1/3", leaving 1/3" additional travel, and you could back off up to 1/6" (under the 1/2 G criterion) for 1/2" total travel.

Belleville springs are interesting, but I am not sure they are the best choice for this. For one thing their working load of range of compression is much less - the above spring will compress for 2/3 of its length (and 1/2 usefully), whereas few Belleville springs compress more than 1/4 at working load, thus for any working distance the Belleville stack will be twice as tall (or taller).

BTW - the bolts don't need to be so heavy, you can get finer threads in 1/4", and if you separate the spring "rail" (which would bear the weight of the mirror plus cell) from the collimation you could go a lot finer in the thread (10-32 maybe).


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starman345
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Re: 14" Hubble Optics Project new [Re: careysub]
      #5265890 - 06/11/12 08:49 AM

I think I'll order some Belleville springs and also some regular compression springs and experiment with both, they really don't cost that much anyway.
I'm mounting everything...triangles/bars, 45 degree whiffletree supports and mirror retaining clips to a 1/4" thick aluminum plate and then have three collimation bolts between the plate and the frame of the cell (1 1/4" angle alum with 1x1 alum sq tube for crossbars) that will be bolted to the mirror box sides.
Finer thread bolts is a sensible approach for collimation, I'll go with that and maybe a bit smaller than 3/8" bolts. I'm having a hard time finding fully threaded bolts locally, probably have to order them too, also the all metal lock nuts that K&B reccommend are rare here...
Thanks for the tips guys


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piaras
scholastic sledgehammer


Reged: 01/26/09

Loc: Niagara Region
Re: 14" Hubble Optics Project new [Re: starman345]
      #5266696 - 06/11/12 04:29 PM

You should be able to find full thread shafts at hardware stores like Kent, Home Hardware etc, or full thread bolts at auto dealers or go to your local scrap yard. Every where else any bolt over 1" will not have full threads.

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