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:
Spiratone 500 f/8 Minitel and 800
Tokina
Sigma (Accura)
Vivitar
Zuiko Olympus
Minolta 250 f5.6, using mangin mirrors
recent new comers:
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:
Tamron SP 500mm f/8 55BB and test review
nikon nikkor 500mm f/8 new , ver C mir
secondhand ref price usd 400 ~ 600
Minolta Reflex ( or Sony )
Ttartisan 250mm f5.6 (minolta clone) review
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.
cajunastro review.
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