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4.5 inch reflector is there a great one?

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#1 vtornado

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Posted 28 May 2018 - 04:05 PM

Hi,  I see tons of 4.5 inch reflectors for sale on craigs list.

I bought a newer one that was a orion mini dob, and I am really happy with it.

 

Since I am so happy with the format, I was wondering if there is a "premium" version of this scope.

What about a celestron/vixen model?  Would that mirror be just a little better?

I have seen some long 4.5's that have a 1000mm f.l.  Would that put up a sharper image than my 900mm?

Did any manufacturer ever parabolize one of these mirrors?

 

I have read mixed reviews about the red tasco's,  If I read between the lines, there were probably different sources, or

some sloppy QA, so some are good and others not so good.

 

Thanks for any help.

 

VT



#2 ShaulaB

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Posted 28 May 2018 - 04:22 PM

Have you thought about grinding your own 4.5" mirror? To my knowledge, there is no "premium" version of a 4.5" reflector out there. But making a small mirror at a focal length longer than 700mm is not an overwhelming process.


Edited by ShaulaB, 28 May 2018 - 04:23 PM.

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#3 CHASLX200

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Posted 28 May 2018 - 04:23 PM

Only good one's were the Celestron- Vixen Polaris versions. Had two of them and they were very sharp.


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#4 davidmcgo

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Posted 28 May 2018 - 04:34 PM

My club supposedly has a 4.5” Takahashi Newtonian.   I haven’t  actually seen it but here is a link :

 

http://www.sdaa.org/loanerscope.htm

 

Dave


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#5 deSitter

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Posted 28 May 2018 - 04:41 PM

Not so. Best view of the Moon I ever had with a small (< 5") scope was with a Meade 114/500NT. Absolutely perfect star test. Excellent mechanics, even the plastic focuser was tight and smooth.  Any 4.5" scope is a good bet because a good spherical f/8 to f/10 mirror is an excellent optic. You can't generalize. I would assume classic era 4.5", e.g. Tasco 11TE, are good bets.

 

Video with that scope - no stacking, no processing.

 

https://www.youtube....h?v=qPw-2stFJhE

 

-drl


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#6 nicoledoula

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Posted 28 May 2018 - 04:44 PM

NO. And why would you grind a 4.5" mirror when you can buy a 6" F/8 for $300, or an 8" for less than $500? 


Edited by nicoledoula, 28 May 2018 - 04:46 PM.

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#7 Jon Isaacs

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Posted 28 May 2018 - 04:45 PM

Only good one's were the Celestron- Vixen Polaris versions. Had two of them and they were very sharp.

 

From a mechanical point of view, the Celestron C-4.5 is certainly at the top of the heap of the commonly imported 114mm F/8's... That Polaris mount is a good one.

 

Jon


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#8 Steve Allison

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Posted 28 May 2018 - 04:47 PM

I see 4 inch Carton mirrors for sale periodically, so this could be an option if you are willing to give up a quarter of an inch of aperture. In my experience, Carton products are uniformly excellent. I have a heavy-duty Carton alt-azimuth mount that can hold just about anything...

 

steve



#9 memento

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Posted 28 May 2018 - 04:51 PM

I have / had the Vixen R-114S and a "run of the mill" 114/900 (Tasco etc.).

 

Optics in both are good.

 

If you want nice handling, a really adequate mount, and a good 1 1/4" focuser I would suggest the Vixen GP-R114 (with Great Polaris) or NP-R114 (with New Polaris). But be careful if you buy one on a New Polaris as some versions have a REALLY REALLY STUPID tube ring design. Instead of then sourcing new tube rings, you'd better get one with the proper ones in the first place.

 

Here is a thread on my NP-R114S that had the horrible tube rings:

https://www.cloudyni...the-same-mount/

 

Here is a Vixen NR-R114M with proper tube rings:

http://www.photoinfo.../VixenR114M.htm

 

Here is a thread on my current Circle T 114/900:

https://www.cloudyni...-aka-tasco-11t/

 

Optically I find them all very nice. Nothing to complain at all.

 

But the mount is not as good and it has a .965" focuser. Of course you can modify everything but then ... why not just get the Vixen.

 

I only have the Circle T as it was dirt cheap and also a nice project to clean up and bring back to life. Whereas I could buy another classic (that I personally wanted more) for the money that I got for the Vixen. Thomas


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#10 CHASLX200

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Posted 28 May 2018 - 05:44 PM

From a mechanical point of view, the Celestron C-4.5 is certainly at the top of the heap of the commonly imported 114mm F/8's... That Polaris mount is a good one.

 

Jon

They are great mounts. I am sure a Tak 130 Newt would be a even better killer.



#11 DAVIDG

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Posted 28 May 2018 - 05:55 PM

 The best is  a 4" Fecker Celestar from the 50's. They show up from time to time on Ebay and here for sale. Here is  a picture of the one I restored.

 

                        - Dave 

 

4celestar.jpg


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#12 vtornado

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Posted 28 May 2018 - 08:46 PM

I see 4 inch Carton mirrors for sale periodically, so this could be an option if you are willing to give up a quarter of an inch of aperture. In my experience, Carton products are uniformly excellent. I have a heavy-duty Carton alt-azimuth mount that can hold just about anything...

 

steve

I see those carton mirrors here too.  They look nice.  Too bad it is not a drop in replacement.  I would have to source a tube, maybe i could mod the 114 mirror cell.

I was hoping for a garage sale / craig's list where  maybe all I have to do is clean and regrease.

So far there are some nod's for the celestron/vixens here.

 

I have a carton 60mm f/17 objective.  The word superlative comes to mind.



#13 vtornado

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Posted 28 May 2018 - 09:04 PM

Have you thought about grinding your own 4.5" mirror? To my knowledge, there is no "premium" version of a 4.5" reflector out there. But making a small mirror at a focal length longer than 700mm is not an overwhelming process.

Shaula, one day I dream of grinding my own mirror, especially under someone's guidance.    I'm sure if I tried it myself, I would screw up about 3 times before I got one right.    There are courses downtown at our planetrium, but that is a 2 hour trip each way for me.It would be cool though and then I could parabolize it.    If I hit the lottery, or if I can ever slow down at work ...

 

My hope was that maybe one of the classics took a little more time to make a superior intrument.

 

According to Dave Knisley's post In order to achive diffraction limited optics a 4 inch mirror should be f/10.5, so parabolizing should be a benefit.

https://www.cloudyni...erical-mirrors/


Edited by vtornado, 28 May 2018 - 09:06 PM.


#14 Steve Allison

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Posted 28 May 2018 - 09:04 PM

Dave-

 

Love that Fecker! Beautiful and top quality, too.

 

I almost hit the bid button last time one was on eBay.

 

I assume you have seen Dave Trotts YouTube video?

 

Steve


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#15 memento

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Posted 29 May 2018 - 05:51 AM

 

According to Dave Knisley's post In order to achive diffraction limited optics a 4 inch mirror should be f/10.5, so parabolizing should be a benefit.

https://www.cloudyni...erical-mirrors/

The real 114's are 4.5" and f/8. Not a big difference to a 4" with f/10.5. Not worth to think too much about it.

 

If you want a "perfect" mirror and care about these things like parabolic mirror and perfection and such, go for minimum 6" instead. Then it all might be worth the hassle. Plenty of really good 6" Newtons available. Does not need to be a classic.

 

Just my 2 cent ... and again: My two 114/900s showed great images. If I wanted more, I'd certainly go bigger in aperture. Thomas


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#16 Jon Isaacs

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Posted 29 May 2018 - 06:17 AM

Shaula, one day I dream of grinding my own mirror, especially under someone's guidance.    I'm sure if I tried it myself, I would screw up about 3 times before I got one right.    There are courses downtown at our planetrium, but that is a 2 hour trip each way for me.It would be cool though and then I could parabolize it.    If I hit the lottery, or if I can ever slow down at work ...

 

My hope was that maybe one of the classics took a little more time to make a superior intrument.

 

According to Dave Knisley's post In order to achive diffraction limited optics a 4 inch mirror should be f/10.5, so parabolizing should be a benefit.

https://www.cloudyni...erical-mirrors/

 

The standard analysis has been that of Texereau.  This is from David's post:

 

APERTURE TEXEREAU MINIMUM F/RATIO
3 inch f/6.4
4 inch f/7.1
6 inch f/8.1
8 inch f/8.9
10 inch f/9.6
12 inch f/10.2

The above f/ratios might be fairly usable for an astronomical telescope's
spherical primary mirror, as they do just barely satisfy the 1/4 wave
"Rayleigh Limit" for wavefront error."

 

I think if you read the rest of the thread you will find that these are the accepted numbers.  For example, Vladimir Sacek of the Telescope-Optics pages takes issue with David's numbers.  From the Telescope Optics webpages:

 

F=(90.17D)1/3 for D in inches

 

http://www.telescope.../reflecting.htm

 

The equation I use is 

 

wf = 22.55 x D/Fr3    (D in inches)

 

Both these equations say that a spherical 4.5 inch F/7.4 mirror is 0.25 wave.  A 4.5inch F/8 spherical mirror is 1/5 wave.

 

Edit:  A spherical 4 inch F/10.5 would 1/13 wave. 

 

Edit #2:  Those C-4.5 rings are a disaster..  


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#17 bremms

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Posted 29 May 2018 - 11:31 AM

Have or had five of them. Two different Tasco 11TR's one with a decent mirror and one with an very good mirror. Put in a better diagonal and focuser on the one. Gave it to my flight instructor for his grand kids. Good images that one. Was given another 11TR back in the late 80's and gave it to a friend in our club. He used it a good bit. I remember good images of Jupiter and it was better than the 60mm F15 refractor.    Currently have a Vixen/ celestron 130mm and it's pretty good. Machined a couple bits for the sled focuser and it has less no slop now. Have an Edmund 4.25" F6 that is good too. That one shows close to dead straight smooth lines on DPAC. I use the same Vixen/ Celestron Polaris mount for both.


Edited by bremms, 29 May 2018 - 11:32 AM.

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#18 Tenacious

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Posted 29 May 2018 - 01:04 PM

 

If you want a "perfect" mirror and care about these things like parabolic mirror and perfection and such, go for minimum 6" instead. Then it all might be worth the hassle. Plenty of really good 6" Newtons available. Does not need to be a classic.

 

Just my 2 cent ... and again: My two 114/900s showed great images. If I wanted more, I'd certainly go bigger in aperture. Thomas

+1   The 6" f/8 shows more with great fidelity.  Mine ( Harden's Deep Space Hunter) is Dob mounted.  This makes it very comfortable to use, I can look at any part of the sky while seated in an ordinary lawn chair.  It is easy to carry in one trip.  It natively takes 1.25" EPs.


Edited by Tenacious, 29 May 2018 - 01:06 PM.


#19 Bomber Bob

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Posted 29 May 2018 - 01:06 PM

Here's the one you really want:  1960 Hino Mizar P-100

 

Hino Mizar P-100 S12.jpg

 

F10 parabolic mirror, nice helical focuser, decent finder, and very stable pedestal mount -- and easy to tote the whole rig around the yard when you're dodging trees and./or clouds.  APO views at a fraction of the price, and fine imaging, too:

 

P100 - Jupiter (GRS) 20170513V04SD21.jpg

 

Competes well with the Celestron/Vixen C114 Polaris:

 

C114 - Jupiter 20170110V08X02.jpg


Edited by Bomber Bob, 29 May 2018 - 01:10 PM.

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#20 vtornado

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Posted 29 May 2018 - 01:40 PM

The standard analysis has been that of Texereau.  This is from David's post:

 

APERTURE TEXEREAU MINIMUM F/RATIO
3 inch f/6.4
4 inch f/7.1
...
spherical primary mirror, as they do just barely satisfy the 1/4 wave
"Rayleigh Limit" for wavefront error."

 

Thanks for your help Jon,

 

When I said a diffraction limited mirror I was referring to  a little farther down in Dave's post.

 

The above f/ratios might be fairly usable for an astronomical telescope's
spherical primary mirror, as they do just barely satisfy the 1/4 wave
"Rayleigh Limit" for wavefront error. However, amateurs looking for the best
in high-power contrast and detail in telescopic images (especially those
doing planetary observations) might be a little disappointed in the
performance of spherical mirrors with the above f/ratios. Practical
experience has shown that at high power, the images produced by spherical
mirrors of the above f/ratios or less tend to lack a little of the image
quality present in telescopes equipped with parabolic mirrors of the same
f/ratios.

 

For example, the typical "department store" 3 inch Newtonian frequently uses
a spherical f/10 mirror, and should give reasonably good images as long as
the figure is smooth and the secondary mirror isn't terribly big. For common
apertures, the following approximate minimum f/ratios for Diffraction-Limited
Newtonians using spherical primary mirrors can be found below:

APERTURE F/RATIO FOR DIFFRACTION-LIMITED SPHERICAL MIRRORS
-----------------------------------------------------------------------------
3 inches f/9.6 (28.8 inch focal length)
4 inches f/10.6 (42.4 inch focal length)

...
 

 

So perhaps ... A 4.5 inch f/8 parabolic mirror could outperform, a 4.5 inch f/8 spherical for high detailed planetary observations.

Although that rests on my newb ability to some day grind my own mirror, and get a parabola better than a very nice spherical mirror.

 

===========

 

Also unsaid in my original post is the hope that one of these better 4.5 inchers might appear in a craig's list post, for the $50.00 or less

that these scopes fetch.

 

================

 

Maybe I should go to a 6 inch f/8 as others have said and forget the 4.5.   Yes I could dob it, but I was also hoping to throw the tube onto CG5 class mount,

to get no brainer tracking.  6" f8, might be a bit shakey.



#21 Bomber Bob

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Posted 29 May 2018 - 01:58 PM

Maybe I should go to a 6 inch f/8 as others have said and forget the 4.5.

 

Well, I got my RV-6 for $150...

 

RV6 Rebuild S43 - COMPLETE.jpg

 

And, the OTA rides well on a vintage Vixen Polaris mount (pedestal or tripod).


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#22 CharlieB

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Posted 29 May 2018 - 02:52 PM

This one is a great 4" scope.  Optica b/c (Astro Optical) LN4-E.  I need to shoot some new photos of this.  The clock drive is quite accurate.  It has an excellent parabolic mirror and the focuser handles all of my older orthos, but it doesn't like any of the Antares Plossls.  Too bad there weren't more of these sold n the USA.  I think this one is from the early 1970s judging by the clamshell style.

 

I think Sheldon Faworski still has some 4" f/10 spherical mirrors, which would make for a very nice scope.

 

Charlie

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#23 steve t

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Posted 29 May 2018 - 03:25 PM

Back in 1973 my family lived  in a small house and I shared a bedroom with a little brother. Since storage was always limited a 4" F/8 seemed most pratical as my first attempt at telescope making.

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#24 CharlieB

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Posted 29 May 2018 - 03:42 PM

Back in 1973 my family lived  in a small house and I shared a bedroom with a little brother. Since storage was always limited a 4" F/8 seemed most pratical as my first attempt at telescope making.

Very, very nice!


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#25 Jon Isaacs

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Posted 29 May 2018 - 11:42 PM

When I said a diffraction limited mirror I was referring to  a little farther down in Dave's post.

 

 

And I was referring to what others had said in the thread in response to what David had written. In particular, Vladimir Sacek who is the author of the Telescope-Optics webpages.

 

This is his first reply:

 

https://www.cloudyni...s/#entry3947494

 

"You are talking two different "diffraction-limited" criteria. The 1/4 wave p-v of lower order spherical, or 1/13.4 wave RMS, or 0.80 Strehl is conventionally accepted criterion, and is actually based on diffraction calculation (Marechal).

 

The raytracing "diffraction-limited" criterion based on all the rays contained within the Airy disc, is informal and arbitrary. In fact, it has nothing to do with diffraction (which, as we all know, is a wave phenomenon, that cannot be described by rays). In this particular case, even the choice of focus is incorrect; the smallest blur location has more than twice the error of best focus location. No one focuses on that point, so what is the rationale of using it as the basis for a quality criteria?"

 

And here:

 

https://www.cloudyni...s/#entry3948050

 

"The utility of spot size vs. Airy disc criteria is in assessing gross level of correction of raytraced systems, usually in the process of designing. It was never meant to be an accurate predictor of optical quality.

Vla"

 

Vlad also uses the equation I use, wf = 22.55 x D/F3   to compute the spherical wavefront error of a spherical Newtonian.  It's worth noting that a 6 inch F/8.2 is diffraction limited and that according to David G, the Criterion RV-6s used spherical optics of this configuration.

 

Edit:  What i should have said is that I use Vlad's equation. 

 

Jon


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