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Making a 6 inch F3 Mirror/Dob

ATM lens making mirror making optics reflector equipment
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1112 replies to this topic

#1 Lognic04

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Posted 19 August 2018 - 07:06 PM

Hi all!

I was reading through Mel Bartels website, and the site and sketches from his 6 inch f/2.8 were just too cool - i have to see those views!

The 6 inch f3 will also be TINY! like a tabletop/ handheld dob!

The question is - do i want to make it f/3 or f2.8?

I already have the blank, grit and tool!

Will start hogging tonight, its gonna be a long time! But i do have a metal tool

Pyrex blank, 22mm.

 

IMG_20180820_071634_739.jpg


Edited by Lognic04, 19 August 2018 - 07:23 PM.

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#2 Pinbout

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Posted 19 August 2018 - 07:28 PM

F3 or slower

 

it better to get a couple under your belt before going that fast but hey 

 

question is do you have a paracorr II and a 21 ethos- ya need that to get his views


Edited by Pinbout, 19 August 2018 - 07:30 PM.

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#3 Lognic04

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Posted 19 August 2018 - 07:48 PM

An Ethos is an eyepiece that i have always wanted, but no paracorr - i have a GSO CC though?



#4 wrnchhead

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Posted 19 August 2018 - 09:27 PM

Wow I am learning tons from his website, great tip! I now understand the reason for parabolic vs spherical. Still reading, his super fast 6in is definitely fascinating!



#5 ccaissie

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Posted 19 August 2018 - 09:45 PM

f/3 ... nice even number. 

You ought to be able to hog that out pretty quick.  But if your tile tool is flat, that's going to thwart the metal tool hogging process.  Looks flat.  Would be ok for an f/8.  

 

1.  hog out the glass with the metal tool to the approximate radius.

2.  make the tile tool to fit this curve.

3.  Smooth and fine the curve for polishing.

 

I admire Mel's observing and his scope building too!


Edited by ccaissie, 19 August 2018 - 09:49 PM.


#6 Pinbout

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Posted 19 August 2018 - 10:58 PM

An Ethos is an eyepiece that i have always wanted, but no paracorr - i have a GSO CC though?

Cough, cough



#7 Lognic04

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Posted 20 August 2018 - 06:14 AM

Made a rubbish start today - only 6 wets or about 10 mins on the glass, but I got 0.1mm hogged!
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#8 Oberon

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Posted 20 August 2018 - 06:39 AM

Cough, cough

funnypost.gif



#9 Pinbout

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Posted 20 August 2018 - 08:20 AM

An Ethos is an eyepiece that i have always wanted, but no paracorr - i have a GSO CC though?

Is that an evil CC?

 

giphy.gif


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#10 calypsob

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Posted 20 August 2018 - 10:39 AM

An Ethos is an eyepiece that i have always wanted, but no paracorr - i have a GSO CC though?

a gso coma corrector will not work the same because a paracorr acts partially as a barlow by 15% I believe. The gso is better than nothing but in the long run you should definitely keep an eye out for an upgrade. 


Edited by calypsob, 20 August 2018 - 10:40 AM.


#11 stargazer193857

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Posted 20 August 2018 - 12:46 PM

Wouldn't the central obstruction be too big?

#12 Pinbout

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Posted 20 August 2018 - 01:25 PM

Wouldn't the central obstruction be too big?

its widefield so it shouldn't matter

 

what matters is how big an exit pupil can he stand...that's why MB uses a 21mm 100*

 

the eyepiece will be bigger than the scope, well the p2 and 21mm... ha ha ha


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#13 MitchAlsup

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Posted 20 August 2018 - 04:17 PM

I was reading through Mel Bartels website, and the site and sketches from his 6 inch f/2.8 were just too cool - i have to see those views!

The 6 inch f3 will also be TINY! like a tabletop/ handheld dob!

The question is - do i want to make it f/3 or f2.8?

Shoot rough grinding at F/2.9. Then use fine grinding to move the FL ini one direction or the other.

 

F/2.8 may allow wider FOVs

F/3.0 may allow better correction of the FoV with P2

So the question devolves into:

a) are you more interested in widest possible FoVs, or

b) merely really wide FoVs and more pippoint stars?

It is a fine line to tread.

 

Lockwood says stay below F/2.75 and both of your are.


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#14 Pinbout

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Posted 20 August 2018 - 05:08 PM

Did you see mel’s Sketch up model in 3D warehouse?

 

waytogo.gif


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#15 Lognic04

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Posted 21 August 2018 - 03:26 AM

Sag is now 0.6mm after 25 wets.



#16 Steve Dodds

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Posted 21 August 2018 - 06:31 AM

You are 1/5 of the way there.


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#17 Lognic04

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Posted 21 August 2018 - 07:42 AM

You are 1/5 of the way there.

Goes really quick!!

I might switch to tile tool at 2.5mm or so to break the tool in?



#18 Lognic04

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Posted 21 August 2018 - 06:23 PM

For the tube i wonder if i could just make literally a plywood box?

Tonight hopefully i can hit 1mm sag and i have the day off school tomorrow wink.gif grin.gif

Should be able to get to 2.5mm on the day off :) Then switch to tile tool


Edited by Lognic04, 21 August 2018 - 06:37 PM.


#19 Lognic04

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Posted 22 August 2018 - 03:32 AM

1mm sag!
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#20 Lognic04

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Posted 23 August 2018 - 04:21 AM

1.3mm!
What do you guys waterproof plaster of Paris with? I couldn't afford dental plaster...
Should I pour the tool with tiles embedded inside?
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#21 Pinbout

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Posted 23 August 2018 - 05:29 AM

Epoxy resin


Edited by Pinbout, 23 August 2018 - 05:30 AM.


#22 Lognic04

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Posted 23 August 2018 - 06:32 AM

These tiles have an extremely hard ceramic side and a softer back - which side do I use?

#23 Pinbout

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Posted 23 August 2018 - 08:35 AM

you have glazed tiles?



#24 Lognic04

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Posted 23 August 2018 - 03:50 PM

I think so

#25 Lognic04

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Posted 23 August 2018 - 04:52 PM

Should I make a coin tool? The coins here have no zinc in them though, only copper/nickel


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