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Nstiesi's 4" f/10 Newt

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#26 Garyth64

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Posted 11 October 2019 - 11:05 AM

Will a spherical f10 mirror be pretty close to a parabolic f10 mirror in performance?

 

A 4" f/10 mirror could be made spherical and give some very good views. 

A parabolic 4" f/10 mirror will be just a bit better.

 

On the 4¼" f/13 I made a couple years ago, I made it as close to a parabola as I could.  At f/13 it too, could have been left spherical, and would have been a very good mirror.  I got the correction very close to the theoretical  values.  It was an excellent mirror.


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#27 dogbiscuit

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Posted 11 October 2019 - 02:26 PM

4" f:10

If it's a sphere, it's a parabola.



#28 Garyth64

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Posted 11 October 2019 - 03:45 PM

Ah, that would be a no.  You can do the math.



#29 dogbiscuit

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Posted 11 October 2019 - 05:05 PM

Everyday we refer to 1/4 or 1/8 or 1/10 wave mirrors of all common diameters and f# amateur mirrors as parabolas.

A 4" f:10, if it is a sphere, wavefront error is 1/11 wave.  That is fairly good parabola.


Edited by dogbiscuit, 11 October 2019 - 05:06 PM.

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#30 Garyth64

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Posted 11 October 2019 - 06:17 PM

Sorry, a sphere is a sphere, a parabola is a parabola.  At those slow f ratios, a sphere can remain a sphere, and does not have to be parabolized to have a good figure. 

a 4" f/10, if it is a sphere, with a wave from error of 1/11 wave, is a very good sphere, still not a parabola.

 

For my 4¼' f/13, testing the different zones, the theoretical values for those zones as a parabola were:

.000, .006, .019, and .030.

For that mirror for it to be a sphere, the numbers for those zones would be:  .000, .000, .000, .000.

 

But that mirror I made was a parabola, not a sphere.

 

It's not much, but there is a difference between the values, and that is what I'm talking about.  smile.gif


Edited by Garyth64, 11 October 2019 - 06:45 PM.

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#31 nstiesi

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Posted 11 October 2019 - 07:56 PM

That is interesting...out of curiosity I did the math today (well, I cheated and sketched it in CAD)...I was surprised how much deviation there was between a perfect sphere and parabola just 2 inches from the central axis (.025"). Again, I'm new to optics but not to math....I would have blindly guessed the deviation would be less.

However, in practicality we can all agree that this mirror is likely good enough...especially for the 6 year old that it is being built for.

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#32 dogbiscuit

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Posted 11 October 2019 - 08:11 PM

The reason a sphere is ok for a 4" f:10 (or a 4.25" f:13) is that it is a parabola within reasonable optical tolerances.

It is as much a parabola as a 10" f:3 of the same wavefront error.

1/10 wave error is 1/10 wave error.

 

Figuring that sphere is not necessarily any easier than the parabola.



#33 dogbiscuit

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Posted 12 October 2019 - 12:04 AM

That is interesting...out of curiosity I did the math today (well, I cheated and sketched it in CAD)...I was surprised how much deviation there was between a perfect sphere and parabola just 2 inches from the central axis (.025"). Again, I'm new to optics but not to math....I would have blindly guessed the deviation would be less.

However, in practicality we can all agree that this mirror is likely good enough...especially for the 6 year old that it is being built for.

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That .025" difference in ROC amounts to about 25 nanometers height difference on the mirror surface.



#34 nstiesi

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Posted 15 October 2019 - 07:48 PM

So, back on topic.

Made alot of progress in the last two days....but....I guess my patience has worn thin. Not in a bad way, but precision has gone out the window on this build.

I cut the tube clamps by hand apart from the circular portion. I had to shim the altitude bearings unevenly to make up for the poorly cut clamps.

The worst part is the GROSS underestimation of the weight of the mirror (and it's plastic mirror cell). Because this is for my son, I wanted to keep the whole thing short (it is currently at his eye level at zenith). But, this means I cannot place the tube clamps at the COG.....so it is very nose heavy.

With a cheap red dot and a modest eyepiece, it looks like I'll need about 2 lbs at the bottom of the tube. I haven't come up with a good way to add the weight yet...I don't even have the end of the tube closed yet.7d210fbb8472f2f8d14c66f57c294b77.jpgIMG_20191015_203054622.jpg

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#35 Gordon Waite

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Posted 15 October 2019 - 11:19 PM

Sorry, a sphere is a sphere, a parabola is a parabola.  At those slow f ratios, a sphere can remain a sphere, and does not have to be parabolized to have a good figure. 

a 4" f/10, if it is a sphere, with a wave from error of 1/11 wave, is a very good sphere, still not a parabola.

 

For my 4¼' f/13, testing the different zones, the theoretical values for those zones as a parabola were:

.000, .006, .019, and .030.

For that mirror for it to be a sphere, the numbers for those zones would be:  .000, .000, .000, .000.

 

But that mirror I made was a parabola, not a sphere.

 

It's not much, but there is a difference between the values, and that is what I'm talking about.  smile.gif

Actually, if you are going to be so fussy, you probably didn't make a parabola.  Unless you hit those "theoretical" numbers exactly perfect, then you probably have either an ellipsoid or a hyperboloid.  The odds that you actually made a paraboloid are vanishingly small.  If your eccentricity is on the low side of 1, you made an ellipsoid.  If greater than 1, you made a hyperboloid.  Do you think your eccentricity is really 1.0000000... on the mirror you made?  Live by the picky-picky, die by the picky-picky. ;)


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#36 dave brock

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Posted 16 October 2019 - 04:46 AM

Also, when in use, with the night time temperature falling, the 4" f/10 or f/13 sphere will correct toward a parabola anyway so will probably perform better.

Edited by dave brock, 16 October 2019 - 04:50 AM.


#37 nstiesi

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Posted 16 October 2019 - 06:28 AM

Guys, I think the sphere vs paraboloid horse is dead now.  I am much more concerned with balancing the scope so it is usable than the figure of the $30 mirror.


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#38 Garyth64

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Posted 16 October 2019 - 08:20 AM

No dying here.  I made a statement that I made a mirror as close to a parabola as I could.  It was not I who first started being picky-picky.

 

nstiesi, I am sorry for the sides, I didn't know there would be such a fuss.  (but this is CN)



#39 Garyth64

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Posted 16 October 2019 - 08:25 AM

Guys, I think the sphere vs paraboloid horse is dead now.  I am much more concerned with balancing the scope so it is usable than the figure of the $30 mirror.

How much room do you have before the mirror end hits the mount?

 

Since you made rings from plywood, could you make a couple more and put them at the mirror end?

 

Is it possible to raise up the two sides by adding in wood spacers at the bottom?



#40 nstiesi

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Posted 16 October 2019 - 08:51 AM

How much room do you have before the mirror end hits the mount?

 

Since you made rings from plywood, could you make a couple more and put them at the mirror end?

 

Is it possible to raise up the two sides by adding in wood spacers at the bottom?

The tube is shoved as far south as it can go without the collimation bolts hitting the edge of the rocker box.  That edge is only 3" tall.  I'd rather not raise it so my kid wont need a step stool.  I'm going to stop by Walmart tonight and get some exercise wrist weights and velcro straps.  Not sure if that will be the permanent solution though...

 

I tested it out last night by using a bungee to strap some steel flat stock to the end of the tube.  The motion seemed nice...moved easily (maybe a little too easily) and stayed put when stopped.  I may also end up needing an extension spring, though I am hoping to avoid that.  I was actually fairly pleased with it despite rushing through and hacking up the 2nd half of this build....


Edited by nstiesi, 16 October 2019 - 08:55 AM.


#41 nstiesi

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Posted 17 October 2019 - 08:57 PM

The wrist weights actually worked very well. Does t look great, but at least it is easy, soft, and adjustable.

I feel like the altitude bearings need more friction. I haven't had a good sky to go test it, but I am worried about the scope moving while turning the helical focuser. Need to try it out to know for sure.18953fd42a32d8d5fb40ebf0a244805d.jpg

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#42 ed_turco

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Posted 19 October 2019 - 11:00 AM

Your son will love this telescope. 



#43 nstiesi

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Posted 22 October 2019 - 06:24 AM

A few days ago I printed and installed the end cap. That about does it for this build! Although I may add extension springs one day, or paint the plywood edges, this is for all intents and purposes, finished.

All that is left is to get it under the sky for real testing. The only testing I had done so far was very quick and crude as I was holding the tube in a bear hug and trying to look at the moon.

I'll report on the optics once I get a chance. Florida has been rainy and cloudy for weeks!75fe30dd50affa6e0802e88f34a97af1.jpgb11fe563791818172489890703ec8cc0.jpg

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#44 nstiesi

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Posted 22 October 2019 - 06:24 AM

Completed pics:12449bdd0ba7c12a764cc0033b1ac652.jpg2e8f3077ba52fd5f4c52c114fee72ad0.jpg

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#45 m. allan noah

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Posted 22 October 2019 - 06:38 AM

I'd suggest cutting off the excess collimation bolts, and adding some feet to the bottom around the edge, so it can stand on something other than the collimation screws when it is out of the rocker box.

 

Now its time to use it, and figure out what needs changing. No one ever gets it right the first time :)

 

allan



#46 nstiesi

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Posted 22 October 2019 - 06:42 AM

I'd suggest cutting off the excess collimation bolts, and adding some feet to the bottom around the edge, so it can stand on something other than the collimation screws when it is out of the rocker box.

 

Now its time to use it, and figure out what needs changing. No one ever gets it right the first time smile.gif

 

allan

Thanks!  Definitely going to add that to the upgrade list.



#47 nstiesi

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Posted 05 November 2019 - 07:59 PM

I finally got a break in the clouds to take a quick peek. What a nice little scope! Despite the thin cloud veil, the moon was sharp and bright withy modest eyepieces.

Stars were pretty tight, though I didn't have any bright ones visible in cloudy, light polluted skies.

We got a quick look at saturn as well, which also looked good. Seeing wasn't great, as it was pretty shimmer-y, but the rings were pleasing at 43x and 100x. I thought the cassini division may have been just visible at 100x. It was my son's first look at saturn which was cool.

The focuser performed well. The focal plane is in a good place to accommodate my EPs. The mount was smooth and easy to point. I think the biggest issue I'll have is holding collimation...I know the thin plastic spider is the biggest offender. The mirror cell SHOULD be pretty tight, but time will tell. The good news is that it is very easy to collimate, I just need knobs for the spider.

It occurred to me too late to get a picture of my boy using it, but there will be time for that.

All in all I'm very pleased.... especially for about $75 total cost!

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#48 Sully606

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Posted 09 November 2019 - 07:55 PM

Great build, your son is a lucky boy to have such a father! 
These nice inexpensive mirrors are intriguing. I know nothing about bino-scopes but could these be used for such a purpose? It might be a fun build if the mirrors are appropriate. 
 

Sully




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