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telephoto mirror lens, 甜甜圈反射鏡,波波鏡

 


The optical design is based on gregory-maksutov or spot maksutov telescope, and often called as CAT ( catadioptric) lens which is a combination of mirrors and refractive lens



most of these lens are manual focus. The aperture is fixed. It is virtually free ( marketing bluff beware) of chromatic aberration, coma, and off-axis aberrations. However, due to central obstruction of secondary mirror, contrast is normally lower. However, lateral color aberration ( different image size for different colors) still exist, except on the Zeiss mirotar, which use dual corrector plates

The overall image quality depends mainly on the secondary mirror aberrations. So in lower priced models, if the secondary mirror is a separate element, it has more room for improvement.

Exposure:

The secondary mirror blocks about 1/3 of the area of the lens aperture. This in itself would result in the loss of about 2/3 stop in effective lens speed (i.e. a T-stop that was 2/3 stop slower than the f-stop). The actual T-stop would also include light losses at the reflecting surfaces and light losses (including surface reflection and bulk glass absorption) when the light passes through the refractive optical elements.

This would indicate that both the Pro Optic 500/6.3 and the Tamron 500/8 are lenses with T-stop of around 10. or 1.33 EV dimmer than f/6.3

Experience:

in modern days, user will find it difficult to use:

Manual Focus

no Image Stabilisation on lens

slow aperture

auto exposure will not work properly

with all these in one same lens, you definitely need a sturdy tripod to make a decent sharp picture, in today's DSLR standards.

conventional spot maksutov design

1000mm FL MTO and on youtube review

rubinar:


 

secondhand ref price usd 60 to 80 for these below:

Yashica Tomioka

Spiratone 500 f/8 Minitel and 800

Tokina

Sigma (Accura)

Vivitar

Zuiko Olympus

 


Minolta 250 




recent new comers:

pro-optic 

Opteka

Kelda 

300mm aperture: Super-Paragon/Ohnar/Cambron

for better optics and "made in Japan" brands,secondhand ref price usd 200 to 300

Those design with "spot mirror" on the back , will have better reliability. If the secondary mirror is located on top of front meniscus glass, this mirror ( and may have also an corrector element, is cemented together with transparent optical grade adhesive ( canadian balsam ?). Over time, or due to high temperature, this adhesive may dry up or delaminate.


with mangin mirror:

canon FD reflex 500mm

Tamron SP 500mm f/8 55BB and test review


 

nikon nikkor 500mm f/8 new

secondhand ref price usd 400 ~ 600

Minolta Reflex ( or Sony )


OTOH, the price of the real Mak scopes are very competitive.

Acuter Mak 60mm focal length 750mm : ~ USD 95 

Celestron C90 focal length 1250mm : USD 240 (HKD1880)

Bosma 102mm focal length 1200mm : USD 280, second hand ~ HKD1300 or lower

for Skywatcher SkyMax Maksutov telescopes:

90mm  focal length 1250 mm :  USD 220

102mm  focal length 1300 mm : USD 265

127mm  focal length 1500 mm : USD 435

180mm  focal length 2700 mm : USD 1200

CCDT67 0.67x telecompressor

or celestron 0.63x focal reducer or similar

Although Celestron C6 is a Schmidt Cassegrain, not Matsutov Cass, you can a get a hang of difference from this comparison:

what is still missing is the theoretical spot diagram of these CAT lenses.

Jim Chung  

dcview  

dcview 2

dcview 3

dchome archive 

dchome 反射鏡二營

dpreview on 300 cat

yashica board

advancedphototech.  

fotosaurier

mikeeckman

photo_net

celestron knowledge base

cajunastro review. 

xitek 

repair

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