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Getting some confusing messaging from Celestron about the shape of their primary mirrors

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#1 CrazyPanda

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Posted 21 October 2020 - 11:31 PM

I've been curious if the new Celestron StarSense 130mm F/5 reflector has a parabolic mirror or a spherical mirror.

 

The Celestron Astromaster 130mm F/5 has a spherical mirror. This is stated in Celestron's own knowledge base: https://www.celestro...-the-difference

 

The Celestron Nexstar 130SLT (also a 130mm F/5 reflector) is reported to have a parabolic mirror, and a few retailers list it as such. The higher cost does seem to indicate a parabola would be likely.

 

But given the StarSense line has the same 114mm Bird Jones as the AstroMaster line, I'm hesitant to say that the StarSense 130DX is actually the same OTA as the Nexstar 130 SLT, rather than the AstroMaster 130.

 

As such, I contacted Celestron, and they first told me that *all of their reflectors* use parabolic mirrors.

 

Finding that strange, given their own knowledge base explicitly states the AstroMaster 130 has a spherical mirror, and the fact that they offer two Bird Jones scopes (PowerSeeker 127EQ and AstroMaster 114EQ), I asked them to clarify.

 

They said that all of their "true" Newtonians have parabolic mirrors, as spherical mirrors would degrade performance too much otherwise. They just ignored the fact that their knowledge base says otherwise for the AstroMaster, and I'm fairly certain the PowerSeeker 114EQ is spherical, as is the AstroMaster 76 (though at that focal ratio it's just as good as a parabola).

 

So what actually *is* the verdict on all of Celestron's reflectors?:

 

  • Nexstar 130SLT (parabolic, retailer advertised)

  • AstroFi 130 (parabolic, unconfirmed - appears to be a rebrand/electronics upgrade of the 130SLT)

  • SkyProdigy 130 (parabolic, unconfirmed - also appears to be the 130SLT with a different mount/chasis)

  • StarSense Explorer 130DX (parabolic? spherical?)

  • StarSense Explorer 114 (spherical, Bird-Jones - undisputed by Celestron)

  • PowerSeeker 114EQ (parabolic, retailer advertised, though I thought all of these 114/910 mirrors were spherical these days. Probably irrelevant given the focal ratio.)

  • PowerSeeker 127EQ (spherical, Bird-Jones - undisputed by Celestron)

  • AstroMaster 130 (spherical, both confirmed in knowledge base AND disputed by Celestron support...)

  • AstroMaster 114 (spherical, Bird-Jones - undisputed by Celestron)

  • AstroMaster 76 (parabolic, retailer advertised)

  • FirstScope 76 (spherical, unconfirmed but most likely given the other variations of this scope. Disputed by Celestron support...)

  • FirstScope 76 Signature Series (spherical, confirmed in specs - disputed by Celestron support...)

  • FirstScope 76 National Park Foundation Edition (spherical, confirmed in specs - disputed by Celestron support...)

  • Cometron 76 (spherical, confirmed in specs - disputed by Celestron support...)

  • Cometron 114 (parabolic, confirmed in specs - undisputed by Celestron support)

  • ExploraScope 114 (spherical, Bird-Jones - undisputed by Celestron)

  • LCM 114 (spherical, Bird-Jones - undisputed by Celestron)

  • Advanced VX 8" (parabolic, confirmed in specs - undisputed by Celestron support)

  • Advanced VX 6" (parabolic, confirmed in specs - undisputed by Celestron support)

  • Omni XLT 150 (parabolic??, potentially confirmed in retailer advertising using weird language) - this is an odd duck. They won't explicitly say "parabolic". Instead, they say "aspheric shaping technology" and "virtually no spherical aberration". So is this like... a semi-parabola???

Anyone able to shed more light on this?

 

Also, the three scopes with parabolic listings are all advertised as such by only B&H Photo. No other retailer lists them as parabolic, which makes me wonder if B&H may just be adding "parabolic" to make the scopes more appealing.

 

The reason I question it is because the AstroMaster 76 with an F/9.2 focal ratio *doesn't need to be parabolic*, and yet the FirstScope/Cometron 76 lines with ultra short focal ratios are spherical. Something is amiss here. It's odd that the short focal ratio scopes would be spherical, but then Celestron decides that the long focal ratio AstroMaster 76 needs to be parabolic. This makes me take B&H's listings with a grain of salt.


Edited by CrazyPanda, 22 October 2020 - 12:14 AM.


#2 sharkmelley

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Posted 22 October 2020 - 01:23 AM

Maybe Celestron thinks users aren't particularly bothered by whether the mirror is spherical or parabolic.  That's why the info is not readily available, even to their own support team.

 

Mark



#3 TOMDEY

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Posted 22 October 2020 - 01:57 AM

Alas, sometimes factory reps and even field reps are one, two or three hand-offs removed from the actual designer... so that many iterations of clueless. And, for some reason, they are astonishingly-loathe to inquire up the chain, when in doubt. Instead they double down with their pontifications. Celestron is a small company... so you would think the tech staff is all on the same page. I'll bet they're farming out or rebranding so much of the work that... they may not even know what in on the inside of the tube, anymore. The days of owner, management, design, production, sales, shipping and customer service all being in the same building... or even on the same continent... is a bygone era. That's my guess.   Tom



#4 Jon Isaacs

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Posted 22 October 2020 - 06:59 AM

Alas, sometimes factory reps and even field reps are one, two or three hand-offs removed from the actual designer... so that many iterations of clueless. And, for some reason, they are astonishingly-loathe to inquire up the chain, when in doubt. Instead they double down with their pontifications. Celestron is a small company... so you would think the tech staff is all on the same page. I'll bet they're farming out or rebranding so much of the work that... they may not even know what in on the inside of the tube, anymore. The days of owner, management, design, production, sales, shipping and customer service all being in the same building... or even on the same continent... is a bygone era. That's my guess.   Tom

 

Celestron is actually a branch of a very large company, Synta/Skywatcher.

 

My guess is similar to yours, the tech support staff is not well paid and not knowledgeable, it's just a job.  They may not even know what spherical and parabolic mean.  They probably have some knowledge base in front of them, they do a search and that's what they know.

 

Of course, Crazy Panda is not really buying this for the OTA..   

 

Jon



#5 stargazer193857

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Posted 22 October 2020 - 12:49 PM

Makes you wonder if f5 is the system f# or the primary f#. If it is the system f#, that is too fast for a bird Jones.

But many companies sell products and know nothing about them.

I asked a glass company what their standard sheet sizes are. The rep replied they can cut them to whatever size I want. I replied I'm trying to save money on cuts and would like to know the sizes in stock so I can better plan the cuts without scrap and not need several quotes. The rep had to transfer me. All the rep knew how to do was take down a request and forward it, no knowledge of the product.

#6 CrazyPanda

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Posted 22 October 2020 - 12:54 PM

Makes you wonder if f5 is the system f# or the primary f#. If it is the system f#, that is too fast for a bird Jones.

But many companies sell products and know nothing about them.

I asked a glass company what their standard sheet sizes are. The rep replied they can cut them to whatever size I want. I replied I'm trying to save money on cuts and would like to know the sizes in stock so I can better plan the cuts without scrap and not need several quotes. The rep had to transfer me. All the rep knew how to do was take down a request and forward it, no knowledge of the product.

Well as far as I know, any of their F/5 reflectors are normal Newtonians, just that some evidently have spherical mirrors and some have parabolic mirrors (no doubt of varying accuracy, but at least an attempt was made to parabolize them). And yes, if you're thinking that a 130mm F/5 spherical reflector is a travesty due to 0.88 waves of spherical aberration, you're right, and it's downright predatory (and arguably counter-productive) for Celestron to even offer such a scope. A 130mm aperture scope *should* offer reasonably nice views of the planets at magnifications in the 130-200x range as long as the atmosphere is steady and provided its mirror is parabolized. But with 0.88 waves of spherical aberration, you'll just never find good focus with the scope, leading to disappointing planetary views that may turn you off from astronomy altogether. 

 

Their Bird-Jones scopes often list the net system focal ratio. So the 127mm PowerSeeker with 1000mm focal length is advertised at the system focal length of about F/8. The primary mirror of those Bird-Jones scopes is god knows what focal ratio though. Probably less than F/4.


Edited by CrazyPanda, 22 October 2020 - 12:58 PM.


#7 stargazer193857

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Posted 22 October 2020 - 01:18 PM

My ballpark math says 1 wave of error. But I did not formally calculate it.

I've looked through 70mm f3.9, and it was soft at 15x. Saturn's rings could not be resolved, and showed me clear spherical aberration at 30x. It was the minidob newt sold as a first scope.

#8 rhetfield

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Posted 22 October 2020 - 04:01 PM

Celestron is actually a branch of a very large company, Synta/Skywatcher.

 

My guess is similar to yours, the tech support staff is not well paid and not knowledgeable, it's just a job.  They may not even know what spherical and parabolic mean.  They probably have some knowledge base in front of them, they do a search and that's what they know.

 

Of course, Crazy Panda is not really buying this for the OTA..   

 

Jon

My skywatcher heritage is a parabolic mirror.  I would think that it would make sense from a manufacturing standpoint for all of the Synta 130mm/f5 mirrors to be the same.  Similar thing for the other common mirror sizes.  Does anybody know if Synta and GSO make their own mirrors or do they source them from some mirror manufacturer?



#9 sixela

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Posted 22 October 2020 - 05:36 PM

I bought a Celestron 130SLT tube once. I star tested it, and if it was parabolised, then whoever did that stopped well before the job was done ;-).

It was shipped back, and the vendor star tested 5 different ones and they were all looking the same.

So it was replaced by a Skywatcher BlackLine (I did pay the difference), and _that_ one is a paraboloid.

Rumour has it these aren't even Synta mirrors -- according to a vendor, Synta uses a cheap subcontractor for the optics.

The Omni XLT 150 is definitely parabolic. That series is basically the Skywatchers in a different colour.

Edited by sixela, 22 October 2020 - 05:44 PM.

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#10 CrazyPanda

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Posted 22 October 2020 - 08:39 PM

I bought a Celestron 130SLT tube once. I star tested it, and if it was parabolised, then whoever did that stopped well before the job was done ;-).

It was shipped back, and the vendor star tested 5 different ones and they were all looking the same.

So it was replaced by a Skywatcher BlackLine (I did pay the difference), and _that_ one is a paraboloid.

Rumour has it these aren't even Synta mirrors -- according to a vendor, Synta uses a cheap subcontractor for the optics.

The Omni XLT 150 is definitely parabolic. That series is basically the Skywatchers in a different colour.

Seems to me the only way anyone will know for sure which Celestrons are spherical, which are good parabolas, and which are bad parabolas, is to either just star test or bench test a couple samples of each model.



#11 ed_turco

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Posted 24 October 2020 - 08:49 PM

My ballpark math says 1 wave of error. But I did not formally calculate it.

I've looked through 70mm f3.9, and it was soft at 15x. Saturn's rings could not be resolved, and showed me clear spherical aberration at 30x. It was the minidob newt sold as a first scope.

and probably the last.
 



#12 Gregrox

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Posted 31 October 2020 - 05:45 PM

and probably the last.
 

I'm not so sure. Though the FirstScope is a very poor performer on planets and the Moon, it does well for DSOs at its size. The variants sold with Kellner eyepieces instead of AWFUL Huygens and Ramsdens (Orion FunScope, Celestron Cometron & National Parks FirstScopes) are fine. I use my FirstScope with kellners and sometimes I even like how the moon looks in it. The Galileoscope definitely comes out instead when I want to look for planets with a smallscope.

 

It might be different for a beginner expecting bright and clear views, but I think for what it is, if you understand its limitations, it is a good ultraportable grab-n-go tabledob. I might even recommend it over the oft-recommended pair of binoculars, as it's fine for wide-field scanning, and unlike binos it stays where you leave it.

 

The paradox of the thing is you need decent eyepieces to get it to work well, but the cheapest version (Moon & IYA-spiral-names wraparound versions) has some real awful ones. It's less of a good First teleScope and more of a decent supplementary telescope for someone who is already has a good large telescope.

 

I think the important thing to remember about the FirstScope is that no telescopes of that price or cheaper (about $50 USD) have great optics, and more importantly, the tabletop-dob mount on the FirstScope is excellent compared to almost any cheap tripod you find on department store hobbykillers. In cheap telescopes, the mount is the real hobbykiller, not the optics.
 

Oh no! My first two posts on this forum are in defense of a crappy little toy telescope! I promise I'm not a shill, it's just a coincidence.



#13 RobertMaples

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Posted 04 November 2020 - 03:15 PM

My ballpark math says 1 wave of error. But I did not formally calculate it.

I've looked through 70mm f3.9, and it was soft at 15x. Saturn's rings could not be resolved, and showed me clear spherical aberration at 30x. It was the minidob newt sold as a first scope.

I wonder if it was collimated.  While you cannot collimate the primary mirror (at least not easily), you can collimate the secondary.  I have one and can clearly see Saturn's rings and the gap between the planet and the rings and two cloud bands on Jupiter.



#14 sixela

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Posted 04 November 2020 - 06:02 PM

No, those FirstScopes realy have spherical mirrors. There's so much spherical aberration that collimation is the least of your worries.

#15 stargazer193857

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Posted 04 November 2020 - 11:23 PM

I wonder if it was collimated. While you cannot collimate the primary mirror (at least not easily), you can collimate the secondary. I have one and can clearly see Saturn's rings and the gap between the planet and the rings and two cloud bands on Jupiter.


Yes, I collimated the secondary. I even off centered the primary screws to get closer to good collimation there too.


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