*** It is slightly OT ***
Does Mewlon have coma issues due to the DK design? What is your evaluation comparing with 8" SCT or Mak?
Anat
Your Ideal Scope Collection?
hi anat,
Star images are better than other Cassegrains except when comparing to RCs.
However, at the edges, the stars get as bad as do accompanying Cassegrains likewise but not as bad as a SCT.
Also note that coma is easily corrected in this case, although the Mewlon optional reducer will
still introduce noticeable vignetting across an emulsion frame or modest-sized CCDs or similar chip sizes. Note that this
effect has nothing to do with optical quality of the Mewlon, but more like a trade-off.
Yes, in most literature, we are educated with the comatic side effects brought about by the Dall-Kirkham design, but
with reference to Takahashi's Mewlons, they offer slightly bigger sized prolate ellipsoid primaries (for example in the M210,
the primary's diameter is 220mm) and then it is masked, to prevent further aberrations.
Also, the Mewlons operate at a default f/12 focal ratio and with smaller central obstructions than SCTs, off axis coma is
less prevalent of an issue.
The design of the Mewlons pays heed to excellence in contrast, as avident in having a long baffle central tube at the
center of the primary AND also a shorter baffle tube from the secondary. And everywhere internally is matt blackened
to reduce internal reflections as much as possible (including the internals of the adapters), and thereby improving contrast.
From close inspection of previous Mewlon samples I've had, the long central tube is also internally baffled with knife-edge
ridges to further reduce internal reflections!
The Mewlons like commercial SCTs also employ primary mirror shifting for focusing, but focus shift is much lesser in them
than equivalent SCTs.Note that larger Mewlons, 250 and 300 use the secondary mirror for focusing (electronically).
Mewlons are light as well and the well made sturdy finder bracket enables the finderscope to be used as a carrying handle as well!
Next, the field of view is also sharper and flatter than a SCT but worse than the RC.
Also, have heard that the mirror surface accuracy is at least 1/20th wave and mirror coatings have higher
reflectivity (>96%).
With a smaller secondary obstruction than equivalent SCTs, more light energy is concentrated on the Airy Disk.
Further, some may not notice but the popular SCTs have primary and secondary ratios of f2 and f5 respectively (for f10 SCT).
But Mewlons utilise f3 primary and f4 secondary. Hence the resulting narrower cone of light emitting from the primary
onto the secondary is less suseptible to local turbulence and collimation is also made easier on the secondary due to its lower
multiplicative factor.
Not forgetting other "minor" advantages like the better stock 7X50 Takahashi finder!
Star images are better than other Cassegrains except when comparing to RCs.
However, at the edges, the stars get as bad as do accompanying Cassegrains likewise but not as bad as a SCT.
Also note that coma is easily corrected in this case, although the Mewlon optional reducer will
still introduce noticeable vignetting across an emulsion frame or modest-sized CCDs or similar chip sizes. Note that this
effect has nothing to do with optical quality of the Mewlon, but more like a trade-off.
Yes, in most literature, we are educated with the comatic side effects brought about by the Dall-Kirkham design, but
with reference to Takahashi's Mewlons, they offer slightly bigger sized prolate ellipsoid primaries (for example in the M210,
the primary's diameter is 220mm) and then it is masked, to prevent further aberrations.
Also, the Mewlons operate at a default f/12 focal ratio and with smaller central obstructions than SCTs, off axis coma is
less prevalent of an issue.
The design of the Mewlons pays heed to excellence in contrast, as avident in having a long baffle central tube at the
center of the primary AND also a shorter baffle tube from the secondary. And everywhere internally is matt blackened
to reduce internal reflections as much as possible (including the internals of the adapters), and thereby improving contrast.
From close inspection of previous Mewlon samples I've had, the long central tube is also internally baffled with knife-edge
ridges to further reduce internal reflections!
The Mewlons like commercial SCTs also employ primary mirror shifting for focusing, but focus shift is much lesser in them
than equivalent SCTs.Note that larger Mewlons, 250 and 300 use the secondary mirror for focusing (electronically).
Mewlons are light as well and the well made sturdy finder bracket enables the finderscope to be used as a carrying handle as well!
Next, the field of view is also sharper and flatter than a SCT but worse than the RC.
Also, have heard that the mirror surface accuracy is at least 1/20th wave and mirror coatings have higher
reflectivity (>96%).
With a smaller secondary obstruction than equivalent SCTs, more light energy is concentrated on the Airy Disk.
Further, some may not notice but the popular SCTs have primary and secondary ratios of f2 and f5 respectively (for f10 SCT).
But Mewlons utilise f3 primary and f4 secondary. Hence the resulting narrower cone of light emitting from the primary
onto the secondary is less suseptible to local turbulence and collimation is also made easier on the secondary due to its lower
multiplicative factor.
Not forgetting other "minor" advantages like the better stock 7X50 Takahashi finder!
-
HoO_Germany
- Vendor
- Posts: 107
- Joined: Sat Mar 04, 2006 8:41 pm
- Location: Heinsberg
- Contact:
Hi Remusrcj wrote:hi anat,
Star images are better than other Cassegrains except when comparing to RCs.
However, at the edges, the stars get as bad as do accompanying Cassegrains likewise but not as bad as a SCT.
Also note that coma is easily corrected in this case, although the Mewlon optional reducer will
still introduce noticeable vignetting across an emulsion frame or modest-sized CCDs or similar chip sizes. Note that this
effect has nothing to do with optical quality of the Mewlon, but more like a trade-off.
Yes, in most literature, we are educated with the comatic side effects brought about by the Dall-Kirkham design, but
with reference to Takahashi's Mewlons, they offer slightly bigger sized prolate ellipsoid primaries (for example in the M210,
the primary's diameter is 220mm) and then it is masked, to prevent further aberrations.
Also, the Mewlons operate at a default f/12 focal ratio and with smaller central obstructions than SCTs, off axis coma is
less prevalent of an issue.
The design of the Mewlons pays heed to excellence in contrast, as avident in having a long baffle central tube at the
center of the primary AND also a shorter baffle tube from the secondary. And everywhere internally is matt blackened
to reduce internal reflections as much as possible (including the internals of the adapters), and thereby improving contrast.
From close inspection of previous Mewlon samples I've had, the long central tube is also internally baffled with knife-edge
ridges to further reduce internal reflections!
The Mewlons like commercial SCTs also employ primary mirror shifting for focusing, but focus shift is much lesser in them
than equivalent SCTs.Note that larger Mewlons, 250 and 300 use the secondary mirror for focusing (electronically).
Mewlons are light as well and the well made sturdy finder bracket enables the finderscope to be used as a carrying handle as well!
Next, the field of view is also sharper and flatter than a SCT but worse than the RC.
Also, have heard that the mirror surface accuracy is at least 1/20th wave and mirror coatings have higher
reflectivity (>96%).
With a smaller secondary obstruction than equivalent SCTs, more light energy is concentrated on the Airy Disk.
Further, some may not notice but the popular SCTs have primary and secondary ratios of f2 and f5 respectively (for f10 SCT).
But Mewlons utilise f3 primary and f4 secondary. Hence the resulting narrower cone of light emitting from the primary
onto the secondary is less suseptible to local turbulence and collimation is also made easier on the secondary due to its lower
multiplicative factor.
Not forgetting other "minor" advantages like the better stock 7X50 Takahashi finder!
it is trueth, the DK-design brings the coma at the edges of the optics. And it is trueth, they work against with a bigger apperture mirror where it is masked down on a smaller diameter. But it is also trueth, the customer pays for a big size mirror and he use only a smaller one. This is a big point for me to think about to buy a really good made optic as a Mak-Cassegrain. There can be also made a coma correction, and the whole size of the main mirror can be used as aperture(where is edged by the meniscus lens).
So in direct comparance to a MC or a SC (and when I mean SC, I don´t mean the "cheap" systems made by Celestron or MEADE) the DK have only a real advantage when you use the corrector and visual by using really expensive acessories. It is a really good foto-machine, but there are other scopes also good, see please the Mak-Newtons.
I am shure, a Tak Mewlon can be good compared with the high-end telescopes from russia in its optical quality.
I agree, when you set the Mewlon over the "cheap" SC made in USA
Clear skys
Robert
House of Optics Germany
Intes-Micro/Ylena/STF/LOMO/Synta
www.hoo-germany.de
e-mail: info(@)hoo-germany.de
Tel.: +49 2452 976144
Intes-Micro/Ylena/STF/LOMO/Synta
www.hoo-germany.de
e-mail: info(@)hoo-germany.de
Tel.: +49 2452 976144
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Canopus Lim
- Posts: 1144
- Joined: Fri Jun 03, 2005 12:46 pm
- Location: Macpherson
I have just re-read an test report on 200mm-class cassegrain telescopes (CN-212, Mewlon 210, C8, LX90, VC200L, VMC200L) published in Temmon guide (Sept 2005 issue). Even though it is in Japanese, 80% of pages are figures of ronchi tests, spot diagrams, internal design, baffle tests, and simulated planetary images. I think it is very useful for deciding your DREAM cassegrain.
From spot diagrams, the following shows how wide the spots are for all scopes. (I am not sure if it is called "coma")
VC200L < CN212 <<< C8 and LX90 < Mewlon 210 < VMC200L
Note that VC200L has a built-in flattener. On-axis stars are pin points in VC200L, CN212, and Mewlon 210. VC200L does not give high contrast because of large obstruction (42.5%). In summary, CN-212 seems to be the best in rendering contrasty planetary views
Anat
From spot diagrams, the following shows how wide the spots are for all scopes. (I am not sure if it is called "coma")
VC200L < CN212 <<< C8 and LX90 < Mewlon 210 < VMC200L
Note that VC200L has a built-in flattener. On-axis stars are pin points in VC200L, CN212, and Mewlon 210. VC200L does not give high contrast because of large obstruction (42.5%). In summary, CN-212 seems to be the best in rendering contrasty planetary views
Anat
Last edited by anat on Sat Mar 25, 2006 8:49 pm, edited 2 times in total.