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6" F3 Mirror and Scope Build

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#1 Aaron_tragle

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Posted 02 December 2019 - 09:57 PM

Hello CN ATMers!

 

I have been doing visual astronomy since Christmas 2016 when I got my first telescope, a 8" F6 Dobsonian. Now, almost 3 years later (wow, that went by fast) and I am now a senior in high school and looking to the future.

 

Sadly, I have realized that fitting a 8" solid tube dob in a dorm room is simply impractical, especially alongside my astrophotography rig. I have been wanting to learn about optics for a while now and have even considered as a career path, so with the encouragement of Zane (Augustus) and Logan (lognic04) I have decided to make myself a telescope!

 

I’ve chosen to build a 6” f/3 for four reasons:

#1. It will be a great visual instrument for viewing large star clusters and hopefully integrated flux nebulae. My planned college is in the mountains with plenty of Bortle 2 and 3 dark sites within a 30 min drive.

#2. The scope can (hopefully) function as an astrograph for both my own personal projects as well as ASASSN supernova work (just got my first supernova confirmation 2 days ago, as a matter of fact!).

#3. As has been shown by Logan, a 6” f/3 should be achievable for a beginner, so I believe I can make this mirror relatively high quality on my first go.

#4. My dorm room is tiny and I need something as compact as possible.

 

This thread will serve to document the process as well as for me to inevitably ask for advice when there are hurdles I need to jump.

 

I will continue to post updates as this goes!


Edited by Aaron_tragle, 02 December 2019 - 09:58 PM.

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#2 Augustus

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Posted 02 December 2019 - 10:03 PM

Good luck Aaron! Excited to see how this goes.



#3 MitchAlsup

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Posted 03 December 2019 - 12:17 PM

Mel Bartels has documented his 6" F/3 build on his web site.



#4 Aaron_tragle

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Posted 03 December 2019 - 12:21 PM

bow.gif  I didn’t know about this! Thank you so much.



#5 Aaron_tragle

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Posted 04 December 2019 - 06:16 AM

As I am starting to create the list of things I will need to grind the mirror itself, a few questions have been raised about some of the components. One of the major questions is where I will get a metal tool and what could honestly work as one. I talked with Logan Nicholson yesterday (His 6" F3 thread is on the ATM forum too) about the making of an F3 mirror and he told me that pretty much any bit of metal would work, but I was going to ask if there were any recommendations that I could pick up at a hardware store?


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#6 m. allan noah

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Posted 04 December 2019 - 07:19 AM

You are not going to find a 6 inch disc of metal at a hardware store, you'll have to visit a metals supplier or buy it online. I recommend speedymetals.com. As for which metal, it would seem to me that there are pretty big differences in performance and cost, so I think you need more guidance than just any metal. If you have some thread you are following, it is helpful to link to it, so we can be on the same page.

 

BTW, there are a few members of RAS that build scopes, myself included. We can help with tools, etc as you get further along into scope construction.

 

allan



#7 Aaron_tragle

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Posted 04 December 2019 - 08:08 AM

Sorry, in hindsight a link to that thread would be helpful: https://www.cloudyni...h-f3-mirrordob/ . I probably will need some help/tools as I get further into construction, I am pretty sure I am a member of the RAS. I’ve been doing outreach with Terry Barker for my school almost 3 years now so I am familiar with the club, just either too busy to make it to the meetings downtown or I simply didn’t have a driver when I was younger. My schools physics dept. has a closet full of optical equipment that the school has cleared me to look through and see if there is anything I could borrow/use as long as I enter the scope into the state science fair. This scope project is going to be one heck of a ride!

 

-Aaron Tragle 


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#8 davidc135

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Posted 04 December 2019 - 01:23 PM

If you decide on metal for a tool get a ring, not a disc. Even soft metals grind down very slowly compared to glass. I used a 4'' ring of aluminium with the metal about 1/2 inch deep and wide to generate both concave and convex curves on 8'' glass discs. If the circular strokes take the tool barely over the glass edge I get concave. Conversely, with the same tool, if the strokes are as wide as possible and the glass centre is barely covered the result is convex. I thought the ratio of wear, glass and metal was around 50:1. Very rough measurements. 

Buying another 6'' glass disc as a tool is possible too and could be used all the way through to fine grinding at the expense of more 80 grit being needed to rough grind.  David

 

Edit: A disc is fine, my mistake. As the glass becomes concave only the edge of the metal is in contact and so becomes a ring. Try a 3'' cast iron plumbing flange. I see one online for 3.50.


Edited by davidc135, 04 December 2019 - 01:43 PM.

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#9 Pinbout

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Posted 04 December 2019 - 01:33 PM

a 6in f5 would be easier. and still small enough... and can break down to a tiny package.

 

https://www.youtube....h?v=Qr5-12Bvb80

 

 

but so can a 10inf5

 

https://www.youtube....u8CNBe0k&t=107s


Edited by Pinbout, 04 December 2019 - 01:34 PM.


#10 Augustus

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Posted 04 December 2019 - 01:45 PM

a 6in f5 would be easier. and still small enough... and can break down to a tiny package.

https://www.youtube....h?v=Qr5-12Bvb80


but so can a 10inf5

https://www.youtube....u8CNBe0k&t=107s


He wants f3 for an astrograph and so he can do IFN.

I did an f4.3 on my first try. I think f3 is totally doable
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#11 Pinbout

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Posted 04 December 2019 - 01:51 PM

He wants f3 for an astrograph and so he can do IFN.

I did an f4.3 on my first try. I think f3 is totally doable

yeah I've see a lot of f6's figured like f3's... lol.gif


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#12 Augustus

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Posted 04 December 2019 - 01:59 PM

yeah I've see a lot of f6's figured like f3's... lol.gif


My f4.3 is over corrected by about 1/6 of a wave.....

This thread makes me want to stick it back in a scope. Probably will when the 14 is done

#13 dave brock

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Posted 05 December 2019 - 12:37 AM

He wants f3 for an astrograph and so he can do IFN.


Ok, what's IFN?
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#14 Aaron_tragle

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Posted 05 December 2019 - 12:49 AM

Ok, what's IFN?

IFN also known as Integrated Flux Nebulae are large structures illuminated by the light from all the stars in our galaxy. It is ridiculously faint and ultra low magnification, like I’d get from this scope, are ideal for seeing it.
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#15 dave brock

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Posted 05 December 2019 - 01:30 AM

Ta

#16 Dale Eason

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Posted 05 December 2019 - 03:00 AM

Ok, what's IFN?

https://www.bbastrod...els Ghosts.html


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#17 Aaron_tragle

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Posted 06 December 2019 - 04:27 PM

Getting back on-topic, the mirror and blank have been acquired! Zane (Augustus) has also generously sent me a spherometer for this project. I will be building the grinding machine this weekend based on Lognic04's design.


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#18 Aaron_tragle

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Posted 08 December 2019 - 12:29 PM

Spherometers have arrived! I started building the fixed post machine last night, please excuse my horrid handwriting. I went to the hardware store and picked up the materials for the machine, I was really lucky to find an 18" wood disk so now this machine is capable of making up to a 16" mirror! I also got pretty lucky and found a 4" pipe flange that I am going to use as a metal tool. It doesn't look great, but for the first woodworking project that I have done on my own other than a little project I did in woodshop a few years ago I am pretty proud of it! Today I am finishing the fixed post and getting the turn table done, I bought too long of screws for the 1" thick wood disk.

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#19 Lognic04

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Posted 09 December 2019 - 06:23 AM

Can't wait to see how it goes! Looking forward to hopefully meeting you with the completed scope at Stellafane next year!! waytogo.gif


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#20 Augustus

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Posted 09 December 2019 - 10:56 AM

This is going to be interesting to watch. Hopefully I get to see the finished product at Stellafane!

 

Honestly this is making me consider making another mirror myself.


Edited by Augustus, 09 December 2019 - 11:06 AM.


#21 DAVIDG

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Posted 09 December 2019 - 01:52 PM

yeah I've see a lot of f6's figured like f3's... lol.gif

Danny ie Pinbout gives great advice. An f/3 mirror is very difficult to figure and has a very narrow range to be within 1/4 wave. You need the right test methods ( note the plural)  and to fully understand how they are done and what the sources of errors are in them so they can be corrected for. If not what think is  a good mirror when tested may turn out to be far off. 

There is 8" f/3 in the classified right now  selling for $500 and with test image showing that is very well corrected. The price in my opinion is bit low for how difficult it is to figure such a fast mirror to the level of correction shown in the image.  So why a 6" f/3 can  do done as  a first mirror don't be mislead  that it is not going to be a challenge to figure it to be diffraction limited. 

 

                   - Dave 


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#22 davidc135

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Posted 09 December 2019 - 04:07 PM

Sounds like the OP has low power in mind (?) so a bit of error can be tolerated.   David


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#23 DAVIDG

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Posted 10 December 2019 - 02:39 PM

Sounds like the OP has low power in mind (?) so a bit of error can be tolerated.   David

 If the goal is to observe these faint low contrast nebula you want the very best figure to increase the contrast to the max. Here is what Mel Bartels wrote when he compared a couple of different RFT including his own 6" f2.7  https://www.bbastrod...els Ghosts.html

 

" The f5 and f3.8 telescope did give an identical view, but the f2.7's view was more vivid and brighter. The only differences we found was that the f2.7 had enhanced coatings and the 'insane' quality of the mirror."

 

  So  some spherical aberration maybe tolerated but the problem is at F/3 the tolerances are very very tight. So even to have a "bit of error" is going to be  a challenge. So getting to a 1/4 wave is going to be challenge.  Doable as a  first mirror I would say yes, but you have to have good test methods  Not just one method ! ) to show the true figure so you know how much error you have. 

 

              - Dave 


Edited by DAVIDG, 10 December 2019 - 11:17 PM.

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#24 Aaron_tragle

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Posted 13 December 2019 - 04:17 PM

Sorry for the lack of updates, I have been working really hard to get ready for exams at school. In this hiatus, I finished the turntable of the mirror machine, received the mirror and grits also did the initial sagitta measurement of the mirror.

 

Mirror Machine : https://www.youtube....eature=youtu.be

Mirror Blanks: https://youtu.be/xRPxUkoDidE


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#25 Gordon Waite

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Posted Today, 02:31 AM

A possible bit of advice for your fixed-post grinding machine.  With such a small motor, you don't want it to have to do more work than necessary.  So you might consider inverting the caster wheels and mounting them to the table top, instead of mounting them under the turntable.  The way you have it now with the wheels on the turntable, the motor has to turn the weight of all of the casters, instead of just the weight of the turntable itself.  The turntable should ride on top of the casters.  Good luck!


Edited by Gordon Waite, Today, 02:31 AM.



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