Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Walks like a duck, Squawks like a duck, Looks like a duck.... but it’s NO DUCK? ...Help need here!

cassegrain mirror making optics
  • Please log in to reply
23 replies to this topic

#1 Spacedude4040

Spacedude4040

    Vostok 1

  • -----
  • topic starter
  • Posts: 146
  • Joined: 09 Nov 2018
  • Loc: Ontario, Canada

Posted 27 January 2020 - 07:54 PM

OK today I took the day off and disassembledcrazy.gif  my GSO 6" Classical Cassegrain because the other day I used the flashlight testing technique and found my thru put was only 140mm instead of 150mm. shocked.gif Here's the link  for how to do this  .https://www.cloudyni...illustrated/   This also was found out by another members calculation for his 150mm scope was also working at 140mm.

 

I was quite surprised when I took the measurements with a vernier and used "Corel Draw" to lay it all out to scale.doah.gif   You can pm me if you have the program or want to download the trail version and I'll gladly send the file over.

Also I use some gentleman excel program (can't give him credit, don't remember how I got thisconfused1.gif ) to see where it all landed and if I'm correct which I can't see being wrong this is no duck!

 

 

6inch CC Drawing.jpg

 

 

The secondary is a Hyperbolic Mirror and if it's right and I wish somebody with more knowledge can correct me.

The scopes working at  f/14.74  @ 2212mm not a f/12   @1824mm. The four yellow blocks basically match my numbers on my drawing!

 

 

14.75 print out.jpg

 

Here's the excel sheet if you want.  Attached File  6 inch Classical Cassegrain-Parameters 2.xlsx   26.06KB   28 downloads

 

I was looking for where it was vignetting it down to 140mm but couldn't find it either. This is nuts or maybe its just me..

Anybody willing to help or explain my mistake.foreheadslap.gif

 

Mike


Edited by Spacedude4040, 27 January 2020 - 08:03 PM.


#2 Geo.

Geo.

    Vendor - Nexstar Parts

  • *****
  • Vendors
  • Posts: 5,522
  • Joined: 01 Oct 2008
  • Loc: Upstate NY

Posted 28 January 2020 - 04:44 PM

GSO used a smaller than optimum secondary to provide better planetary resolution. See the user test in "Articles and Reviews."

 

Then put it back together.



#3 Spacedude4040

Spacedude4040

    Vostok 1

  • -----
  • topic starter
  • Posts: 146
  • Joined: 09 Nov 2018
  • Loc: Ontario, Canada

Posted 28 January 2020 - 06:18 PM

Hi Geo,

-Well putting a 60mm secondary in front of a 150mm primary is not designing it for higher resolution . That's secondary huge!!

-If you look at my drawing it's not undersized, well maybe if your worry about the 75% rays from the parabolic primary so you might see a slight dimming at the outer edge of a wide field eyepiece.

-Also its operating at f14.75 which once again still makes that secondary huge.

 

I have the scope back together and I'm happy with it but I didn't just fall of the turnip truck and I know GSO didn't either but something is really fishy here!!!

Mike



#4 Spacedude4040

Spacedude4040

    Vostok 1

  • -----
  • topic starter
  • Posts: 146
  • Joined: 09 Nov 2018
  • Loc: Ontario, Canada

Posted 28 January 2020 - 06:31 PM

Somebody can correct me on this but I once read that a RC scope will have a larger secondary then the same focal length Classical Cassegrain.

So is this a f/15 RC that has its secondary reduced to the size of a f/12 CC so this is where the light loss is?

I remember someone stating that the field was too flat to be a CC  more like a RC and people thought he was nuts but now I'm wondering myself.

So if the primary is hyperbolic it explains why the aperture is knocked down from 150mm to 140mm due to the secondary being sized for a Classical Cassegrain and not the wider light cone of a RC Design. They would also need to increase the focal lenght of the secondary to get it to come out the back of the primary because they moved it away from the primary also.

So not only do you get less light because of the fatter cone is missing the secondary, it's now compounded by the secondary increase in obstruction to 61mm over 140mm which is 44%

So we ordered a f/12 150mm  scope with a 33% obstruction and got a f/14.75  140mm scope with a whopping 44% obstruction.

 

Mike


Edited by Spacedude4040, 28 January 2020 - 10:15 PM.

  • Terra Nova likes this

#5 mclewis1

mclewis1

    Thread Killer

  • *****
  • Posts: 21,074
  • Joined: 25 Feb 2006
  • Loc: New Brunswick, Canada

Posted 29 January 2020 - 09:11 AM

Mike, You measured the focal plane distance (using the flash light test?) and calculated a focal ratio of f14.75 using the reduced aperture number. Have you verified the focal length number by actually measuring the focal point position behind the rear cell using a celestial object focused at infinity? Or perhaps doing a plate solve with a known camera and calculating the focal length?

 

Do you know for a fact that the secondary is not affecting the angle of the light cone (and therefore the actual focal length)?


Edited by mclewis1, 29 January 2020 - 09:11 AM.


#6 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 27,522
  • Joined: 01 Feb 2006

Posted 29 January 2020 - 11:35 AM

 actually measuring the focal point position behind the rear cell using a celestial object focused at infinity?

Yes, this was what I was going to say. Important to have the eyepiece and focuser set for infinity focus.

 

As to the OP's concern, if the test was done right, this would not be the first time we have seen scopes that were working at less aperture than specified. I can remember this happening in four different models.

 

The first Stellarvue scopes were 80mm achromats and they were working at reduce aperture because the front of the focuser tube was sticking into the light cone.

 

The 127MCTs were measuring 118mm 

 

The early Orion 120EDs were working at 118mm but hey, that is a tiny difference really and this was corrected in later runs)

 

The other one I don't remember, but I think it might have been the 6" MCT?  I forget, but I am pretty sure that there was one other scope as well.

 

Now why would the focal length change though????  That has nothing to do with the aperture. You could reduce the aperture to 4" and the focal lenght would still be the same. So unless you measured it as being longer than stated, the aperture loss does not change the focal length of the scope...

 

(In scopes with a negative secondary and a moving primary, yes, the focal length can change but this is not the case in the cassegrain unless the mirror spacing changes.)


Edited by Eddgie, 29 January 2020 - 12:00 PM.


#7 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 27,522
  • Joined: 01 Feb 2006

Posted 29 January 2020 - 11:59 AM

And while the aperture may be reduced, the other consequence is that the obstruction now becomes larger (as a percentage of the remaining aperture) and yes, the focal ratio gets slower. 

 

Even without these things, at 6" you are kind of in a transition zone where the performance of the reflectors at this size is getting borderline.  Smaller than this and the advantage quickly shifts to refractors.

I did calculations and measurements on my own 6" scope, but it is nominally f/2.8, though the actual brightness is probably more equivalent to f/3.6. 

 

With these small scopes, to get any kind of meaningfully large fully illuminated field, the secondary has to be much larger (as a percentage of obstruction) than for a larger scope with the same size fully illuminated field.  At f/2.8, to fully illuminate a 25mm APS-C size sensor, the secondary mirror in my scope has to be 40% of the aperture. In my 8" at f/4, it only has to be about 35% of the aperture, and in a 10" scope, it only has to be about 30% of the aperture.   So as the scope gets larger, the secondary can get smaller and still provide the same level of illumination.

 

In my example, the light gathering of the 6" aperture itself is about 28 square inches.   The obstruction though shades about 4.5 square inches of that figure, so now the true light collection area is only 23.5mm

 

Next, we factor in the mirror transmission and we are looking at maybe 90% of that figure (about 21 square inches), so now we have turned the 6" f/2.8 scope into a 4.6" refractor level of light gathering, and an effective focal ratio of more like f/3.6.  (Now no one sells a 6" f/3.6 refractor though so while this scope is not really providing the exposure time that the geometric focal ratio suggests, it is sill considerably faster than any refractor of this size you can buy). 

 

In the OPs case, if the aperture is really being reduced, then when secondary shading and transmission are factored in, the effective focal ratio (as compared to the true brightness) is probably much higher than the sales figure.

 

But this is not an indictment of ES.  It is industry practice to only provide the geometric focal ratio on these instruments and not the effective focal ratio.

 

The point though is that if the scope is working at the 140mm stated aperture, the effective brightness is far less than the sales figures suggest. 

 

I would double and triple check the figure ensuring that the measurement was done with the eyepiece at infinity focus though. but except for the error in focal length change, the evidence does point to reduced aperture.


  • Spacedude4040 likes this

#8 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 27,522
  • Joined: 01 Feb 2006

Posted 29 January 2020 - 12:04 PM

 

 

Now why would the focal length change though????  That has nothing to do with the aperture. You could reduce the aperture to 4" and the focal lenght would still be the same. So unless you measured it as being longer than stated, the aperture loss does not change the focal length of the scope...

 

 

And just as an Einstein like thought experiment, suppose you mask a 100mm f/10 refractor down to 50mm.  The focus position does not change at all.  The scope will still focus in exactly the same spot so the focal length remains 1000mm.  What does change though is the focal ratio and now rather than being f/10, you have a 50mm f/20 telescope, but it is still 1000mm in focal lenght, yes?

 

We see this in large reflector used with stop down masks as well.  Stopping down the aperture in those scopes does exactly the same as in the refractor example above.  If you stop down an 18" dob to 6", it does not change the focal length. The aperture is reduced and the focal ratio is increased but the eyepiece still reaches focus in the same place.

 

And because your scope has fixed primary and secondary, the same will be true there as well. Scope is still working at the same focal length as before.  The aperture and focal ratio, and the percentage of obstruction are all different, but the focal lenght remains unchanged.


Edited by Eddgie, 29 January 2020 - 12:19 PM.

  • Spacedude4040 likes this

#9 Spacedude4040

Spacedude4040

    Vostok 1

  • -----
  • topic starter
  • Posts: 146
  • Joined: 09 Nov 2018
  • Loc: Ontario, Canada

Posted 29 January 2020 - 01:06 PM

Mike, You measured the focal plane distance (using the flash light test?) and calculated a focal ratio of f14.75 using the reduced aperture number. Have you verified the focal length number by actually measuring the focal point position behind the rear cell using a celestial object focused at infinity? Or perhaps doing a plate solve with a known camera and calculating the focal length?

 

Do you know for a fact that the secondary is not affecting the angle of the light cone (and therefore the actual focal length)?

Hi Mark
-The flash light test was to look for any reduction in aperture only which showed approximately 140mm. The focus position was where it was while looking at a focused star at the eyepiece. The funny issue is it didn't matter where the focuser was positioned or how far the LED flash light was held to the eyepiece. So to me the light cone was being limited by an aperture stop or a very undersized secondary.

-The focal position behind the telescope I measured with a ruler and figured it to be 253mm. I remembered somewhere it listed as 150mm passed the focuser so the focuser is at 105mm. Add them both and you get 255 and mine of 253 so I went with 250mm.

The numbers all add up to an f/14.75 focal ratio but could be wrong, that really is not my complaint and I would prefer it longer. It could be a f/12.1 to f/15 scope, what I don’t like is it’s a 140mm telescope with a 61mm ring at primary which is 3mm larger than the secondary.  So this telescope has just under 44% obstructions.

So this is what they are selling compared to what you are getting.

 

They say:
-Classical Cassegrain (parabolic primary and hyperbolic secondary)

-150mm telescope so should have the light grasp of a 150mm not worrying about coatings.

-33% secondary obstruction and some places list even less, they say better contrast then SCT because of it...

 

What you get:

-Some variant of a Cassegrain.
My thoughts are it’s likely a RC with a longer focal length. So how can this be done. Move the secondary father away from the hyperbolic primary to reduce its size then increase its magnification to move the focal point out to the desired distance behind primary. But it’s still a larger secondary, so reduce the size again which reduces the aperture from the primary to the eyepiece.

-140mm usable light gathering ability with reduced resolution compared to a 150mm.

-44% secondary obstruction which will impact and reduce its ability on the use on planets. This is the reason we are mainly buying theses scopes. So your resolution is less because of the smaller primary and what left is further reduced by the huge impact from the secondary.

 

 

Agena Astro Products (Who I really Like) if they a truly testing these telescope before shipping will know what they really are. Every other dealer just resend what they received to us.

If you visit their website you will see their description as A Modern Take on a Classic Telescope Design. The modern take is they are not a true Classical Cassegrain  LOL.

 

I still like my scope and it’s a keeper, there are a lot of pluses to this scope which I like. Just wish it was what I ordered!

 

GSO just needs to be honest with us and let us make an educated decision.

 

Mike


Edited by Spacedude4040, 29 January 2020 - 01:11 PM.

  • Live_Steam_Mad likes this

#10 Spacedude4040

Spacedude4040

    Vostok 1

  • -----
  • topic starter
  • Posts: 146
  • Joined: 09 Nov 2018
  • Loc: Ontario, Canada

Posted 29 January 2020 - 01:16 PM

So in simple terms you order a Corvette convertible for weight saving with a V8 and they ship you a hard top with a V6 LOL

Mike


Edited by Spacedude4040, 29 January 2020 - 01:17 PM.


#11 mclewis1

mclewis1

    Thread Killer

  • *****
  • Posts: 21,074
  • Joined: 25 Feb 2006
  • Loc: New Brunswick, Canada

Posted 30 January 2020 - 08:36 AM

Mike,

 

Thanks for that explanation. 

 

You've measured the focal length well and then with the aperture reduction measurement you indeed have a scope working at almost f15. Visually on it's own this spec change this means virtually nothing (vs an f12 scope) since we're not talking about an actual change in focal length. 

 

But I totally agree with your underlying theme - GSO should be more transparent in their description of the scope. The problem is that many scopes exhibit some aperture reduction (so it's not something generally discussed in the marketing literature), it's just that this particular scope design exhibits quite a substantial amount of aperture reduction in actual use.



#12 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 10,219
  • Joined: 12 Oct 2007
  • Loc: Melbourne, Australia

Posted 30 January 2020 - 05:03 PM

If the diagram is correct and it is clear that nothing is clipping the on-axis cone of light from the primary, then the aperture is 150mm and the result from the flashlight test is not valid.  There are many reasons why the flashlight test could give a bad reading.  It is not a reliable test.

 

If the aperture were less than 150mm there would need to be a baffle somewhere clipping the cone.  If it is clear there is no such clipping going on - the primary acts as the aperture stop and the entrance pupil - and its diameter is the aperture of the system.

 

Frank



#13 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 10,219
  • Joined: 12 Oct 2007
  • Loc: Melbourne, Australia

Posted 30 January 2020 - 05:55 PM

Instead of doing a full ray trace of the design - or using the flashlight test - there are some simple measurements you can make.

 

If you can measure the mechanical layout of the system, measure the following:

 

Diameter of primary

Diameter of secondary

Inner diameter of the front of the secondary baffle

location of front of focus tube baffle and its inner diameter

location of focal point when focused on infinity

 

You may not know the exact optical prescription but you can work backwards from focus assuming the operating f/number.

 

From focus draw an f/12 cone toward the secondary - i.e. for every 12 steps forward it becomes one step wider.  Draw it to where it hits the secondary.  If the secondary is too small or if that cone hits the focus tube baffle, you know it can't be operating at f/12.

 

Now mark the two points where the cone hits the secondary and draw lines from them to the edge of the primary.  Do those lines hit the secondary baffle?  If so then the secondary baffle is limiting aperture.

 

If your drawing shows nothing hitting a baffle - you know it *could be* f/12 with aperture 150mm and not get clipped.

 

And if it is f/12 with aperture 150mm, you know the focal length is 1800mm.

 

Frank

 

Addendum:

 

Note that the above *assumes* it is f/12 - and shows that it could operate as 150mm f/12 system without clipping.  But it could also be operating at full aperture yet much slower.  So it could instead be 150mm f/15 and much longer focal length.  But you have confirmed that it can be full aperture at f/12 without clipping.

 

If you can measure the focal length accurately you should use the f/number based on f/150mm and use that instead of f/12 when doing the diagram.


Edited by freestar8n, 30 January 2020 - 06:00 PM.


#14 Spacedude4040

Spacedude4040

    Vostok 1

  • -----
  • topic starter
  • Posts: 146
  • Joined: 09 Nov 2018
  • Loc: Ontario, Canada

Posted 31 January 2020 - 10:43 AM

This is what I was thinking but came out wrong in the beginning.

(f/13.2-f/13.5) or so with a 140mm-138mm scope with a whopping 44% obstruction.

 

The light cone is missing the secondary so no 100% illumination. This is my guess why the flash light test didn't matter because the secondary is the limiting factor.

 

 

 

6inch CC Drawing   f 3.3 primary.jpg

 

The numbers off the excel spreadsheet are very close.


Edited by Spacedude4040, 31 January 2020 - 04:07 PM.


#15 Spacedude4040

Spacedude4040

    Vostok 1

  • -----
  • topic starter
  • Posts: 146
  • Joined: 09 Nov 2018
  • Loc: Ontario, Canada

Posted 31 January 2020 - 10:47 AM

The numbers off the excel spreadsheet are very close. These numbers are for a Classical Cassegrain and if its a RC it will be different of course. We will never know as we don't have to equipment needed to measure a RC hyperbola .

 

new 6inch cass with small secondary.jpg


Edited by Spacedude4040, 31 January 2020 - 04:38 PM.


#16 Spacedude4040

Spacedude4040

    Vostok 1

  • -----
  • topic starter
  • Posts: 146
  • Joined: 09 Nov 2018
  • Loc: Ontario, Canada

Posted 31 January 2020 - 10:48 AM

Frank,   here is the f/12 light cone from focus.

All the measurements are on the print. Red are actual numbers from my scope. The black numbers are as close as **** is to swearing. Corel Draw is like a 2D CAD program.

 

f12 light cone.jpg

 

Mike


Edited by Spacedude4040, 31 January 2020 - 11:45 AM.


#17 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 10,219
  • Joined: 12 Oct 2007
  • Loc: Melbourne, Australia

Posted 31 January 2020 - 04:59 PM

If you're sure the distance from secondary to focus is 614mm then you know the clear area of the secondary would need to be 51.2mm if it were f/12.  If instead it is 48.5 or less then you know it has to be slower than f/12 - from simple geometry and without knowing details of the design.

 

But it could still be full aperture of 150mm if the focal length is longer than expected.  Do you have a direct measurement of the focal length?  You mention 2212mm above but I'm not sure where it comes from.

 

If the focal length is 2212mm then the f/ratio is 14.74 and the clear area of the secondary would need to be 614/14.74 = 41.6mm - and that does appear to fit.

 

Can you clarify what the focal length is and how you measured it?  I prefer using a plate solve with a camera, where the pixel size is known accurately.  But you can also use drift in an eyepiece if you do it carefully.

 

It looks like the largest diameter cone the secondary can support is about 46mm, which means the fastest the scope could be is 614/46 = f/13.3.  That means if you measure a focal length greater than 13.3*150 = 2000mm, it should be able to operate at full aperture.

 

Note that this simple use of geometry is easy to do from focus to the secondary as long as there are no lenses in that path.  And on the primary mirror side it's also easy if there are no lenses in front of the primary.  So a Maksutov is more complicated since it has the big meniscus in front.  But even for a Maksutov the secondary needs to be large enough to support the desired f/ratio cone to focus.

 

Frank



#18 Spacedude4040

Spacedude4040

    Vostok 1

  • -----
  • topic starter
  • Posts: 146
  • Joined: 09 Nov 2018
  • Loc: Ontario, Canada

Posted 01 February 2020 - 09:43 AM

If you're sure the distance from secondary to focus is 614mm then you know the clear area of the secondary would need to be 51.2mm if it were f/12.  If instead it is 48.5 or less then you know it has to be slower than f/12 - from simple geometry and without knowing details of the design.

 

But it could still be full aperture of 150mm if the focal length is longer than expected.  Do you have a direct measurement of the focal length?  You mention 2212mm above but I'm not sure where it comes from.

 

If the focal length is 2212mm then the f/ratio is 14.74 and the clear area of the secondary would need to be 614/14.74 = 41.6mm - and that does appear to fit.

The measurements that I took from my scope on the first image will indeed work, that why I knew something was fishy and didn't add up.

 

Can you clarify what the focal length is and how you measured it?  I prefer using a plate solve with a camera, where the pixel size is known accurately.  But you can also use drift in an eyepiece if you do it carefully.

Off spreed sheet using my measurements of optics and placement of them

 

It looks like the largest diameter cone the secondary can support is about 46mm, which means the fastest the scope could be is 614/46 = f/13.3.  That means if you measure a focal length greater than 13.3*150 = 2000mm, it should be able to operate at full aperture.

13.3*140mm = 1862mm. The yellow light cone is the 140mm, blue is 150mm primary.

 

Note that this simple use of geometry is easy to do from focus to the secondary as long as there are no lenses in that path.  And on the primary mirror side it's also easy if there are no lenses in front of the primary.  So a Maksutov is more complicated since it has the big meniscus in front.  But even for a Maksutov the secondary needs to be large enough to support the desired f/ratio cone to focus.

 

Frank


Edited by Spacedude4040, 01 February 2020 - 09:44 AM.


#19 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 10,219
  • Joined: 12 Oct 2007
  • Loc: Melbourne, Australia

Posted 01 February 2020 - 09:36 PM

For topics like aperture reduction I would focus on things you can measure directly.  Like spacings, diameters, and focal length.  Measuring the focal length from a ray trace assumes properties of the system, whereas measuring the actual system with a plate solve or eyepiece drift is direct.

 

You may be given the diagram and dimensions of a duck, but the actual specimen is platypus.

 

Frank

 

To summarize:

 

If the measurements of the layout are correct, ignoring focal length, then you know it can't be f/12 because the clear area of the secondary is too small.

 

But if the focal length is in fact long enough to increase the f/number and narrow the cone, it could be operating at full 150mm, but slower speed.

 

No matter what - it's easy for the flashlight to give a misleading result.  And it's odd, but not too surprising if the focal length is longer than stated.

 

But it is unfortunate if it is described as f/12, yet you can tell right away that the secondary is not large enough to support an f/12 cone at that distance from focus.

 

And it's also unfortunate if the secondary really is limiting aperture, and makes it a smaller aperture and slower system than it should be.

 

Frank


Edited by freestar8n, 01 February 2020 - 10:45 PM.


#20 Vinny1980

Vinny1980

    Vostok 1

  • -----
  • Posts: 178
  • Joined: 03 Oct 2017
  • Loc: Aversa, Italy

Posted 20 May 2020 - 07:34 AM

Sorry for late writing. You might find intersting my findings on the other thread about GSO Casses:

 

https://www.cloudyni...ain/?p=10201696


Edited by Vinny1980, 20 May 2020 - 07:35 AM.


#21 jgraham

jgraham

    ISS

  • *****
  • Posts: 21,843
  • Joined: 02 Dec 2004
  • Loc: Miami Valley Astronomical Society

Posted 20 May 2020 - 09:45 PM

I bought an 8" GSO Classical Cass after reading several reviews including some discussion of the under-sized secondary. For me, hitting the number was less important than getting a fine telescope that does what it is supposed to do and brings me joy when I use it. For me, the GSO checks all of those boxes.

 

Atlas GSO CC8 (5-8-2020)-1.jpg

 

Atlas GSO CC8 (5-12-2020)-1.jpg

 

Beautiful photo/visual scope!

 

We are blessed to have such problems. :)

 

Enjoy!

 


  • Starman27, stevew, eros312 and 3 others like this

#22 Darth_Takahashi

Darth_Takahashi

    Explorer 1

  • -----
  • Posts: 66
  • Joined: 06 Mar 2020

Posted 14 June 2020 - 05:03 AM

I bought an 8" GSO Classical Cass after reading several reviews including some discussion of the under-sized secondary. For me, hitting the number was less important than getting a fine telescope that does what it is supposed to do and brings me joy when I use it. For me, the GSO checks all of those boxes.

 

attachicon.gifAtlas GSO CC8 (5-8-2020)-1.jpg

 

attachicon.gifAtlas GSO CC8 (5-12-2020)-1.jpg

 

Beautiful photo/visual scope!

 

We are blessed to have such problems. smile.gif

 

Enjoy!

Exactly, enjoy it and make full use of it.



#23 RalphMeisterTigerMan

RalphMeisterTigerMan

    Surveyor 1

  • *****
  • Posts: 1,709
  • Joined: 01 Nov 2016

Posted 14 June 2020 - 07:01 AM

I strongly suggest reading the review on the CN home page, the GSO 8", a true cassegrain. This gentlemen worked out that he was not getting the true "brightness" and light thru-put that an 8" should be giving. After doing a lot of research and measurements of his "8-inch" cassegrain was closer to that of a 7". 

 

It sounds as though GSO has done the same thing with your scopes, where the diameter of the primary advertised is almost an inch smaller than stated. He actually contacted GSO and they admitted as much.

 

So I'm afraid that your "duck" is chicken in disguise which has been trained somehow to "walk like a duck", "quack like a duck" but is unable to "fly like a duck". Sorry! So be like Sheldon Cooper who strongly suspected that the "orange chicken" he has been eating is actually "tangerine chicken". "...I have oxen in my bed...many, many oxen...show me your mucous!..."

 

LOL! BTW, didn't the KitKat bar once have "5" fingers?

RalphMeisterTigerMan



#24 quilty

quilty

    Vostok 1

  • -----
  • Posts: 156
  • Joined: 07 Oct 2019

Posted 26 February 2021 - 05:37 AM

facing the possibility that everything of matter has already been uttered at this topic I try to summarize the findings

 

What I take for granted:

true aperture is about 138 mm

focal length of primary is about 50 cm, so it's an f/3,3, instead of f/3 and that's the reason for the secondary to be too small or too narrow. 

But you cannot just increase the mirror distance nor just replace the primary by a true f/3 because then the focus would shift into the tube.

the overall focal length is about 1.8 m, so if you suppose a 150 mm aperture it's still an f/12, but when you apply true aperture it is f/13

To have everything neat you'll also have to replace the secondary, for now it is about 3.6x and you need 4x

Then you had a true 150 f/12 CC at full aperture. (Not quite 6 inch, that would be 154 mm)

CO is 44%

Guesswork:

If the primary is hyperbolic or parabolic. And if that makes a big difference


Edited by quilty, 26 February 2021 - 08:17 AM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: cassegrain, mirror making, optics



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics