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Orion 127 Maksutov - Back focus and aperture

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#326 astro_baby

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Posted 01 July 2012 - 08:17 AM

You'll notice in the previous pic there is no image coming off the card - thats because by the time I took some pics the torch was going flat and it wasnt kicking enough photons to get an image onto the card.

Heres a shot of the card with a white light torch being put into the EP to give an idea of what it all looked like from the front end.

The card is marked with an earlier pencil mark.

Attached Thumbnails

  • 5297520-MRT003.jpg


#327 astro_baby

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Posted 01 July 2012 - 08:35 AM

The results were as follows;

NO ADAPTERS/EYEPIECES - Used for Lining Up only
Light applied directly to telescope rear, no adapters or lenses.........Trace 175mm Dia. Scope unusable in this condition.

1.25 DIAGONAL and 1.25" ADAPTER
Light applied to 1.25" 25mm Erfle Eyepiece with 1.25" diagonal and 1.25 adapter supplied with scope.......Trace Diameter 171mm

Light applied to 1.25" 25mm & 7mm Orthoscopic Eyepieces with 1.25" diagonal and 1.25 adapter supplied with scope.......Trace Diameter 171mm

MOONLITE FOCUSER FITTED / 1.25 DIAGONAL
Light applied to 1.25" 25mm Erfle Eyepiece with 1.25" diagonal.......Trace Diameter 170mm

Light applied to 1.25" 25mm & 7mm Orthoscopic Eyepieces with 1.25" diagonal .......Trace Diameter 170mm

MOONLITE FOCUSER FITTED / 2" DIAGONAL
Light applied to 1.25" 25mm Erfle Eyepiece with 2" diagonal.......Trace Diameter 170mm

Light applied to 1.25" 25mm & 7mm Orthoscopic Eyepieces with 2" diagonal .......Trace Diameter 170mm

IN SUMMARY
Based on the work done by others and my own testing I conclude the Skymax 180 operates with an aperture of circa 170mm. I do not know why.

Adding the Moonlite removes approximately 1mm of aperture with a tolerance of around 1mm in measuring so its a bit too close to call for the limited facilities I have available.

For me I will continue to use the Moonlite - I really seriously doubt I am going to miss 1mm of aperture in a country where the atmopshere is almost never really steady anyway :) Any small loss of aperture is more than offset by the smoother focusing and removable of image shift issues.

I will have to defer to people with bigger brains than me as to exactly why the scope is limited this way.

#328 GlennLeDrew

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Posted 01 July 2012 - 11:50 AM

Melanie (astro_baby),
When doing the flashlight test for aperture, always have an eyepiece installed, and try to ensure the focus is set reasonably near to infinity. When adding/removing components at the back end, the eyepiece can end up well removed from infinity focus.

Here's why you must set the focus. The eyepiece forms a tiny image of the light at its focus. This little image acts like a focused image of a star, and the light therefrom, when traveling in reverse through the system, traces out the full light bundle the system can accommodate as co
Ing from a distant source. If the eyepiece is too far inward, the light exiting the scope's front end is somewhat divergent, and the effective aperture is likely to be artificially enlarged. Conversely, if the eyepiece is too far back, the light exiting the front end will be convergent, thus artificially reducing the effective aperture. The errors thus introduced may well be small enough to ignore, depending on where the aperture reduction is occurring, but it's always best to be rigorous.

A green laser makes this testing easier if you couple it with a small finder. Tape the laser to the finder's eyepiece, and coming out the finder's obejective will be a collimated beam enlarged in diameter by an amount equal to the magnification. This application is called a beam expander. The purpose is to ensure filling the scope eyepiece's exit pupil with light, which results in a fully illuminated disk of light exiting the scope's front end. If the laser beam is smaller than the exit pupil, only a corresponding fraction of the entrance pupil is filled, requiring to 'wiggle' the laser about in order to sample opposite sides of the exiting light bundle for diametric measurement.

Note that the laser can be in contact with the finder's eyepiece, and the finder's objective can be in contact with the scope's eyepiece. This is because the laseright is collimated. A plain old flashlight does not emit properly collimated light, thus the requirement that it be placed no closer to the eyepiece than at least 10 eyepiece focal lengths.

Soon after introducing this refinement to my 'flashlight test for aperture' here on CN, some wag coined the handle, BELT, for "Beam Expanded Laser Test."

#329 astro_baby

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Posted 01 July 2012 - 12:22 PM

Yup the scope was refocosed each time for infinity by using a very distant radio mast thats about 8-10 miles away. I normally find that works ok and is within a tweak of perfect focus.

I'll try with a green laser when I can borrow one off my sis in a few weeks time but given the data from other tests in this thread I doubt it would show a huge variance and I dint really have the facilities to build a test rig that would be accurate enough to get down to millimetre perfect measurements.

I kind of proved, at least to my own mind, that the external focuser has no apparent effect on the scope which was all I was really trying to acheive.

Just thought given the direction the thread took that it might be of interest.

If the clouds in the UK ever push off, we are currently in the wettest, cloudiest summer I can recall, I will do another eyeballs test but the Mak 180 is something of a cloud magnet so it may be some time.

#330 Ed Holland

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Posted 02 July 2012 - 03:18 PM

Nice setup!

I did see your other thread, and intended to respond, but never got around to it. Being unfamiliar with that particular scope, and unwilling to start another mega thread, I hesitated to get involved - Sorry!

Your approach to the measurement was very similar to mine: no purpose built jig, just careful arrangement of the parts with "common household materials" and proper focusing. I believe your results, if my example of the 127mm scope was anything to go by. There is little, if any, effect on aperture by increasing the back focus distance. The effective focal length (and f/ratio) will change, of course and these factors can be determined with a little care, if you so wish.

I'm sorry the weather back in good old Blighty is so uncooperative. Moving to California does have its astro-benefits :)

Ed

#331 dweller25

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Posted 30 November 2014 - 09:06 AM

I just used the flashlight test on my Blue Skywatcher 127mm Mak.

 

The primary measured 120mm amd the secondary obstruction measured 46mm

 

So it has a 38% secondary obstruction.

 

In terms of contrast transfer I find subtracting the secondary from the primary to roughly equal the contrast equivalent of a refractor, so in my case 120-46=74mm

 

So in my mind I have the performance of a 3" APO in a light grab and go tube for very little money. :)



#332 Live_Steam_Mad

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Posted 26 August 2015 - 08:26 PM

Has anyone done this aperture / BELT test on the 7" Meade Maksutov?

 

The secondary obstruction on this 7" Meade Mak is 39 per cent, this is due to the conical secondary baffle.;-

 

https://picasaweb.go...sutovCassegrain

 

https://picasaweb.go...390628396915218

 

https://picasaweb.go...orBeingRemoved#

 

The primary is Ellipsoidal in surface figure, and is oversize at 207mm ;-

 

https://picasaweb.go...397674737744546

 

https://picasaweb.go...EMOVAL20130831#

 

BTW also just thought I would point out that (it seems) most (all?) Maksutov correctors that are commercially produced or made by amateurs have got straight sides, not conical (slanted sides), and that in Telescope Optics (Rutten / Van VenRooij), every Maksutov corrector is shown as having the light diverging as it enters the corrector (obvious I suppose as the corrector is a negative lens), so I take it that the very edge of the corrector cannot be used to gather light as the light would be deflected into the corrector holding ring as it enters?

 

The Meade Mak configuration does not appear in the book, it seems (it's NOT a Gregory, or Rumak, etc), it seems to be almost or actually unique for a commercial instrument?

 

Best Regards,

 

Alistair G.


Edited by Live_Steam_Mad, 26 August 2015 - 08:28 PM.


#333 Asbytec

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Posted 30 August 2015 - 02:22 AM

IN SUMMARY
Based on the work done by others and my own testing I conclude the Skymax 180 operates with an aperture of circa 170mm. I do not know why.

 

 

 

Many of us have beat this up a few years ago - hijacking Ed Holland's thread at the time (this one?), for which I am both grateful and apologetic for. :)

 

It's either the secondary mirror being the same diameter of the meniscus, it needs to be larger so as not to loose aperture. Or, as in my case, the secondary baffle cut into the light cone (and was simply, but cautiously removed with no observable ill effect.) These two seem to be the primary causes or reduced effective aperture. 




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