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New AT 6" and 8" Classical Cassegrain

1301 replies to this topic

#1251 Garyth64

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Posted 26 October 2020 - 07:44 AM

Quilty:

50 pages?  isn't it more like 63?  It is amazing, and after going thru it all, I can see that about 80% of it could be eliminated.

The formula is straight forward.  You can use the nomenclature in #1245 for explanation.

If the secondary is not receiving all light from the primary, and moving it away from the primary 14mm to 18mm, causes the secondary to collect all the light from the primary, doesn't matter.  It doesn't matter because the focus will be inside the tube, and you will not be able to focus anything.

Yes, the secondary will have to have a different radius of curvature for a new set of parameters that will allow all light from the primary to be seen, and have the focus well behind the primary for things to come into focus. The curvature of the secondary will have to be changed, and that doesn't mean that you will be increasing, or even decreasing the power of the secondary.

What is 21 (7x3)mm?  You have to stop using that, whatever it is.  It doesn't give you any relevant information, it is meaningless.

What is "ray set"?  What ever that is too, just stop using it.  It is giving you wrong answers, or maybe you aren't using it correctly (?).

Edited by Garyth64, 26 October 2020 - 07:55 AM.

#1252 Garyth64

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Posted 26 October 2020 - 07:52 AM

Thanks for the math Gary, if I was designing a system that's the way I would go. When I'm setting up the optics knowing the mirror spacing and back focus I just use the secondary magnification times itself to guide me to move the secondary a mm or two to bring the moon into focus. Works for me

Mike

Thinking about it now.................. I would search out a Cassegrain spreed sheet because I'm lazy

Yes, it can work.  Moving the secondary a little, causes the focus to shift a lot.  It is not a direct ratio, as there is more than one variable.

There are Cassegrain programs on the internet, where you can plug in all the parameters of a system, and you can see what I have said.

haha, in those programs, how are they coming up with the information.  All the formulas, and many more, are already plugged in to the program for the user.

I like doing the math on paper.

#1253 quilty

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Posted 26 October 2020 - 08:47 AM

HI Gary,

yes most could be eliminated including most of my stuff.

I'm well aware that the focus will be inside the tube and that's exactly why I didn't administer that operation though all mechanic parts were ready to start

I remember the numbers. mirror distance is 324 mm, sec diameter is 48.5 mm, primary diameter 150 mm

applying ray set the optimum (full aperture minimum CO) spacing is 305mm (1-48.5/150)x450mm. (This doesn't consider CO by secondary baffle). The secondary was even 6 mm too large at 324mm distance

But the CC shows just 138 mm aperture at a mirror distance of 324mm, which means real focal length of primary is more than 450mm, correct?

My primary would need a 150/138 times larger secondary, yielding 53mm or a distance of 150/138 x 324 mm, yielding a forward shift by 18 mm to 352 mm, so would do, correct?

And primay focal length is not 450 but 352/324 x 450 mm yielding 489 mm, correct? And this is quite what I read directly when measuring the primay focal length in the moonlight. And this is proof enough for me to state the primary to be an f/3.25 one, the two independent approaches.

Well then, what power is the secondary to yield an overall f/12? First calculation

And, what power would be the secondary at the correct distance of 352mm? to focus maybe 3-4 inches behind the tube? second calculation

Gary, you seem to not only beeing able to tell, you seem to have access to appropirate mirrors as well.

So, why not doing the whole thing? Calculating maximum mirrors without changing dimensions.

160 mm primary focussing at 460 mm distance f/2.875, keeping the 48.5 mm secondary at 324 mm distance, what power would be the secondary then?

And then getting those mirrors, maybe drilling the thread into the primary center if only newtonian mirrors are available and we can hope for a significant enpowerment, can't we?

So, just three examples to calculate

Edited by quilty, 26 October 2020 - 10:42 AM.

#1254 Garyth64

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Posted 26 October 2020 - 10:38 AM

It is hard to understand your calculations and what you want, but here goes.

I'm not going to show all the math, and just give you the answers and what I see.

In you first part, using a f/3 primary with a separation of the mirrors at 324mm.  I come up with a secondary diameter  as 44.8mm.  You say the diameter is actually 48.5mm, so I don't know what the problem is there.

first calculation if the primary is an f/3.26, the amp would be  12/3.26 = 3.69x

But with the fl of the primary at 489mm (f/3.26), I don't see how you got that, but you say you measured it, so ok.

With this one, you want a separation of the mirrors at 352mm with about 100mm focus behind the primary.  And p = 137mm, and you give p' as 452mm (separation plus back focus).  In this one, the  amp is 452/137 = 3.29x, for a system of f/10.75.

Your number do not work, as this is not a f/12.

In your last one, with a primary of 160mm diameter, a fl of 460mm, and the separation of the mirrors at 324mm.

You left out the back focus distance, so I will use 100mm from your other example.

In this one, p = 136,  (460 - 324).  p' = 324 + 100, or 424mm.  So the amp = p'/p = 424/136 = 3.11x.

But for this example, the secondary needs to be 50.1mm in diameter.

As I said, your calculations don't make sense to me.  Just don't use that program.

Edited by Garyth64, 26 October 2020 - 11:14 AM.

#1255 quilty

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Posted 26 October 2020 - 12:00 PM

first calculation if the primary is an f/3.26, the amp would be  12/3.26 = 3.69x

This was my calculation as well.

In you first part, using a f/3 primary with a separation of the mirrors at 324mm.  I come up with a secondary diameter  as 44.8mm.  You say the diameter is actually 48.5mm, so I don't know what the problem is there.

Well, I do. As I said in this constellation I would have useless CO, the sec is 6 mm too large, or 4 mm in your calculation. But this was the case if the primary mirror was a f/3 which supposition you find everywhere and this is not reality.

Primary f/3.25In this one, the  amp is 452/137 = 3.29x, for a system of f/10.75.

Your number do not work, as this is not a f/12.

As I already said, I don't mind if the resulting scope is f/12 or f/11 or f/13 I just want no castrated one. And at f/11 maybe a hyper primary would do better, I don't mind that neither, for I had more aperture and less CO THOUGH f/11 instead of f/12. And this shows why the actual design is not perfect. And why doesn't that make sense?

In this one, p = 136,  (460 - 324).  p' = 324 + 100, or 424mm.  So the amp = p'/p = 424/136 = 3.11x.

But for this example, the secondary needs to be 50.1mm in diameter.

The last two examples are puzzling me. When you calculate f primary times power secondary and f primary is f/2.85 then the secondary power should be more than 4x,shouldn't it? And is logical for curvature must be higher to better collimate the mor inclined light rays from the primary, correct?

But in the last example an f/2.85 primary needs just a 3.11x secondary, that I cannot understand.

Or, this way, 3.11x2.85 would yield an f/8.8 total wouldn't it? Great, faster than the RC while less CO and more aperture!

Again, my secondary inner diameter is 48.5 mm, mirror distance is 324 mm (depending a bit on collimation setting), the primary dia is 150 mm. The true aperture is 138 mm. The rest is calculated using ray set (do you say so, just considering at what distance the circumference light cone reduces to the sec size and to nought at the focal length. And this yields a focal length of 489 mm while direct measurement yields 49 +-1 cm which matches very good.

And again, designing the scope mirrors while mirror distance and scope dimensions is fix given would mean, the resulting f factor may be not at free choice, ok then, I don't care. Better having an f/13 or f/11 6.3 inch scope at 38% CO than an exact f/12 5.4" scope with 44% CO, true?

Better dropping a good friend than skipping a bad joke

Edited by quilty, 26 October 2020 - 12:48 PM.

#1256 Garyth64

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Posted 26 October 2020 - 02:27 PM

It is your example!  I think you just don't know how things work in these compound telescopes.

You just have to believe, and trust in the math.

primary = 160mm in diameter

primary focal length = 460mm

the primary is a f/2.87

separation of the mirrors = 324mm

you left out the back focus, so I made it 100mm as in your other example.

with the focal length at 460mm, and the separation of the mirrors at 324mm, that means that p, or the distance of the secondary is inside the focus of the primary, is 136mm.  (460 - 324 = 136)

p' is the separation of the mirrors, 324mm, plus the back focus of 100mm.  324 + 100 = 423mm, which is p'

p = 136mm, p' = 424mm.  amp = p'/p  = 424/136 = 3.11x

So with the primary at f/2.86, and the amp of the secondary at 3.11, the system is a f/8.89.

But as I said the secondary in this example comes out to being 50.1mm in diameter.

When you look into the focuser without an eyepiece, do you see the entire circle of the primary?  I would think that you should, if you don't something is definitely wrong, and take the scope back.    There is nothing else you can do to make it right, without redesigning the entire system.  We can keep going over and over this, but nothing is going to change.  I am sorry if you don't understand.

Edited by Garyth64, 26 October 2020 - 02:35 PM.

#1257 quilty

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Posted 26 October 2020 - 05:56 PM

yes, redesigning a bit.

That would be close to it. 160 primary at 460 mm focal length. (1-324/460) x 160 yields just 47.3 mm. So one mm spare left. Was a first estimation, and there's still the secondary baffle, so let's try 470 mm focal length: (1-324/470) x 160 = 49.7 mm, this sounds better, only 3.5 mm of primary circumference left out, that would be it, 156 mm primary at 470 mm focal length yielding (1-324/470) x 156mm = 48.5 mm demand of secondary, a perfect match at f/3.01, quite close to f/3 and trying your math yields secondary power 2.90. (I guess even and exact f ratios for the primaries are sort of pointless for in any scope their focal length counts, whereas their size just matter concerning aperture and CO, so on a less exact scale)

(p now is 146 mm, p' unchanged).

the scope then was an f/8.7 156mm with 39% CO. instead of my 138 mm f/12 scope with 44% CO without changing size, weight or anything else. A significantly faster scope with significantly less CO, isn't that worth it? And what I heard, hyper secondaries are the harder to produce the higher their power, specially exceeding 4x. If anyone might know, a real cass design might be overstretched at that speed, he is free to choose a hyper primary thus producing a new RC scope but still with 39% instead of 50% CO, which kept me off purchasing the GSO 6 inch RC in spite of its better advertised angular defnition. The new one wouldn't.

So this 2.9x secondary should be fine. I chose the 156 mm just to fill it completely into the secondary, a slightly different combination of primary size and focus would do as well. This is the optimum mirror setting when mirror distance and secondary size is fix.

A redesign, why not when the factory design is below optimum. I would try if I got the proper mirrors. And sure I don't need the collimation ring on the secondary.

I think everything is fine with my scope, no I'm sure of it. Just the mirrors could be optimized. And if someone knows how to get them and could tell me..

Another question I don't know at all, is about the mirror surface quality. A smooth 138 mm mirror might be better than a 156 mm mirror with inferior smoothness. So, if this effect is likely to be bigger than 10% it could overcompensate the ray set size optimizing. And that was my question how the CC scopes perform compared to other cat and cass scopes of the same nominal size (aperture).

I have just one to compare, the 5" Mak, sort of a two-point measurement in which the CC falls short a bit even regardless to size difference.

#1258 glend

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Posted 26 October 2020 - 06:15 PM

Time of move on folks. GSO is not going to redesign their CCs based on opinions of a few obsessives. It is what it is, and what it is, is just fine with the vast majority of buyers.

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#1259 Garyth64

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Posted 26 October 2020 - 06:42 PM

yes, redesigning a bit. That would be close to it. 160 primary at 460 mm focal length. (1-324/460) x 160 yields just 47.3 mm.

No, no, no!  It does not work like that.  Please refer to the equations I posted yesterday.  the 47.3mm only represents the diameter of the cone of light at 136mm.

136mm is the distance the secondary is inside the focus of the primary.  This is p.

The secondary needs to be larger than that.  It needs to be larger.

You also have to take in concideration of the diameter of the focal plane at the primary's focus.  Texereau uses the diameter of the Moon in the sky.  He multiplies the focal length times .009.  So for a 460mm focal length, the Moon will be 4.14mm in diameter at the image plane.

Then in using that number into his equation, where a = 4.14mm.

diameter of the secondary = ( D - a )  x p    + a  =    (160 - 4.14) x 136     + 4.14  =  50.22mm

fl                                    460

In this system of yours, the secondary should be a minimum of 50.22mm.

You should have made a new thread for all this discussion of yours a long time ago.  You have burned me out.

I've explained things to you, given you the equations, there is not much for me to do for you.  Especially when you start theory crafting.  You're just going to have to learn it on your own.  I am sure that all the other 6" f/12 CC are just fine and are not having any of the trouble you are experiencing.  Nothing has to be optimized.  It's time to put this one to rest.

I agree glend, it is time to move on.

Edited by Garyth64, 26 October 2020 - 06:56 PM.

#1260 Spacedude4040

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Posted 26 October 2020 - 10:26 PM

@ spacedude

Mike, I remember you did a detailed sketch of the 6 inch CC with exact dimensions which we finally agreed and which I cannot find again. It was a thread, would you share the link? My notes were not planned to endure 50 pages

Here it is Quilty

#1261 quilty

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Posted 27 October 2020 - 03:32 AM

@ Mike: thanks for the dimensions, and you used the word "verner" which I couldn't find. Is it the thing below? Always better now

@ glend: Surely so. I already said to leave this thread some pages ago, but couldn't resist, a bit obsessive.

And I'm afraid neither GSO nor the suppliers will bother to serve us with optimized equipment or even telling truth in their specs. The providers continue lying while knowing better and that is lying. Though GSO answered to consider a redesign, but probably just curtesy.

And my consequent answer would be to send back the thing for it isn't what they pretend it to be....

@gary: In this system of yours, the secondary should be a minimum of 50.22mm.

Well, in my very real, physically existing scope, the secondary should be minimum 52.7 mm, so an improvement anyway, isn't it?

Yes, maybe better consider a certain extent of focal plane instead of a spot but would this change things significantly? Nope

I could do a sketch for you in which you could try and find the flaws but you feel burnt out.

So,  better have a stop here without having solved a thing.

And I read between the lines that you know how to get the mirrors wanted. So, when I had them I might assemble a thing while others claim it to be impossible. I did so before in a totally different field (not at all optic)... and lost that job. For there cannot exist what mustn't exist.

Or Gary, in this case speaking in general: I say the actual 6 inch CC which is subtimal can be redesigned without changing dimensions except primary diameter and mirror curvature and without intruding high-end stuff to yield a significant performance improvement and you say either impossible or not worth it, which of them I haven't found out yet.

So, time to stop here.

Stephan

almost forgot. When writing this, was thinking about an extent of focal pane in the background and my first idea about this is

geometric optics (what we do when estimating mirror size) operates rays (one-dimensional) and spots (zero dimension) and blurring the focal spot might involve blurring straight thoughts as well. Or the other direction, light path is reversible as you might know :-) blurred ideas are likely to miss the point. Just first impression

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Edited by quilty, 27 October 2020 - 07:28 AM.

#1262 Spacedude4040

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Posted 27 October 2020 - 07:24 PM

Yes Quilty, the proper name is a Vernier Caliper. I use a digital one that makes it quick and easy.

Mike

#1263 doug mc

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Posted 27 October 2020 - 07:43 PM

Yes Quilty, the proper name is a Vernier Caliper. I use a digital one that makes it quick and easy.

Mike

As a retired machinist, we used to call them very nears. In comparison to micrometers.

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#1264 Terra Nova

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Posted 28 October 2020 - 08:34 AM

As a retired machinist, we used to call them very nears. In comparison to micrometers.

I always just heard them referred to by their slanguage name, Mike.

#1265 quilty

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Posted 28 October 2020 - 11:36 AM

Yes, quite creative that verynear, guess I'll keep it in mind. And specially more precise for us Germans. When we read 5.0 mm we'll stick to that exactly and reject anything slightly differreing, while very near means 5.0 +- 0.05 mm, sort of.

The following might be slightly off topic.

Boring. Would that relate to bore as hole or thread? In the sense of "steady drop hollow the stone" or just steady drop bores? Sounds so fine that I'm nearly sure of it.

Edited by quilty, 29 October 2020 - 05:45 AM.

#1266 quilty

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Posted 03 November 2020 - 05:24 AM

quite dead at moment. Maybe due to the elections you're starring at like the rabbit at the snake. Keep cool, Trump wili loose desperatly.I guess.

@ gary: ray set I found as a translation for our "Strahlensatz" though sounding odd a bit. But it doesn't seem to be. I mean just applying geometric optics, calculating lengths, for example if the secondary is too close to an f/3 primary with the effect that it is 4 mm too small a shift by +12 mm will sort that out. Any shift in x dimension causes a shift of 1/3 extent in y dimension, just because light uses to move along straight rays and the primary f ratio is f/3. Simple as that.

Edited by quilty, 03 November 2020 - 05:56 AM.

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#1267 quilty

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Posted 09 November 2020 - 07:30 AM

Watching mars still. 1/8 turn in 2.5 hours, which is 20 hours a day. More exact is finding the same cape (looks like india) ponting into the same direction (straight north) next day + 30 minutes. Which is a bit more than 24 hours a day. Still more exact would be the research of Mars rotation period in  the www, but not that funny.

And wathcing half Moon. At 300x just to see what happens. Try this, with 1.25" 6mm 66° eyepiece on terminator, you'll see a very bright golden ring shaped like the collimation ringmark. Next time I'll  to do a pic of it. So, seems to me that ringmark is for no good.

Apart from that at 300x at the terminator, you beautifully experience that you've reached  the scope's limit, yet not overstretching it. You distinguish more detail or any detail more easily than at lower power and at the same time it's obvious that higher power won't help anything, rather the opposite

#1268 quilty

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Posted 06 February 2021 - 06:01 AM

About size and weight. The CCs are quoted to be small and lightweighing considering their aperture. But at direct compare everything is depending. On the quest for the most powerful optics my NEQ3-mount would bear I've now own three of them. The first pic compares the CC to a Meade 2080 SC with the result that it is slimmer but a bit longer, without the focusser. And both weigh 5.5 kg. With its focusser the CC is still longer and about 6 kg. On the right side you see a homemade vixenrail from an alloy tin 70x350x3 mm.

The second pic compares three primary mirrors. As shown before, the size step form the 5" Mak to the "6"" CC is small not to say negligible. And as said above the Mak primary mirror is in fact a bit bigger than 5" or appears at least so through the front lens whereas the CC mirror physically is just 150 mm (not quite 6 inch). (The effiective aperture is still less, as was discussed many times before, but this restriction cannot be seen in the pic.)

So when comparing directly the CC is still a small scope and the step to a true 8 inch scope is really considerable, and the 8 inch scope is really medium size.

Q.

Edited by quilty, 06 February 2021 - 10:33 AM.

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#1269 Russkiy

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Posted 10 February 2021 - 02:41 PM

Да, диафрагма 137-138 мм ...

https://yadi.sk/i/TYjBKbjmsFcLZw

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#1270 Russkiy

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Posted 10 February 2021 - 02:45 PM

Flashlight Test

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Edited by Russkiy, 10 February 2021 - 02:46 PM.

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#1271 quilty

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Posted 23 February 2021 - 04:06 AM

oh yes, my magical torch test which you can assess with CO and true aperture. The issue of the GSO CCs

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#1272 ttc359

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Posted 28 February 2021 - 01:25 PM

Just my newb 2 cents after slogging thru most , not all, of this enormous thread.. the scope isn't for everyone and isn't it good we have so many choices in scopes today?   Personally, my eyes pick up false color way too easily (unfortunately) and it just pops out at me.   So I like the idea of a 5.2" CA free scope for \$500.   A C5 is \$630 (how??).  An Orion 6" mak is \$600.   But the deciding factor between these 3 scopes is not the price.. the white paint job on the AT CC6 just looks fantastic!

Q for any CC6 owners please.   Can the focuser be rotated?  It looks like maybe the silver bezel at the base can be loosened independently of the focuser, you can turn the focuser, then retighten the bezel?  I want to put this on an manual alt-az, but dont want the knobs pointing up.

Edited by ttc359, 28 February 2021 - 02:17 PM.

#1273 Thandal

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Posted 01 March 2021 - 08:33 AM

@ttc;

Yes, the focuser can be loosened and rotated to any orientation before re-tightening.

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#1274 quilty

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Posted 05 March 2021 - 06:43 AM

(The effiective aperture is still less, as was discussed many times before, but this restriction cannot be seen in the pic.)

well, maybe it can be suspected. The second pic was taken in a distance of about 1m and the three of them don't differ too much in focal lenghts. And you see the reflection of the secondary mirror in the primary and you see that CC's secondary appears the smallest indicating that the CC primary's curvature is the least and you might guess that when moving away while watching that reflection unlike the other two the CC sec's reflection would never reach the primary's rim.

#1275 quilty

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Posted 09 March 2021 - 07:18 AM

@ specedude

hello Mike,

there were thoughts about a small improvement.

As we know, the secondary baffle could be 3mm wider before it starts defining the obstruction. If you shorten the baffle by 2cm and replace that section by a tin of 0.5 to 1 mm thickness around it, that wouldn't be harmful. And when the sec fixing ring can be removed and the sec is fully and entirely aluminated to the edge it could be fixed with silicone or similar without that restraining ring. That operation might release the full 150 mm aperture at least.

Would you think, that improvement is considerable, or would it just help to feel better?

Stephan

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