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New AT 6" and 8" Classical Cassegrain

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#1101 Thandal

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Posted 22 September 2020 - 02:15 PM

<snip>
It just so happens that I'm conducting a study on the compressive stress relaxation of O-rings right now ...

 

 

Of course you are!  lol.gif

 

I love how there are CNers involved with all sorts of esoteric work that has astro-applicability in unexpected ways.  waytogo.gif


Edited by Thandal, 22 September 2020 - 02:15 PM.

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#1102 quilty

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Posted 23 September 2020 - 04:42 AM

ok, another post, but for general concern. You might call it nerdsstuff and esotheric but it matters. At least when you want to have a scope that's worth a look through. The cc is an easy-access-device where you could ask what's the point of doing half a turn of fixing ring in order to fix the problem. But what about more complicated scopes? I just don't want to open a Mak or an SC scope or maybe a big refracor the lens of which is fixed by a plastic protected srew ring before using just to make sure of that everything is ok. I don't want to fix the negligences by the producer, but I'm doing. And research on non-drooping rubbers does as well, as long as the clients do it.


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#1103 RichA

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Posted 26 September 2020 - 12:13 AM

I've found every post helpful in some way, truly have. I like to see both sides. I like folks tearing into scopes physically and reporting what they find and how they interpret the results. Discrepancies. Inaccuracies. It's good to knock the gloss off something when you have one in hand - I find it commendable and of good value.  

 

Back to the Moon =)

No one is well-served by reviews that gloss-over or lie by omission about specifics on complex telescopes.  Compound scopes require more "work" than refractors and the like, but some people may not know that. 



#1104 Peter Besenbruch

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Posted 26 September 2020 - 02:06 AM

The cc is an easy-access-device where you could ask what's the point of doing half a turn of fixing ring in order to fix the problem. But what about more complicated scopes? I just don't want to open a Mak or an SC scope or maybe a big refracor the lens of which is fixed by a plastic protected srew ring before using just to make sure of that everything is ok. I don't want to fix the negligences by the producer, but I'm doing. And research on non-drooping rubbers does as well, as long as the clients do it.

I haven't had an SCT open, but the 180mm Mak is a different story. I had to remove the dovetail in order to install Reflectix. Unscrewing the screws is easy, but then you have to collect the loose nuts floating around on the inside. I removed the corrector to reach in and get them. The dovetail helps hold the ends of the insulation in place. Putting the nuts back, along with the tube reinforcements, requires a hand inside the tube, holding things in place. With the dovetail back in place, installing the corrector was easy. Even better, the collimation was still true.

 

I didn't snug down the primary, because I haven't noticed any mirror flop, yet. If I ever do, I'll just snug things down. Things just come loose, sometimes.



#1105 Thandal

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Posted 26 September 2020 - 08:36 AM

No one is well-served by reviews that gloss-over or lie by omission about specifics on complex telescopes.  Compound scopes require more "work" than refractors and the like, but some people may not know that. 

 

No need to go this far off-topic.  No one here is lying, (that I can tell) by omission or otherwise.

 

This forum is for Cats and Casses: by definition not "simple" telescopes.  (Although a classic cassegrain is pretty simple if we're only talking about operating, not making, one. lol.gif )

 

Even a basic newtonian takes more "work" (e.g. collimation) than a galillean refractor.

Here on CN, outside the "Beginners" forum, that's sort of a given.


Edited by Thandal, 27 September 2020 - 12:06 AM.

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#1106 Terra Nova

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Posted 26 September 2020 - 06:36 PM

Reading all of this I think the absolute minimum size of one of these to make it worthwhile is the 8”. The 6” seems severely compromised. Too bad, 6” Newtonians have always been a very well thought out mainstay in amateur astronomy, especially prior to the coming of SCTs (the C8) and the Dobsonian revolution. But the poor 6” classical cass of the current incarnation seems very poorly designed.



#1107 Thandal

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Posted 27 September 2020 - 12:00 AM

@Terra Nova; Don't know why you would say the 6" is poorly designed.  It's the exact same design as the 8".

 

But it is f/12, (same as the 8") so... high magnification, narrow FOV, for lunar and planetary observing.  That 6" Newtonian you reference is probably f/8 or f/6, or maybe faster.  Different capabilities for different purposes.

 

Sure, the 8" has the advantage(s) of the larger aperture, at 50% greater weight and almost 100% greater cost.

 

I was impressed with the 6", but jumped on a great deal to trade up to the 8".  So have experience using both. waytogo.gif


Edited by Thandal, 27 September 2020 - 12:05 AM.


#1108 teechinghong

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Posted 27 September 2020 - 01:57 AM

@Terra Nova; Don't know why you would say the 6" is poorly designed.  It's the exact same design as the 8".

 

But it is f/12, (same as the 8") so... high magnification, narrow FOV, for lunar and planetary observing.  That 6" Newtonian you reference is probably f/8 or f/6, or maybe faster.  Different capabilities for different purposes.

 

Sure, the 8" has the advantage(s) of the larger aperture, at 50% greater weight and almost 100% greater cost.

 

I was impressed with the 6", but jumped on a great deal to trade up to the 8".  So have experience using both. waytogo.gif

Hi, have you done any planetary imaging with the 6" CC previously? Or your current 8" CC?



#1109 Thandal

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Posted 27 September 2020 - 07:56 AM

Hi, have you done any planetary imaging with the 6" CC previously? Or your current 8" CC?

 

I don't *do* imaging.  I'm strictly a visual observer.  grin.gif 


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#1110 Terra Nova

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Posted 27 September 2020 - 10:16 PM

An old school 6” F8 to F12 Newtonian designed for visual use rather than designed to illuminate a large ccd or cmos chip, and optimized for planetary viewing would have had a much smaller central obstruction and been capable of utilizing all of the primary surface area beyond the shadow of the secondary. A truly classical CC design would have again utilized a smaller secondary and a shallower primary of a longer focal length and hence producing a final focal ratio closer to F20 than F12 and hence again utilized much more of the total aperture. These F12 CCs IMO just do not seem to scale down well. While the 8” seems to do a good job and has an effective aperture of better than 7”, the 6” CC is so compromised by the scale down that it is working more like a 4” or 5” scope and one would do far better with a refractor or a 6” F8 Newtonian it seems to me. It just seems to be neither fish nor fowl. I think in dropping down to 6” they should have changed other factors as well and created a better scope. Maybe something on the order of an F18. There’s a reason why the older makers of classical Cassegrains back in the day pretty much stayed away from anything smaller than an 8” and the few 6” CCs that were produced were on the order of F20 or higher. 


Edited by Terra Nova, 27 September 2020 - 10:38 PM.

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#1111 t.mihai147

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Posted 28 September 2020 - 12:04 AM

It just happens that last evening I run a comparison between GSO 6" CC and TAK FC100DC.

I was not able to see anything in the CC that TAK was not able to show with a better (more contrast) view that CC.

I compared Jupiter, Saturn, Mars and Moon at 100-180x in both instruments.

Due to poor transparency, I was not able to go above 180 with any image improvement but the  TAK was more crisp, even if the image was dimmer. On the moon the difference was not big, but on Saturn and Jupiter the view had much more contrast in the TAK. Mars was showing just a faint darker area in the central zone.

In theory, the resolution and contrast of CC would be 6"-35% = 3.9" while the TAK was 4" but the visual difference was much higher.

I have also compared one high-power star de-focused image in both and while the TAK was very close to text book image, the CC was not able to show symmetrical rings or single dot with one ring - it was more like one focus point spread over multiple adjacent zones (due to unstable air cells maybe?)

CC was well colilmated but I think I was also hinting some air turbulence caused by the central mirror/shield?? 

Anyhow, TAK is much more expensive but again I am convinced the 4" APO is better that 6" CC except maybe some DSO (which I cannot see in Bortle 8 sky from my location)


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#1112 Thandal

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Posted 28 September 2020 - 04:06 AM

<snip>

TAK was more crisp, even if the image was dimmer. On the moon the difference was not big, but on Saturn and Jupiter the view had much more contrast in the TAK.

 

<snip>

 

Anyhow, TAK is much more expensive but again I am convinced the 4" APO is better than 6" CC except maybe some DSO (which I cannot see in Bortle 8 sky from my location) ...

 

Well, you've described the major difference between a near-top-of-the-line (f/7.4) apo refractor and almost any regular ("built for the cost-conscious") multi-mirror instrument.

 

And if you are able to ignore the price difference of around US$1,200... let's say your criteria are going to be very different from mine!  lol.gif

 

 

EDIT: Oh, and an f/12 classic cassegrain is a poor choice for most DSOs.


Edited by Thandal, 28 September 2020 - 04:07 AM.


#1113 quilty

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Posted 28 September 2020 - 06:33 AM

@t.mihai147: Before your post I would have stated the actual cc6 could compete with a good 4 inch f/7 ed or apo refractor beeing heavier and providing reduced contrast but more light and beeing much cheaper. So it is impossible to do a choice unless it is totally an individual one. 

@Terra Nova. The cc6 design might be a bit fishy but I still think a slight mirror adaption could reduce obstruction significantly without exceeding an f/15 ratio. A smaller secondary would then need the collimation ringmark to be removed, but I think we could deal with that. Yes, and of course the cc6 should be less bulky than the 8 inch and then it makes sense, for some people. And I would replace the double speed friction focusser by for example a Bresser 2.5 inch Hexafoc, wich weighs less than the Crayford. I never touch the low speed button, I think dual speed is an issue at low focal lengths. But in vertical position the Crayford might slip when heavy stuff is hanging. The double speed Crayfords are great at Dobsonians where they're always level. 

What is DSO?


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#1114 Thandal

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Posted 28 September 2020 - 08:06 AM

<snip>

What is DSO?

 

 

DSO: "Deep Sky Object".  Nebulae, galaxies, etc.  While many of the more familiar DSOs are fairly bright (e.g. most of the Messier Catalog) the vast majority are much more faint, have low-contrast, and/or are extended rather than compact targets.

 

All of these factors mean f/8 or faster instruments with their wider FOV for a given aperture are better suited for observing (and especially imaging) DSOs than an f/12 or slower one.

 

EDIT:  For compact targets, (e.g. planets, multi-star systems, etc.) higher focal ratios allow higher magnifications (for a given eyepiece) without needing the various "correcting" accessories (e.g. for coma, field curvature, etc.) or even a Barlow that faster ones require to provide an acceptable image, especially in the off-axis portions of the field.


Edited by Thandal, 28 September 2020 - 08:17 AM.


#1115 Terra Nova

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Posted 28 September 2020 - 09:40 AM

Well, This has all confirmed in my mind that I made the right choice in not getting the 6”. I’ll stick with my 4” apos. The 8” would be a better choice, but I think I’ll continue to forgo the experience. I love sharp, crisp, high contrast views, dainty defraction rings, tight Airy disks, and pinpoint stars. And I really hate wasting my time with collimating and cool down issues. That’s why I could never find an SCT that suited me and I have had several (10”, two 8”, 6”, and 5”).


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#1116 Bomber Bob

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Posted 28 September 2020 - 12:50 PM

Well, This has all confirmed in my mind that I made the right choice in not getting the 6”. I’ll stick with my 4” apos. The 8” would be a better choice, but I think I’ll continue to forgo the experience. I love sharp, crisp, high contrast views, dainty defraction rings, tight Airy disks, and pinpoint stars. And I really hate wasting my time with collimating and cool down issues. That’s why I could never find an SCT that suited me and I have had several (10”, two 8”, 6”, and 5”).

Yeah, I tried that very well-made Vixen VMC200L to break into the 8" Cass Class.  Then, I got an 8" F6 Newt, and the Vixen got sold...  I really like the idea of an 8" or larger CC, but the execution matters...

 

BIF:  Old & heavy as it is, my antique Tinsley 6" F20 Cass performs very well.  I attribute that to some features:  All-metal construction (aluminum tube, brass hardware), 2 x 4-rod secondary spider, no-clip primary mirror cell (1mm-deep rim, so even pressure at the edge) + cork-cushioned 3 wide-tongue tabs on the back.  Despite its age, it holds collimation between sessions.  But, at 26# (12 of that in the machined brass primary mirror cell + focuser assembly), it's overkill for a small Cass...


Edited by Bomber Bob, 28 September 2020 - 06:47 PM.

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#1117 quilty

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Posted 29 September 2020 - 04:48 AM

Once again. Consider the Takahashi mewlons, for instance the 7 inch. They might give an impression to where a perfect cc design can be pushed. When it is as perfectly designed by a look through it as it is by a look at it, that's my thing, except the price. Looks like a jet propulsion engine from the sixties, except the fix finderscope should bear a rotating diagonal. The only secondary I know held by three spikes, that looks cool.

@t.mihai147: When you ever are in the position to compare a mewlon 180 to a 4 or 5 inch refractor that would be interesting. The mewlon 180 seems to be less expensive than the TAKFC100DC. 

I'm still considering to get a 8" SCT, a Meade ACF or a C8 but like Terra, till now, I never had a sharp view through any SCT, that kicked my ****. The 180 mewlon seems to perform level to the Skywatcher Mak 180, oh, again a Mak. At 2.7 m focal length, the Mak. The mewlon is about 2,1 m.


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#1118 quilty

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Posted 29 September 2020 - 05:22 AM

EXPLORE SCIENTIFIC MN-152/740 6 "OPTICAL TUBUS Maksutov-Newton

 

Another thing. Was a hot recommending by two connoisseurs independently. Would perform good apo level while short focussing. I had one if there was not the weight and the eyepiece at the wrong position like all newtonians have. And long acclimatisation time.



#1119 Thandal

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Posted 29 September 2020 - 08:12 AM

Once again. Consider the Takahashi mewlons, for instance the 7 inch.

<snip>

The 180 mewlon seems to perform level to the Skywatcher Mak 180, oh, again a Mak. At 2.7 m focal length, the Mak. The mewlon is about 2,1 m.

 

The Mewlon 210 is a wonderful instrument. 

And at US$3,000, it ought to be!

 

Why are people comparing the performance of a GSO CC to that of ones costing 2x to 5x?

Talk about another sub-$1K 'scope that does as well for lunar and planetary (visual) observing and maybe I'd be interested.

 

As for the Explore Scientific MN-152 Maksutov-Newton (the "David H. Levy Comet Hunter" Telescope):

 

if there was not the weight ... And long acclimatisation time.

 

And the dewing.  And the 5" aperture.  And at f/4.8 this instrument was designed for low-power/wide field views to sweep the sky, as it's nickname implies.  (Also, it is no longer being manufactured.)


Edited by Thandal, 29 September 2020 - 09:01 AM.


#1120 Terra Nova

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Posted 29 September 2020 - 09:19 AM

Once again. Consider the Takahashi mewlons, for instance the 7 inch. They might give an impression to where a perfect cc design can be pushed. When it is as perfectly designed by a look through it as it is by a look at it, that's my thing, except the price. Looks like a jet propulsion engine from the sixties, except the fix finderscope should bear a rotating diagonal. The only secondary I know held by three spikes, that looks cool.

@t.mihai147: When you ever are in the position to compare a mewlon 180 to a 4 or 5 inch refractor that would be interesting. The mewlon 180 seems to be less expensive than the TAKFC100DC. 

I'm still considering to get a 8" SCT, a Meade ACF or a C8 but like Terra, till now, I never had a sharp view through any SCT, that kicked my ****. The 180 mewlon seems to perform level to the Skywatcher Mak 180, oh, again a Mak. At 2.7 m focal length, the Mak. The mewlon is about 2,1 m.

I have had better luck with the Maks I’ve owned, but only one of them has succeeded in putting a smile on my face every time I have looked through it or at it! My Questar has satisfied the criteria I listed in my earlier post in this thread (#1115) 100%. The views of that little 3.5” Mak closely match those of my Takahashi FC-76 3” Fluorite apo refractor. ;)

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#1121 Thandal

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Posted 29 September 2020 - 09:35 AM

I have had better luck with the Maks I’ve owned, but only one of them has succeeded in putting a smile on my face every time I have looked through it or at it! My Questar has satisfied the criteria I listed in my earlier post in this thread (#1115) 100%. The views of that little 3.5” Mak closely match those of my Takahashi FC-76 3” Fluorite apo refractor. wink.gif

 

 

Yes.  A new Questar 3.5" with Broadband Coatings = US$5.5K

And a 3" Tak FC-76 apo is still US$1.8K.

 

Again, why are we talking about these completely different classes (in terms of design, quality, and especially cost) of instruments in the New AT 6" and 8" CC thread?

 

If one prefers the (relatively) smaller, but (perhaps) sharper (better?) views through those, great!  Glad you have them.  My location, (very high humidity most of the year) my observing preferences (high mag./lunar and planetary), and my wallet, (including needing a better mount) led me to a different decision.  Which led me to this forum!  waytogo.gif
 


Edited by Thandal, 29 September 2020 - 09:37 AM.


#1122 Terra Nova

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Posted 29 September 2020 - 09:48 AM

Yes.  A new Questar 3.5" with Broadband Coatings = US$5.5K

And a 3" Tak FC-76 apo is still US$1.8K.

 

Again, why are we talking about these completely different classes (in terms of design, quality, and especially cost) of instruments in the New AT 6" and 8" CC thread?

 

If one prefers the (relatively) smaller, but (perhaps) sharper (better?) views through those, great!  Glad you have them.  My location, (very high humidity most of the year) my observing preferences (high mag./lunar and planetary), and my wallet, (including needing a better mount) led me to a different decision.  Which led me to this forum!  waytogo.gif
 

Well in that case, I would imagine that for $100 more than the 6” CC, either the ASTRO-TECH AT102ED 4" F/7 ED REFRACTOR OTA, or the Orion 150mm Mak (which uses an oversized primary that produces an effective aperture MATCHING it’s advertised aperture), would be a far more trouble free scope producing better images as compared to the GSO 6” CC. I don’t have either of them and happily don’t have the 6” F12 CC, but from reading all 45 pages of this thread and reviews of the two afore mentioned alternatives, I’m pretty sure that my surmise is correct.


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#1123 Thandal

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Posted 29 September 2020 - 10:15 AM

Well in that case, I would imagine that for $100 more than the 6” CC, either the ASTRO-TECH AT102ED 4" F/7 ED REFRACTOR OTA, or the Orion 150mm Mak (which uses an oversized primary that produces an effective aperture MATCHING it’s advertised aperture), would be a far more trouble free scope producing better images as compared to the GSO 6” CC.

 

I don't understand your "far more trouble free" statement?  I've had no trouble at all with mine.

 

And I don't think one should compare a 102mm (4") f/7 refractor to a 6" (or greater) f/12 instrument of any design.  And a Mak (or any other catadioptric 'scope) has it's own issues.

 

The criteria by which one would judge their relative merits would either be:

 

(a) totally different from each other (e.g. light gathering/limiting magnitude vs. contrast vs. highest useful magnification vs. optical aberrations vs. weight vs. acclimation/dewing vs. mirror flop, vs. ... ) or;

 

(b) totally subjective, as in "I just prefer this other one when I look through the eyepiece."  Which is totally fine!  But not really what this thread is about.  wink.gif


Edited by Thandal, 29 September 2020 - 10:16 AM.


#1124 Terra Nova

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Posted 29 September 2020 - 11:08 AM

I don't understand your "far more trouble free" statement?  I've had no trouble at all with mine.

 

To name a few ‘problems’ mentioned throughout this thread.

  1. Need of extension rings when using visually as opposed to off the shelf.
  2. Possible need to change extension rings when using eyepieces of varying design and size (eg. Naglers to orthoscopics, going from 2” eyepieces to 1.25” eyepieces, etc.)
  3. Acknowledged need for collimation when the extension rings are changed out.
  4. Need to be collimated when focuser is rotated because it is not square.

I’ve never experienced these problems with any refractor, Mak, or SCT I’ve owned.

 

I don't think one should compare a 102mm (4") f/7 refractor.
 

Why not when it has been well proven that the scope is operating at an effective aperture in that range when one considers both the central obstruction and the size of the baffled light cone.

 

And I don't think one should compare a 6" (or greater) f/12 instrument of any design.  And a Mak (or any other catadioptric 'scope) has it's own issues.

 

This is perhaps your most baffling statement, (excuse the pun). lol.gif Why the hell not? You’re saying that it’s not fair to compare a 6” F12 cassegrain-type telescope to another 6” F12 cassegrain-type telescope?

foreheadslap.gif

Again, the Orion 150mm Mak uses an oversized primary (~162mm) that actually produces an effective aperture of 150mm aperture when considering the size of the baffled light cone. It would seem to me that it would be a very fair comparison.

 

totally subjective, as in "I just prefer this other one when I look through the eyepiece."

 

One who can read and who values expert opinion (acknowledging that such a person might seem rare today), does not need to drive a Yugo to know they would not want to own one.

 

But not really what this thread is about.

 

I’m sorry, I didn’t notice the word ‘Moderator’ below your name. I shall have to look more carefully in the future. wink.gif

 

However, I am not the first person to make these comparison in the 45 pages of this thread. I think it quite important to make such comparisons. They may just prevent someone from buying something they might be dissatisfied with. I am speaking particularly of the 6” model here as most seem to be happy with the 8”. I for one had seriously considered being an early adopter of the 6” when the two were first announced, nearly a year before they ever hit market. One they did, and more information came out, I am pretty sure that given the telescopes that I do have and the larger number that I have had over the years, I would have been a pretty disappointed customer. Is it not fair to save another from possible disappointment? At any rate, I am gleefully happy that you have been so satisfied with your two. 


Edited by Terra Nova, 29 September 2020 - 11:12 AM.

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#1125 Thandal

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Posted 29 September 2020 - 12:00 PM

@Terra Nova;  It just seems like we're way off the track in discussing the maks and apos of widely different sizes and capabilities.  A few comments about why other designs might be more suitable for a given user/purpose/pocket, sure.  But beyond that?

 

And as for the (nominal) 5" Mak-Cass being comparable, that's why I listed some of the other decision criteria (weight, dewing, mirror flop...)

 

EDIT: Fixed typos, and I noticed you dropped half of my line about the subjective criteria, i.e. "Which is totally fine!"  smile.gif


Edited by Thandal, 29 September 2020 - 12:04 PM.



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