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14.7" f/3 Dob/RFT

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#51 Oberon

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Posted 03 December 2019 - 02:46 PM

Doesn't that require a big expensive flat, or is that a different test?

It requires a bath, a liquid, and a gravity. You can use a cheap pan, oil, and the gravity comes free.


 

#52 Pierre Lemay

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Posted 03 December 2019 - 03:08 PM

Doesn't that require a big expensive flat, or is that a different test?

The Bath and DPAC are different tests. Bath is a simplified, common path interferometer test. You can learn more about this test, and building a tester, in this thread: https://www.cloudyni...-at-the-bottom/  . Using free software to interpret the data it will provide a quantitive, 3D mapping of the optical surface under test. 

 

The Double Pass AutoCollimation Test (DPAC) is one which calls upon the use of an smooth optical flat. See this thread for more details on how it works and is conducted: https://www.cloudyni...4046-dpac-test/ . Unlike the Bath, it’s more qualitative than quantitative but, if the optical flat is smooth, DPAC is capable of revealing 1/13th wave peak-to-valley geometrical defects on the mirror surface instantly, without having to resort to any software or measurements. It can also be used to test complete optical tube assemblies, wether Newtonian , refractor or compound instruments. 


 

#53 stargazer193857

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Posted 03 December 2019 - 03:32 PM

Steve Dodds uses that test. I bet flats that size are expensive.... If you want to test for spherical undercorrection. If you only are looking for lumps and bumps, I wonder if smooth power would be acceptable.
 

#54 a__l

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Posted 04 December 2019 - 12:47 AM

First of all you can determine astigmatism. From the stand (mirror cell) or from the mirror.

Price tag: small lens, flat mirror and beam splitter.

Laser, camera and X-Y-Z table. Plus open software (Dale)

Read what Dale et al writes.

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Edited by a__l, 04 December 2019 - 01:11 AM.

 

#55 BGRE

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Posted 04 December 2019 - 01:39 AM

Just don't try to make accurate measurements using that exact setup unless you're into rigorous diffraction calculations etc.

Either use a lens to image the test surface onto the screen or use a camera focused on the test surface.

 

Someone actually tried the depicted fringe projection setup using a camera to capture the fringes on the screen. 

The results weren't accurate due to diffraction and other effects.

When he was persuaded to use the camera to capture the interferogram directly, the accuracy and consistency improved.

The camera views the test surface through the interferometer and is focused on the test surface. 

 

For large fast test surfaces imaging distortion and other effects can be significant. These effects aren't limited to Bath interferometers.


 

#56 Augustus

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Posted 07 December 2019 - 10:52 AM

Mirror & cell arrive this week. In the meantime I've started on the mirror box.

 

I decided on a solid hardwood construction this time, using maple with bits of other hardwoods. After tackling a few coffee table and cabinetry projects I figure I am ready to try doing a hardwood-only scope. In addition to the simple challenge and the arguably superior aesthetic, using maple actually saves me a few pounds (particularly on the rocker box). 

 

I am well aware of the potential risks in this design choice but I would rather experiment and see how it pans out myself.

 

So far, I have joined together the top and bottom segments of the mirror box and begun to put a finish on them. They have turned out quite flat and the final product is going to look awesome.

 

glueup.JPG

 

I'm not bothering to install fans. The thermal inertia of this scope's thin, relatively small mirror is quite low (and it's quartz).


 

#57 a__l

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Posted 07 December 2019 - 08:56 PM

It must be very well made wood. And probably for the right price.
Otherwise, cracks and curvature (after some time).


 

#58 dave brock

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Posted 07 December 2019 - 09:19 PM

Some mirror makers won't sell thin mirrors unless they sell the cell too.


Who does that?
 

#59 Augustus

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Posted 12 December 2019 - 05:47 PM

As everyone probably predicted I ended up ditching maple. Was unhappy with the size the mirror box was gonna be and had too many joinery issues. 

 

Built the new plywood mirror box, and started gluing the UTA reinforcement rings in.

 

glueuputas.jpg


 

#60 hamishbarker

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Posted 12 December 2019 - 07:43 PM

As I understand, minimum magnification is limited to that which results in an exit pupil no bigger than that of the observer's eyeball. So 5-7mm depending on the eye (age, dark adaption factors etc).

 

If one is making 14.7" mirror, this means minimum magnification would be 53-74x (depending on eyeball.)

 

So at f3 this would mean an eyepiece of not longer than 16-21mm (same).

 

But if the focal ratio was say f5 instead, this eyepiece could be as long as 35mm.

 

Given that wide field 2" eyepieces are available, what's the attraction of going for the ultra short focal ratio which places (I guess) extreme demands on the coma corrector?  


 

#61 Augustus

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Posted 12 December 2019 - 08:00 PM

As I understand, minimum magnification is limited to that which results in an exit pupil no bigger than that of the observer's eyeball. So 5-7mm depending on the eye (age, dark adaption factors etc).

 

If one is making 14.7" mirror, this means minimum magnification would be 53-74x (depending on eyeball.)

 

So at f3 this would mean an eyepiece of not longer than 16-21mm (same).

 

But if the focal ratio was say f5 instead, this eyepiece could be as long as 35mm.

 

Given that wide field 2" eyepieces are available, what's the attraction of going for the ultra short focal ratio which places (I guess) extreme demands on the coma corrector?  

The coma corrector acts as a 1.15x Barlow, which shrinks the exit pupil a little. Also the primary is actually 14.5" clear aperture, with .1" of bevel on the edge.

 

A 10" f/5 can get a max field of 2.2 degrees using a 41mm Panoptic and no coma corrector. If you want coma correction that field shrinks to 1.889 degrees thanks to the amplifying power of the coma corrector.

A 10" f/3 with a 20mm 100-degree eyepiece can get a max coma-corrected field of 2.27 degrees (wider than the f/5 even without a corrector) with the benefits of a wider AFOV, at more or less the same exit pupil as the 10" f/5. My 10" f/3.2 gets a field of 2.13 degrees.

 

Faster focal ratios also mean shorter tubes which is a huge help at big apertures. You need a trailer and a heavy ladder for a 25" f/5 - the darned thing is 10 feet tall. A 25" f/2.6 fits in a Camry and the eyepiece is reachable by most folks even at the zenith without a stepstool or ladder.


 

#62 Augustus

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Posted 12 December 2019 - 09:11 PM

This is my second time using "cabernet" stain. I love it.

 

boxvarnishdrying.jpg


 

#63 Augustus

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Posted 27 December 2019 - 04:40 AM

The primary finally shipped on Christmas Eve. I’ll be home in just under a week to resume work. I have the cell now too.
 

#64 Augustus

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Posted 28 December 2019 - 03:25 PM

Mirror arrives on the 7th
 

#65 Augustus

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Posted 04 January 2020 - 06:24 PM

Painted the inside of the mirror box as well as the entire UTA with Black 3.0. Also cut the main light baffle.

 

Still struggling to find a blade that'll cut through the steel spider vanes, once I do I'll be cutting the vanes and installing the spider and focuser in the UTA. 


 

#66 jtsenghas

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Posted 06 January 2020 - 08:39 PM

Painted the inside of the mirror box as well as the entire UTA with Black 3.0. Also cut the main light baffle.

 

Still struggling to find a blade that'll cut through the steel spider vanes, once I do I'll be cutting the vanes and installing the spider and focuser in the UTA. 

Depending on how thin your vanes are,  you might try scoring them with a sharp utility knife with many passes,  and then flexing and snapping them.  This is how steel shim stock is often cut and this method results in a straight edge with only a slight burr that can be sanded.  The trick is to make all of your scoring cuts exactly coincident,  even if that requires you to clamp a straight edge to the vane.  If you have to sharpen your utility knife during this work once or twice,  so be it. 

 

I also have a bottle of Black 3.0 squirreled away and haven't tried it yet.  How robust is it against scratching and marring? Acrylic paints vary so much. 


Edited by jtsenghas, 06 January 2020 - 08:43 PM.

 

#67 Augustus

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Posted 07 January 2020 - 04:21 PM

Got the mirror from Steve today. 3 months from ordering to delivery ain't bad at all for a premium 14.7" f/2.9 mirror!

 

This thing is utterly surreal. The curve is so deep that you can easily see it looking even slightly horizontally at the mirror.  And of course it's a featherweight!

 

Unfortunately it does not fit the AP cell by about 0.1". Just emailed Nathan. Best fix I can think of would be slightly flattening the edge supports or making them a smaller diameter.
 

Depending on how thin your vanes are,  you might try scoring them with a sharp utility knife with many passes,  and then flexing and snapping them.  This is how steel shim stock is often cut and this method results in a straight edge with only a slight burr that can be sanded.  The trick is to make all of your scoring cuts exactly coincident,  even if that requires you to clamp a straight edge to the vane.  If you have to sharpen your utility knife during this work once or twice,  so be it. 

 

I also have a bottle of Black 3.0 squirreled away and haven't tried it yet.  How robust is it against scratching and marring? Acrylic paints vary so much. 

That sounds mildly terrifying but I'm gonna try that.

 

I'm a little disappointed by my Black 3.0 tbh. For the outer surface of the UTA I am going to use flat black latex paint over the 3.0 because the 3.0 looks.... rather ugly.

 

Of course I failed on my trip to HD today to grab the angle brackets for the spider... sigh.


 

#68 jtsenghas

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Posted 07 January 2020 - 04:48 PM

If your vanes are too thick,  or the steel too mild and flexible,  my suggestion may not work,  but for vanes made from shim stock or similar material this works well. 


 

#69 jtsenghas

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Posted 07 January 2020 - 04:53 PM

Note that Black 3.0 can be thinned with water to lay on a smoother coat.  No doubt that increases drying time. It's not meant to be cosmetically pretty or durable,  just especially non reflective.  I may use some in the UTA of my Chief and Astroscan rebuild. 

 

(Edited for typos with my new phone.  **** Autocorrupt!)


Edited by jtsenghas, 07 January 2020 - 05:13 PM.

 

#70 Augustus

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Posted 08 January 2020 - 06:18 PM

I will try the water thinning, perhaps. Thanks!

 

Turns out my cell has 1/2" posts instead of the 3/8" ones that were supposed to come with it. That extra quarter inch will allow me to fit the mirror no problemo. 


 

#71 Augustus

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Posted 10 January 2020 - 03:32 PM

Of course, mirror box #2 was severely, permanently out-of-square, and it's occurred to me that I probably need more mass in that thing to balance.

 

Time for mirror box #3.......


 

#72 a__l

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Posted 10 January 2020 - 08:34 PM

There must be a proper build sequence. It is likely that there will be a mirror box number 4.
The mirror box should be built last. When the weight of all parts is known.
For example UTA. There are reasonable doubts about its stiffness for a spider with thin blades.

 

Perpendicularity must be checked in all angles. Top and bottom. Tool (photo).

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Edited by a__l, 10 January 2020 - 08:53 PM.

 

#73 avarakin

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Posted 10 January 2020 - 08:55 PM

There must be a proper build sequence. It is likely that there will be a mirror box number 4.
The mirror box should be built last. When the weight of all parts is known.
For example UTA. There are reasonable doubts about its stiffness for a spider with thin blades.

I thought that the bearings should be cut last, once the weight of UTA and box are known. 

I am not an expert, just a thought.

 

Alex


 

#74 a__l

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Posted 10 January 2020 - 08:58 PM

The question is the reasonableness of their size.


 

#75 avarakin

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Posted 10 January 2020 - 09:16 PM

I've seen some pretty large bearings.

For this scope the bearings should be very reasonable, considering very short FL.


 


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