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23.5" f/3.6 Dob

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#51 Augustus

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Posted 18 June 2020 - 08:14 PM

I have a 18" 2" thick mirror with a 2" nylon strap and I see a significant change in collimation from straight up to horizontal, you might want to consider steel cables.

Seatbelts are cheap and easily replaced, also much easier to get adjusted initially. I'm kind of terrified of messing up with a steel cable.



#52 grzesznypl

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Posted 18 June 2020 - 08:19 PM

Out of curiosity, how heavy is that mirror that you need steel cable? Wouldn't whiffletree support be enough and ... better?


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#53 Augustus

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Posted 18 June 2020 - 08:33 PM

Out of curiosity, how heavy is that mirror that you need steel cable? Wouldn't whiffletree support be enough and ... better?

70lbs.

 

Believe it or not, Cruxis calculates a cable sling as better than whiffletrees, of course the nylon sling is a bit worse but eh, we'll see. I feel like the nylon sling is probably going to be near-equal support-wise, just more prone to losing collimation. It's also way easier to make than any of the other options. If it really sucks and I can get rid of some more scope stuff I might just buy the Glatter sling from Rob Teeter.



#54 grzesznypl

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Posted 18 June 2020 - 08:41 PM

70lbs.

 

Believe it or not, Cruxis calculates a cable sling as better than whiffletrees, of course the nylon sling is a bit worse but eh, we'll see. I feel like the nylon sling is probably going to be near-equal support-wise, just more prone to losing collimation. It's also way easier to make than any of the other options. If it really sucks and I can get rid of some more scope stuff I might just buy the Glatter sling from Rob Teeter.

Ouch! That's going to be quite an adjustment from stubby :p


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#55 Augustus

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Posted 18 June 2020 - 08:44 PM

Ouch! That's going to be quite an adjustment from stubby tongue2.gif

Yeah, it's a little disturbing that the 14.7 PLUS all of my other observing equipment/accessories I haul with it weigh altogether about as much as this thing's mirror.



#56 Dale Eason

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Posted 18 June 2020 - 09:39 PM

The belt will be better than the wiffle tree only if you can keep the center of the belt positioned on the edge of the mirror at the mirror edge's center of gravity.  Doable but sometimes not trivial.  So just pay attention to that aspect.  That usually means some sort of keeper holding it in place on the mirror's edge.  Just one more refinement to keep in mind.


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#57 tommm

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Posted 18 June 2020 - 11:16 PM

The work by cruxis (of which Dale was part I think) shows that the force by a sling needs to be applied along a line which is the intersection of the mirror O.D. and a plane through the CoM of the mirror. Just a couple mm off from this can introduce astig.  The problem with a seat belt is insuring this condition, i.e., that much of the support by the belt is not applied away from this line. My first large scope built to K&B's book had one.  My main problem was keeping it from sliding part way off the mirror if bumped around during transport.


Edited by tommm, 18 June 2020 - 11:18 PM.

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#58 Dale Eason

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Posted 19 June 2020 - 02:16 AM

No I did not have a part in his work.  But I do admire it and use it all the time.  However even before it was published I had seen the issues using interferometry and settled on the 90 deg pegs as the simplest solution for my needs of lighter mirrors. 


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#59 Augustus

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Posted 19 June 2020 - 02:31 PM

Mirror is here! Thank you Steve!

 

bigmirror.jpg

 

(Yes, it did have padding, I removed it for the photo).

 

Still looking for a secondary - really need a 5" or 6" one.

 

 

 

 


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#60 Diego

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Posted 19 June 2020 - 05:02 PM

Wow huge mirror!!! Awesome!!!

Regarding the sling, indeed the steel cable may seem dangerous around glass, but you could add a shrink wrap around it for extra protection and prevent accidental scratches or what not.

For the 90 degree edge support, I've considered using linear bearings, that way the mirror is free to move vertically unconstrained. Since they are used in 3 D printers and in regular printers, they are very cheap. You do have to find a way to have them positioned at the mirror COG, but once set they'll hold their position unlike a like a steel or nylon strap that may eventually slip.
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#61 Bob4BVM

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Posted 19 June 2020 - 06:23 PM

Mirror is here! Thank you Steve!

 

attachicon.gifbigmirror.jpg

 

(Yes, it did have padding, I removed it for the photo).

 

Still looking for a secondary - really need a 5" or 6" one.

Nice ! Smart catch on that one :)

 

Hey, BTW, what are the dimensions of that glass tabletop behind you ? ... :)  :)

 

CS

Bob


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#62 Augustus

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Posted 20 June 2020 - 12:11 AM

Nice ! Smart catch on that one smile.gif

 

Hey, BTW, what are the dimensions of that glass tabletop behind you ? ... smile.gif  smile.gif

 

CS

Bob

30", I have two, thought about slumping them maybe but never really made sense. My mom would like to get rid of them at some point, LOL


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#63 Bob4BVM

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Posted 20 June 2020 - 01:29 AM

30", I have two, thought about slumping them maybe but never really made sense. My mom would like to get rid of them at some point, LOL

TWO of them ! ?

 

30-inch Bino-Scope !!!


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#64 a__l

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Posted 20 June 2020 - 02:13 AM

Seeing this mirror, I flinched. I did not expect such destruction. Actually, it makes sense to think, it makes sense to invest money and your own work in a telescope with such a mirror.

 

By the way, I still have not involved 5-inch mirror (pyrex, dielectric coating) with a holder from AstroSystems for my 24". After I replaced it with 5.5 inches quartz. Both from Terri Ostakhovsky. But there the price is not $150, for the last I paid more than $1000. My postage is close to the amount you want for the secondary.


Edited by a__l, 20 June 2020 - 02:54 AM.

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#65 Augustus

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Posted 20 June 2020 - 01:52 PM

I've pretty much finalized the scope design, which will use an Obsession UC-style UTA with a pyramidal 4-vane spider. There are a variety of reasons for doing this including ease of construction, maximum safety for the secondary (compared to a NMT-style UTA with a conventional spider and the focuser hanging below), and overall portability. I'm well aware of the weaknesses of a single-ring UTA but if NMT is able to get away with it and slap on giant finderscopes and filter slides to boot, I think I'm fine. 

 

The truss poles are 1.25" OD aluminum and will be attached using the New Moon CTS system. The bearings will be 1.5" thick, 20" radius crescents with the top 10 degrees cut off for clearance (and let's face it, I never observe anything lower than that anyways, especially with something this big).



#66 Bob4BVM

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Posted 20 June 2020 - 04:30 PM

I've pretty much finalized the scope design, which will use an Obsession UC-style UTA with a pyramidal 4-vane spider. There are a variety of reasons for doing this including ease of construction, maximum safety for the secondary (compared to a NMT-style UTA with a conventional spider and the focuser hanging below), and overall portability. I'm well aware of the weaknesses of a single-ring UTA but if NMT is able to get away with it and slap on giant finderscopes and filter slides to boot, I think I'm fine. 

 

The truss poles are 1.25" OD aluminum and will be attached using the New Moon CTS system. The bearings will be 1.5" thick, 20" radius crescents with the top 10 degrees cut off for clearance (and let's face it, I never observe anything lower than that anyways, especially with something this big).

Hmmm,  i for one would not use the UC UTA design. I have a couple friends with 15 & 18" versions of the UC and they would certainly agree based on their long experience.

Talk to someone who actually uses a UC before you head down that rabbit hole. Even more concern with your larger secondary...

CS

Bob



#67 Augustus

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Posted 20 June 2020 - 04:32 PM

Hmmm,  i for one would not use the UC UTA design. I have a couple friends with 15 & 18" versions of the UC and they would certainly agree based on their long experience.

Talk to someone who actually uses a UC before you head down that rabbit hole. Even more concern with your larger secondary...

CS

Bob

The Obsession UCs use a plywood ring, which by all accounts is vastly inferior to aluminum. I used a UC22 and found it was slightly unsatisfactory but nothing that completely ruined the overall experience.

 

I've yet to meet an unhappy New Moon Hybrid owner and they use aluminum rings for those.



#68 Augustus

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Posted 20 June 2020 - 04:44 PM

Ended up finding that I can get away with a 4" secondary mirror instead of a 5" one if I ignore some very mild vignetting of the outer 1/8" or so of the mirror (so now it's closer to 23.25"); I ended up going with that. $280 for a 1/12 wave vs. $800+ for 1/8 wave....


Edited by Augustus, 20 June 2020 - 04:48 PM.


#69 a__l

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Posted 20 June 2020 - 07:57 PM

You are mistaken again! It will not be vignetting. It will be circumcision of your primary! It makes no sense to use such an aperture with a 4" secondary!
There is a nuance. Do not forget about the lip AstroSystems holder. This is my mistake with a 5 inch mirror. Another reason I bought Denk bino.

 

Ps. It makes no sense to use 1/12 wave secondary for your primary mirror.


Edited by a__l, 21 June 2020 - 02:04 AM.


#70 Augustus

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Posted 20 June 2020 - 08:08 PM

You are mistaken again! It will not be vignetting. It will be circumcision of your primary! It makes no sense to use such an aperture with a 4" secondary!
There is a nuance. Do not forget about the lip AstroSystems holder. This is my mistake with a 5 inch mirror.

UTA is minimum possible diameter (25" ID) and the focuser (which I'd like to add is another HC2) is as far inward as can be so it should be fine. 

 

Capture.JPG



#71 Steve Dodds

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Posted 20 June 2020 - 08:08 PM

My calculations show a min. size of 4.5"


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#72 Augustus

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Posted 20 June 2020 - 08:22 PM

My calculations show a min. size of 4.5"

Distance from the center of the secondary to the fully racked-in top of the focuser is 13.7 inches (the HC-2 is on a 0.25" thick baseplate and 0.965" tall), and I like to position my focal plane so that there's just enough inward travel to accommodate different people's eyesight and the Paracorr takes care of adjustment between eyepieces anyway for the most part. I'm actually not 100% sure how the Paracorr affects field illumination since it basically acts as a relay lens moving the focal plane outward. The 14.7" has an equally minimal secondary size of 3.5" and it doesn't vignette or clip at all.

 

If I'm wrong maybe there might be some really minor clipping (if we move the focal plane out by half an inch I lose 1/8" around the edge - and on smaller mirrors that's the kind of proportional loss you get depending on how wide the bevel is), but seeing as there's conflict as to whether masking down to 23.5" is even enough anyway, I don't really think it's worth worrying about. Honestly any miniscule (and I mean REALLY miniscule) light loss is probably made up for by the superior coating of a brand new Antares vs getting some old AstroSystems or Ostahowski anyway..... plus I saved at least 400-500 bucks. 


Edited by Augustus, 20 June 2020 - 08:25 PM.


#73 a__l

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Posted 20 June 2020 - 08:39 PM

And another nuance. 5 inches heavy mirror. It is dangerous to glue this. There will be a lot of broken glass.
I really do not recommend using one plywood ring. As the aperture grows, problems grow rapidly.
You will understand this when you start building.



#74 Augustus

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Posted 20 June 2020 - 08:43 PM

And another nuance. 5 inches heavy mirror. It is dangerous to glue this. There will be a lot of broken glass.
I really do not recommend using one plywood ring. As the aperture grows, problems grow rapidly.
You will understand this when you start building.

It's going in an AstroSystems holder and the UTA ring is aluminum?



#75 a__l

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Posted 20 June 2020 - 08:58 PM

UTA is minimum possible diameter (25" ID) and the focuser (which I'd like to add is another HC2) is as far inward as can be so it should be fine. 

 

attachicon.gifCapture.JPG

http://www.loptics.c.../diagonals.html

13,8 -12,5 = 1,3" Obviously not enough. You need at least 2 times more. In this case, paracorr-2 will partially block the light path. HC2 it has nothing to do with it. For example, in the case of MoonLite focuser, this will add part of the focuser tube to the light path.

 

See http://www.televue.c...id=61&Tab=_ttop

89 мм (3.5") so that paracorr-2 does not block the light path.


Edited by a__l, 21 June 2020 - 12:42 AM.



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