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StellaLyra Classical Cassegrain & Ritchey-Chrétien Telescopes


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1 minute ago, johninderby said:

Just measured the primary mirror as best I could with it in place and estimate it’s about 204mm in dia. 🙂

That review quoted 7.3" primary based on dim image.

Explanation was that it was undersized secondary mirror that produced effective aperture stop.

We could in fact see if there is any merit for this claim if we knew couple of things:

Primary to secondary separation in mm (does not have to be exact - measuring tube length and then subtracting mirror thicknesses and offsets from OTA start would be enough) and speed of primary mirror.

If I'm not mistaken, speed of primary is something like F/3 for this scope. This means that it has FL of 609mm.

If size of secondary is 60mm (whole obstruction with mirror support and baffle tube is 68.5mm), then distance between between mirrors needs to be 609 - 60 * 3 = 429mm for edge case where only central spot on optical axis is not vignetting and primary is not obstructed for it see diagram:

image.png.08fcfb61d933d20ebea165c159d01941.png

OTA is 536mm long, so there is enough room for scope not to be stopped down, even if size of secondary is 60mm. Primary is probably 20mm thick, there is another 30-40mm for mirror cell and back of the scope - that is 60mm. Secondary is probably less than 10mm in thickness, support is another 20mm and it is about 10mm inside of the tube. That makes total of about 100mm.

536 - 429 = 107mm

This is of course just speculation and there is no way of knowing these things without measuring things.

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You can now buy Classical Cassegrain & Ritchey-Chrétien telescopes at FLO 🙂  StellaLyra Classical Cassegrain & Ritchey-Chrétien telescopes Manufactured by Guan Sheng Optical in Taiwa

Lyra meet Mira. 😁

Just pulled the trigger on a Stellalyra 6” f12 CC. Ooops!   Now to decide which scope must go to make space for it? 🤔

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Interesting that the vendor took it up with the manufacturer who produced an explanation. If Larry's assertion was incorrect surely the vendor and the manufacturer would have quickly pointed that out ?

Setting that to one side, the review was very positive I thought. Here is another positive one:

https://astronomytechnologytoday.com/2020/01/03/gso-eight-cassegrain/

Also these reviews are the GSO branded units. It's quite possible that the specs of the Stellalyra units are a little different.

 

 

 

Edited by John
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5 minutes ago, johninderby said:

Distance between primary and secondary is about 410mm measured from surface of primary to aproximate position of secondary.

Do you have any idea what is the size of secondary mirror itself?

Btw, you can do simple test to see if primary is obstructed - once you have the chance to test under stars (or indoors if you have access to artificial star).

It involves fingers and eyepiece :D

You need to observe defocused pattern of a bright star - like really defocused so that shadow of secondary and spider can be clearly seen - even primary mirror clips. At this point you are actually looking at projection of aperture as seen by eyepiece.

Take a finger and slowly extend it over aperture as to block some of incoming starlight. At some point it will be seen as a shadow at the eyepiece - similar to this image of focuser protruding into light path:

startest.jpg

When this happens - you measure how much of a finger (or ruler :D )  you needed to put over aperture and based on size of aperture and primary mirror - you can figure out if effective aperture is less than size of primary (is OTA opening minus twice finger length over OTA opening less or equal to primary mirror diameter)

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2 minutes ago, johninderby said:

The secondary is 60mm.

So we have 60mm size of secondary, ~204mm size of primary and ~410mm distance between them. We are keeping assumption of F/3 primary.

Let's see how much stop down will we be roughly getting with these specs.

612mm FL of primary, at 410 it will have cone width of (612-410) / 3 = 67.333...mm

204 : X = 67.333... : 60

X = 60 * 204 / 67.333 = 181.78mm = 7.15"

With above we would get even more stopped down primary.

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28 minutes ago, John said:

Interesting that the vendor took it up with the manufacturer who produced an explanation. If Larry's assertion was incorrect surely the vendor and the manufacturer would have quickly pointed that out ?

Yes, I know, but somehow things don't really add up, and I'm just trying to see why is that.

First question is - why would GSO sacrifice some of aperture when they don't really need to do that. Second question is why would reviewer make claim that view in that scope is so much less bright than C8 and comparable to 6" refractor when explanation given above does not support such claim.

There is more details like having fully illuminated field being 15mm in diameter. That is 7.5mm in radius which means that this size is much less on secondary - maybe 1 to 2mm or even less (secondary is magnifying). This means that with only marginally bigger secondary one would not get stopped down primary - at least in same central diameter.

They have 38% CO 6" scope - why push for 33% 8" scope when it would work at full aperture with maybe 35-36%? Alternative is to separate primary and secondary more, change curvature slightly - like F/2.9 or something instead of F/3 and get full aperture with same CO.

In any case - I feel that something is not adding up and I just want to know what (even if it is something silly like - "I don't want to hamper sales of C8 too much so I'll say that CC8 is dimmer").

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5 minutes ago, jetstream said:

@vlaiv, what is the fully illuminated field of this telescope? maybe need to assume a low power widefield ep?

According to the article discussed it is 15mm:

Quote

With the relatively short-focus parabolic primary mirror, a large secondary that would degrade image contrast would not be desirable.  The solution: Use a smaller secondary [still a roughly 33% obstruction] and reduce the effective aperture of the scope to 7.34 inches(186.5mm).  According to the factory specs, the 100 percent illumination circle is 15mm.

 

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I'm feeling rather awkward now that a review that I posted because I thought it might help a prospective purchaser has resulted in a rather technical discussion in this thread that FLO started announcing these new scopes.

Perhaps a mod might snip out some of this and stick it into the scopes discussion section which might be a more apt place for it ?

Apologies to FLO.

 

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17 minutes ago, vlaiv said:

According to the article discussed it is 15mm:

 

Yes, I saw that. I was wondering if you could run the calc. I use Bartels program for newts I play with but I don't think it will work with these. So does the secondary obstruction render this a 7.3" scope? what is the true obstructions effect on true focal ratio?

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Vlaiv, if you redo the maths with manufacturers 200mm and 60mm with the claimed 7.34" stepped down and solve for the mirror distance you get 406.9mm.
This is pretty close to Johninderby's approx. measurement of 410.
So doesn't this uphold, rather than discredit, the reviews stepdown claim?

I agree that this would not have shown itself to the reviewer as "a much dimmer image"; your earlier maths showing a 0.2 mag difference seems more likely closer to the truth.

I suspect his exaggerated dimmer image claim was either caused by some other phenomenon or, more cynically, by wanting to paint a more negative picture in an otherwise quite positive review. Or perhaps it was the only way he could think of bringing up the stepdown point.

Edited by globular
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11 hours ago, John said:

I'm feeling rather awkward now that a review that I posted because I thought it might help a prospective purchaser has resulted in a rather technical discussion in this thread that FLO started announcing these new scopes.

Perhaps a mod might snip out some of this and stick it into the scopes discussion section which might be a more apt place for it ?

Apologies to FLO.

 

John,
You weren't the first to post the review - it was mentioned, together with the 7.3" debate, back on page 2.  You just reignited it!
Personally I think this whole thread, even including this latter technical discussion, highlights the interest and excitement that FLOs new offering is generating. 

If bits are moved elsewhere please post a link so I can find and follow them.

Edited by globular
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Might be a good idea to split off the technical discussion into a new thread as while very interesting this is FLOs thread about their new scopes. Have learned a few things from the discussion though.🙂

Look forward to reading  reviews of this scope by new owners and their real world experiences. Whatever the technical points are the 8” just plain gives great views. 👍🏻

EDIT:

Currious about the effective aperture of other scopes such as the various SCTs and Maks. 🤔

Edited by johninderby
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I was just wondering if these scopes come with the main dust cover at all? and can i check that there is no Bahtinov mask available for the 8" F12 im just making sure my shopping list is in order before i hit the buy it now button 👍

Edited by WbRaDy
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8 hours ago, John said:

I'm feeling rather awkward now that a review that I posted because I thought it might help a prospective purchaser has resulted in a rather technical discussion in this thread that FLO started announcing these new scopes.

Perhaps a mod might snip out some of this and stick it into the scopes discussion section which might be a more apt place for it ?

Apologies to FLO.

 

I'm sure FLO would want to get to the bottom of the exact specs of the scopes they sell, so they can update the descriptions and allow their customers to make informed choices. 

If the 8" CC is reduced aperture I wonder if the same applies to the 6" and 10" CCs

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11 minutes ago, CraigT82 said:

 

If the 8" CC is reduced aperture I wonder if the same applies to the 6" and 10" CCs

And SCTs and Maks? 🤔

Edit:

Came accross this on CN re: the C8 SCT.

With 140mm of back focus, the C8 is effectivly working as a 6.7" telescpe with about a 38% obstruction.

Edited by johninderby
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On the subject of the dimmer image of the CC, I suspect it could be as simple as a different eyepiece being used with different transmission properties and/or a slightly different magnification being used, both of which could make a big difference to perceived brightness. I once bought a specialist planetary eyepiece which significantly dimmed the image compared to my normal eyepiece of the same focal length. 

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17 hours ago, vlaiv said:

First question is - why would GSO sacrifice some of aperture when they don't really need to do that.

Maybe to clean up the messy edges of the mirror figure wise to improve Strehl? It's one guess.

Edited by Lockie
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