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Old 09-07-2019, 04:18 PM
bluesilver (Peter)
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Viewing Jupiter's moons with 14 or 16 inch Dobsonian

Hi, i have been viewing Jupiter with a Skywatcher 10 inch dobsonian.
When i am viewing Jupiter, the best i can get of its moons are sort of like small to medium stars.
Nice and sharp in focus, not blury images.

I was interested to know if i were to go to a 14 inch or 16 inch dobsonian, will the lager aperture allow me to see the moons as tiny round dots rather than stars?
Hope that makes sense.
I was figuring that the larger aperture would allow more light to enter, therefore letting in more detail.
Any information would be appreciated.
Thanks.
Peter.
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Old 09-07-2019, 05:23 PM
astro744
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You need good seeing and good collimation. First time I saw them was with an 8" f9 Newtonian but I have since seen a disc in a 4" refractor. Ganymede is relatively easy now for me. A larger aperture will give you more light but you don't need more light as the moons are bright enough, in fact higher power, less brightness but good seeing and collimation is best.
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Old 10-07-2019, 04:21 PM
bluesilver (Peter)
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Thanks for the reply, appreciated.
I could be thinking wrong here, but maybe an aperture mask on the 16 inch dobsonian could help dull out the brightness while still having the large aperture?
I could be thinking it all wrong here though.
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Old 10-07-2019, 06:03 PM
Wavytone (Nick)
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OP what you need is magnification lots of it, and premium optics with high strehl above 0.95... and there is no substitute for looooong focal length, not just aperture. I have the perfect planet killer, with a focal length of 3100mm... resolves Jupiters moons nicely, on a good night its like a Voyager photo.

If in Sydney PM me.
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Old 10-07-2019, 09:48 PM
astro744
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Quote:
Originally Posted by bluesilver View Post
Thanks for the reply, appreciated.
I could be thinking wrong here, but maybe an aperture mask on the 16 inch dobsonian could help dull out the brightness while still having the large aperture?
I could be thinking it all wrong here though.
Why buy a bigger 'scope only to reduce its aperture? Once you put an aperture mask on that is then your aperture. The focal length remains the same but focal ratio changes. E.g. A 16" f4.5 would become a 4.5" f16.

The exit pupil will be reduced for the same magnification with an aperture mask in place so images will be less bright and this will help but the same can be achieved with your 10" at full aperture given the right magnification. You lose resolution too with your aperture mask but given a good mirror, good seeing and good collimation you should be able to see Ganymede's disk easily at 4.5" but it will resolve more easily at 10" or 16" provided conditions are right.
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Old 10-07-2019, 10:34 PM
raymo
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Being too bright is not the problem, bright is good; your 16" will easily show
the moons as discs, given enough magnification. I'm guessing that you don't
have a suitable high power eyepiece. The 10mm that comes with many scopes
is not enough; you need one around 6 or 7mm giving 300x and 257x respectively. You could also have
4mm [450x] for when the seeing is really good.
raymo

Last edited by raymo; 10-07-2019 at 10:37 PM. Reason: more text
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Old 11-07-2019, 04:26 PM
bluesilver (Peter)
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Thanks for the replies and information.
Sorry if this is another basic question, still trying to get my head around all the right things.

So for planetary viewing, i am better off going for a longer focal length rather than larger aperture?

I thought the larger aperture would give me more detail due to the more light it lets in.

So instead of looking at say the 10 inch dobsonian i might be better of going for something like the Cassegrain Telescopes,
Something like the Black Diamond 180/2700 Mak-Cassegrain, where it has a focal length of 2700 mm ?
Or this might be a tad over the top, but the Celestron C14
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Old 11-07-2019, 06:27 PM
Wavytone (Nick)
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Quote:
Originally Posted by bluesilver View Post
I thought the larger aperture would give me more detail due to the more light it lets in.
In theory resolution is a function of aperture. But turbulence in the atmosphere limits that, most of the time.

The second challenge is the quality of the optics. There is a distinct difference in what you will see though a scope that has what I can only describe as "ordinary" (ie budget grade) optics vs one that has premium optics, certified and tested to meet a very high standard.

The third challenge is how you propose to achieve high magnification. Using a fast (ie short) newtonian and insanely short eyepieces with 10 elements is not the optimum solution.

It is far better to start with a scope with a long focal length and use a more modest eyepiece with as few elements as possible. Ideally, 1 element (they exist but have a small field of view, around 10 degrees).

Quote:
instead of looking at say the 10 inch dobsonian i might be better of going for something like the Cassegrain Telescopes, Something like the Black Diamond 180/2700 Mak-Cassegrain, where it has a focal length of 2700 mm ?
That would be a good start for a budget portable scope, optically they are consistently very good and better than the average 8" SCT. They're a very nice size/weight too for a "quick look". The sheer weight of larger SCT's like C9.25 and C11 (and especially the CPC versions) or 12" LX200 makes them tiresome and many give up as they just can't face packing that up in the dark and in the cold. The Meade 14"/16" and C14 are not one-man portable scopes.

In this respect at 13kg the MK91 is a very good solution, as is a C9.25 on a GEM mount. (not the CPC version).

Quote:
Or this might be a tad over the top, but the Celestron C14
I wouldn't be interested in a C14 even if you gave it to me. My next choice would be something like a lightweight truss 14" f/7 dobsonian with a Zambuto mirror, small secondary, and properly baffled. The snag is the wait time for the mirror as it would have to be made to order, and Zambuto have a very long backlog.

If money is no limit there is a 16" f/15 Maksutov for sale with 1/8 wave optics that will easily out-resolve anything from Celestron/Meade. But that is a tad under 50,000 euros.

FWIW on the planets my MK91 out-resolves all the 10-11" scopes it has been compared with, in side-by-side comparisons on the observing field. C9.25 and Meade 10" aren't even close. In theory something like a 12" f/8 newtonian with a perfect mirror and small secondary should be better, but it would need a Zambuto mirror and will be a fair bit larger. That's in theory, practice is another thing and I have yet to see one.

The one scope that I do know was better is the 16" f/7 newtonian at Crago (Mt Bowen). But that had a very high quality Suchting mirror and the scope is a big beast, with a full tube and permanently mounted in an observatory.

Last edited by Wavytone; 11-07-2019 at 06:51 PM.
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Old 11-07-2019, 07:34 PM
ausastronomer (John Bambury)
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Quote:
Originally Posted by Wavytone View Post
In theory resolution is a function of aperture. But turbulence in the atmosphere limits that, most of the time.

I wouldn't be interested in a C14 even if you gave it to me.
No C14's or big Meade's for me either. They wouldn't even be allowed onto the same observing field as my 14"/F4.5 Zambuto powered SDM.

Quote:
My next choice would be something like a lightweight truss 14" f/7 dobsonian with a Zambuto mirror, small secondary, and properly baffled.
It doesn't need to be F7. My 14"/F4.5 Zambuto powered SDM gives planetary views the equal of anything I have looked through over the past 50 years, which includes Newtonians to 36", Refractors to 15" and Classical Cassegrains to 20". Very closely followed by James Pierce's 16"/F4 Lockwood powered SDM. Rod Berry's 20"/F5 SDM (The Mary Rose) with OMI Mirror is right up there as well. Unfortunately with a 20" x 2" thick mirror it took quite a while and favourable thermal conditions to properly stabilise and deliver its best views. Also with 20" of aperture seeing needed to be good as well. When thermals and seeing conditions co operated, the planetary views of Jupiter and Saturn through the Mary Rose were exceptional.

Quote:
FWIW on the planets my MK91 out-resolves all the 10-11" scopes it has been compared with, in side-by-side comparisons on the observing field.
You haven't compared it to my 10"/F5.3 Suchting powered SDM in that case.

I have time and money, if you care to try it on ?

The days of extremely long focal length Newtonians are long gone. The main criteria to getting good planetary performance from a newtonian are:-

1) Premium Optics (primary, secondary and eyepiece). The optical system
is only as good as its weakest link.
2) Get the secondary obstruction under 20%
3) A thin mirror, which cools and stabilises quickly.
4) Good Collimation.
5) Good Baffling.
5) A good solid telescope structure (motorised tracking helps enormously)

The two advantages to a long focal length Newtonian over a short one is that the long focal length one will have a greater depth of focus, which makes things a bit easier for the observer. It doesn't improve the image quality. Similarly, the diffraction limited field in a long focal length Newtonian is larger than in a short focal length one, due to coma, but with a tracking scope and the image centred in the FOV, that is irrelevant in any case.

The disadvantages of a slow focal length Newtonian are numerous.

Years ago the number of medium to larger aperture high quality Newtonian mirrors in the F4 to F5 class was pretty small, hence people favoured slower F-ratios, as the mirrors were generally better, than the faster mirrors. That doesn't apply these days. Mark Suchting actually independently tested my 14"/F4.5 Zambuto mirror many years ago, at about 1/40th wave.

A well tuned high quality newtonian in the 12" to 20" class between F4 and F6 will give planetary views the equal of anything money can buy.

Cheers
John B
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  #10  
Old 11-07-2019, 07:51 PM
ausastronomer (John Bambury)
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Hi Peter,

A premium 14" to 16" Newtonian will give superior planetary views to your 10" Skywatcher, when thermal and seeing conditions are favourable.

With a mass produced 14" to 14" dob the optical quality will vary. One of the guys who goes to the Pony Club, has or had, a 14" Skywatcher that give exceptional lunar planetary views. Unfortunately most mass produced mirrors won't be at that level.

Cheers
John B
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  #11  
Old 11-07-2019, 08:23 PM
Wavytone (Nick)
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John there are a few reasons I like f/7:

- f/7 maximises the useful range of magnifications possible from eyepieces to about 10:1 between highest and lowest, without using Barlows. Most f/5 scopes are limited to a much lesser range and with f/4 scopes its more like 3:1. Similarly at long f/ratios the range is limited - the lowest useful power of the MK91 is about 100X and it is useful past 600, but that's still only a 6:1 range.

FWIW The Crago 16" provides a perfect example of this.

- f/7 isn't going to challenge low-glass eyepieces selected to give optimal planetary views - I am not a fan of ultrawides, nor eyepieces with far too many elements (Al Nagler I am looking at you). I'm sure I could design the ultimate 3mm 200 degree fishbowl eyepiece the size of a housebrick with 20 elements - and it will be the ONLY eyepiece you'd ever need, but...the image quality won't be great.

- the secondary obstruction is small enough to be neglible even allowing for a slightly oversized secondary to minimise vignetting off-axis at low power. At f/5 this isn't the case and forget f/4 (I've been there done that). Conversely going to f/8 or f/9 doesn't achieve much either (have seen some really long old-school newtonians).

- at f/5 (as indicated in your post in another thread) the compromise between backfocus and secondary mirror obstruction becomes a real issue. At f/7 it isn't an issue, ie. plenty of backfocus can be had without a whopping secondary that's going to hurt image quality.

- in a lightweight 14" f/7 dob the eyepiece height suits me just fine. Shorter dobs are frankly a pain in the back, and pointless as I end up putting a stool under them to raise the eyepiece up.

- the cost of a few inches longer struts is nothing...

And that's aside from the issues with figuring the mirror precisely.

The last issue I have with all newtonians is diffraction spikes from spider vanes. Sure, most of you think it doesn't matter. But actually it does, as I found out when comparing the view of a tight double (separation 0.6") - the MK91 resolved the pair but the Mewlon could not thanks to the spikes. A design with 1 vane or two curved vanes could be better, I guess.

On another note I have had a view of the planets through the 12" f/23 tri-schiefspiegler made by Barry Adcock long ago. That was in a league of its own. This showed just how good an unobstructed, perfectly achromatic scope could be. Newtonians simply don't come close, but the tri-schief was far from portable...

Last edited by Wavytone; 11-07-2019 at 09:06 PM.
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  #12  
Old 12-07-2019, 04:26 AM
astro744
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Your current telescope is quite capable of showing the disks as they are visible in considerabley less. I refer you to some other some other posts on the topic I found by doing a web search.

https://www.cloudynights.com/topic/2...-jovian-moons/

https://www.cloudynights.com/topic/4...sks/#Post65250
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Old 12-07-2019, 03:24 PM
bluesilver (Peter)
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Appreciate the replies, some very good information here.
Sounds like basically if i were to go for a larger Dobsonian, 14 or 16 inch, i would also be best to look at Zambuto mirrors for it.
I am still new to finding out what is budget scopes and what are good scopes.
I was thinking at leaning towards the Black Diamond 180/2700 Mak-Cassegrain, but if there are better options that are easy enough to get here in Australia, i would be interested in looking into them also.
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Old 13-07-2019, 01:37 AM
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Amen & Amen

Quote:
Originally Posted by ausastronomer View Post
No C14's or big Meade's for me either. They wouldn't even be allowed onto the same observing field as my 14"/F4.5 Zambuto powered SDM.



It doesn't need to be F7. My 14"/F4.5 Zambuto powered SDM gives planetary views the equal of anything I have looked through over the past 50 years, which includes Newtonians to 36", Refractors to 15" and Classical Cassegrains to 20". Very closely followed by James Pierce's 16"/F4 Lockwood powered SDM. Rod Berry's 20"/F5 SDM (The Mary Rose) with OMI Mirror is right up there as well. Unfortunately with a 20" x 2" thick mirror it took quite a while and favourable thermal conditions to properly stabilise and deliver its best views. Also with 20" of aperture seeing needed to be good as well. When thermals and seeing conditions co operated, the planetary views of Jupiter and Saturn through the Mary Rose were exceptional.



You haven't compared it to my 10"/F5.3 Suchting powered SDM in that case.

I have time and money, if you care to try it on ?

The days of extremely long focal length Newtonians are long gone. The main criteria to getting good planetary performance from a newtonian are:-

1) Premium Optics (primary, secondary and eyepiece). The optical system
is only as good as its weakest link.
2) Get the secondary obstruction under 20%
3) A thin mirror, which cools and stabilises quickly.
4) Good Collimation.
5) Good Baffling.
5) A good solid telescope structure (motorised tracking helps enormously)

The two advantages to a long focal length Newtonian over a short one is that the long focal length one will have a greater depth of focus, which makes things a bit easier for the observer. It doesn't improve the image quality. Similarly, the diffraction limited field in a long focal length Newtonian is larger than in a short focal length one, due to coma, but with a tracking scope and the image centred in the FOV, that is irrelevant in any case.

The disadvantages of a slow focal length Newtonian are numerous.

Years ago the number of medium to larger aperture high quality Newtonian mirrors in the F4 to F5 class was pretty small, hence people favoured slower F-ratios, as the mirrors were generally better, than the faster mirrors. That doesn't apply these days. Mark Suchting actually independently tested my 14"/F4.5 Zambuto mirror many years ago, at about 1/40th wave.

A well tuned high quality newtonian in the 12" to 20" class between F4 and F6 will give planetary views the equal of anything money can buy.

Cheers
John B
Amen & Amen Bammo -- I've been preaching this here for some time. Leaving aside a similar aperture refractor, in terms of image quality on planets, a well made, optimised Newtonian with high-quality optics and a small secondary obstruction <20% will be the winner all other things (like a good mount) being equal.

One thing to be said for longer focal length newtonians is that it is easy to get a small secondary while maintaining a good-sized 100% illuminated field **and** that to get higher magnifications, a longer focal length eyepiece is allowed providing a bit more comfort on the eye and usually longer eye relief.

Best,

L.
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Old 27-07-2019, 06:57 AM
astro744
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Quote:
Originally Posted by bluesilver View Post
Thanks for the replies and information.
Sorry if this is another basic question, still trying to get my head around all the right things.

So for planetary viewing, i am better off going for a longer focal length rather than larger aperture?

I thought the larger aperture would give me more detail due to the more light it lets in.

So instead of looking at say the 10 inch dobsonian i might be better of going for something like the Cassegrain Telescopes,
Something like the Black Diamond 180/2700 Mak-Cassegrain, where it has a focal length of 2700 mm ?
Or this might be a tad over the top, but the Celestron C14
There is nothing magical about a Mak. The 180mm has a smaller aperture than your 10” Dob but does have longer focal length. The longer focal length just means you can use longer focal length eyepieces to get a similar power that on your 10” requires shorter focal length eyepieces. The maximum capable magnification still goes to the 10” as it will give a higher power at a similar exit pupil.

The Mak has a 30% obstruction by diameter (used to compare contrast) whereas your Dob is closer to 25%. The Mak is a sealed system which will help with tube currents but it takes longer to reach ambient. A C14 has a larger aperture, large obstruction and long focal length but can deliver fine images if of good mirror and properly cooled. The 180 Mak too can do the same but with less bright images and less resolution.

You are correct in saying higher aperture is capable of more detail. Yes there is more light so higher powers are possible for the same exit pupil but also there is more resolution. (Of course good seeing and collimating required). I got the impression though that you believe adding an aperture mask to a larger mirror Dob retains the resolution of the larger mirror but reduces the brightness. No, adding a mask produces a smaller aperture telescope but one that is not obstructed by the secondary and spider vanes. However in moments of good seeing the larger aperture will show more detail even on an obstructed system. (A reasonable mirror assumed but doesn’t have to be first class).

Maks too can be difficult or impossible to collimate by the user and usually have to be sent to the factory. If anyone tells you they hardly ever need collimating ask them what happens when they do. If you do get a fine sample and seeing is good you will get some fine images but your current 10” should be able to do the same and a larger aperture Dob will show more given good seeing, good collimation and have a reasonable but not necessarily first class mirror, (first class excel in best seeing).

Whatever you choose, enjoy!
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Old 27-07-2019, 08:27 AM
Tropo-Bob (Bob)
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Spoting the discs of Jupiter's Moons depends on both seeing and the quality of the optics. Unfortunately, my seeing is a little below average.

I had several run of the mill newtonians which I used for many years without seeing discs. It was only when I brought a high quality 80mm refractor that the discs just popped out at me. Till then, I did not even realise it was possible to see them.
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Old 29-07-2019, 11:47 PM
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ngcles
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Hi Bob & All,

Quote:
Originally Posted by Tropo-Bob View Post
Spoting the discs of Jupiter's Moons depends on both seeing and the quality of the optics. Unfortunately, my seeing is a little below average.

I had several run of the mill newtonians which I used for many years without seeing discs. It was only when I brought a high quality 80mm refractor that the discs just popped out at me. Till then, I did not even realise it was possible to see them.
Given the angular diameter of Ganymede near opposition is about 1.55 arc-seconds (the other Galilean moons range down to about 1.0 arc-seconds) and the Dawes Limit for an unobstructed 80mm aperture is about 1.45 arc-seconds, I'd suggest that what you are seeing here isn't the actual disc of the Galilean moon(s), but "Airy-discs" formed by the telescope of what are, for all practical purposes in that aperture, point sources (ie like stars).

In order to properly resolve Ganymede into a disc, you will need a high-quality telescope of 20cm aperture and near perfect seeing conditions plus high magnification. The smaller Galilean satellites will require additional aperture. In order to see a properly resolved "disc" of the moon (and no, I'm not talking about a shadow on the Jovian cloud-tops-- that's a slightly different bucket of fish), you'll need enough aperture that the angular diameter of the satellite in question is at least two, better three times -- or more, "Airy-discs" across.

I should add that I've seen Ganymede, Callisto & Io resolved into a disc in my old 31cm telescope at x286 and once or twice seen gross detail on Ganymede (ie one side darker than the other or irregularities in surface brightness across the disc) a few times in my 46cm at x317 and above.

Best,

L.

Last edited by ngcles; 31-07-2019 at 10:33 PM.
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Old 30-07-2019, 03:24 PM
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ngcles
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Takahashi Mewlon -- not a Newtonian

Hi Nick & All,

Quote:
Originally Posted by Wavytone View Post
The last issue I have with all newtonians is diffraction spikes from spider vanes. Sure, most of you think it doesn't matter. But actually it does, as I found out when comparing the view of a tight double (separation 0.6") - the MK91 resolved the pair but the Mewlon could not thanks to the spikes. A design with 1 vane or two curved vanes could be better, I guess.
I am not entirely surprised by your comparison between the Mk91 and the Mewlon on a pair of 0.6" separation. Was it the Mewlon 210 or 250 you were viewing with?

The Mewlon is not a Newtonian but instead a Dall-Kirkham and is also a telescope (more correctly astrograph) optimised for photography/imaging -- not visual observing.

Sure, they do have first-class high-accuracy very smooth optics but again, 30% (or just over -- a bit more again than the MK91) central obstruction and the support vanes for the secondary mirror are about 4mm diameter. Not a problem for photography but they do make for thick, bright diffraction spikes.

I don't know which pair you were observing for making this comparison but just to add a bit of spice to the mix, here's a 1997 observation I made of Gamma Sextantis with my old 10" f/6.1 Newtonian that had a baffled tube 50mm secondary, three vane spider (with quite thin spider vanes) at x580. Gamma Sextantis at the time was at 0.54" separation with a difference in magnitudes of 5.1 + 5.4.:

"Well split with a dark hairs breadth between. Clearly split 90 - 95% of the time. In PA 70, diffraction rings surrounding both components virtually stationary. Another faint star 1' away magnitude11 in PA 340. "

The Dawes limit for a 10" telescope is 0.46" so I have split 'em almost down to the practical limit with nothing more than a common "light-bucket" -- as you like to call them.

Best,

L.

Last edited by ngcles; 04-08-2019 at 06:27 PM.
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Old 04-08-2019, 06:06 PM
Rainmaker (Matt)
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Actually the secondary support vanes on the Mewlon 210 are 1.55mm thick
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Old 04-08-2019, 06:22 PM
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Hi Rainmaker,

I'd be interested to know where you arrived at that figure of 1.55mm.

Quote:
Originally Posted by Rainmaker View Post
Actually the secondary support vanes on the Mewlon 210 are 1.55mm thick
They certainly look much thicker than that in this pic:

https://optcorp.com/products/takahas...-telescope-ota

Best,

L.
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