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Achro versus Apo


NGC 1502

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Here’s a short comparison between my elderly Vixen 80mm f=910mm Achro versus a clubmates Vixen 103mm f=795mm Apo.

The winner is obvious from the start, but that’s not what I was trying to find out. What I was trying to evaluate was just how much better the Apo was, given the very large cost difference.

So last evening, 27th June, as twilight started, both these scopes were set up alongside each other both on similar mounts, mine a Sky-Watcher 1.75” tripod with a Giro 3 alt-az, my clubmates on an identical tripod but with a Sabre alt-az fitted.

First up was Jupiter. The seeing was fairly good and the view satisfying in both scopes, mine at  114x, the Apo at 99x then at 133x. We used Vixen LVWs and TV Radians. The difference in the quality of the view was fairly readily apparent, especially in moments of better seeing. But to really get a feel for just how much better the Apo was, then Jupiter really needed to be at a much higher altitude and top seeing conditions, and Jupiter was past its best and getting lower. We viewed the lovely double star Rasalgheti (alpha Herculis) a clean split in both.

Then I asked my clubmate if I could do a star test with the 103 on Vega, he readily agreed. The out of focus diffraction rings were identical to my eye on both sides of focus. The only time I’d ever seen a star test that good was around 10 years ago using another clubmates TeleVue NP101 on Capella. I made another request to attempt to split the difficult double Delta Cyg. I’ve tried on numerous occasions to achieve a split with my Vixen 80mm, but never managed that even in excellent conditions.

The 103, using my 4mm Vixen LV at a natch under 200x gave a clean clear split at this first attempt. Just wonderful  ?

The bottom line ?.............that view could cost me a packet...........if my clubmate gets to read this.......a very large financial gift would be gratefully received.............?..........?...........?

Seriously, with thanks to my clubmate, greatly enjoyed the session.

Ed.

 

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Nice report Ed, and interesting session by the sounds. There is certainly something very special about a high quality 4” apo, glad you had the chance to have a look, an expensive addition to the wish list! :)

 

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Great stuff, but I wonder how much of the difference is "achro" vs. "Apo", or actually 80mm vs. 103mm? Delta Cyg is easy with my 102mm Vixen achro, ditto both my Maks, but almost impossible with my ED80.

If nothing else, the 103mm scope has twice as much light coming through as your 80mm scope, and the companion to Delta Cyg is nearly four mags fainter......

I love these "side-by-side" comparisons.......... keep 'em coming!

Chris

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4 minutes ago, chiltonstar said:

Great stuff, but I wonder how much of the difference is "achro" vs. "Apo", or actually 80mm vs. 103mm? Delta Cyg is easy with my 102mm Vixen achro, ditto both my Maks, but almost impossible with my ED80.

If nothing else, the 103mm scope has twice as much light coming through as your 80mm scope, and the companion to Delta Cyg is nearly four mags fainter......

 

Good point that Chris, thanks,  Ed.

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Achros are good but they are an 18th century idea. They solved the problem of the single-lens refractor, okay, now low-dispersion glass solves the achro problem. Instead of endlessly debating achro versus apo, we should lobby glass and telescope makers to make low dispersion glass more affordable.

TS has two 102mm f/7 low-dispersion glass doublets with identical focusers and tubes, the FPL-51 semi-apo costs 600€, the FPL-53 apo is 1,000€. The price for the mating glass couldn't be that different, so the FPL-53 glass blank alone is worth 400€ more? And only in four-inch size? Is that possible?

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p4964_TS-Optics-ED-APO-102mm-f-7-Refraktor-mit-2-5--R-P-Okularauszug.html

https://www.teleskop-express.de/shop/product_info.php/info/p9868_TS-Optics-PhotoLine-102mm-f-7-FPL-53-Doublet-Apo-with-2-5--Focuser.html

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Nice write up of our session Ed.

It was most enjoyable and importantly fun too, as has been suggested an amount of the difference was the the 80mum to 103mm step; but as you noted the posh glass is making a contribution.

The one thing from last night other than scope comparison that I found was that a shared observing session is so much nicer than a solitary one. 

I was most pleasantly surprised to read the write up and still find it difficult to believe that I am blessed with such a fine instrument, even better I have a Vixen 80mm just like yours too and like that rather a lot as well

Vixen fanboy, you bet ?

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Aah, so it was your new ED103 Alan?!

Sounds like a fun session, using 2 fine scopes. There is no doubt that Delta Cygni is a tough test for an 80mm, but it can be done. I've split Delta in my 80mm (Circle T objective) F15 achromat in good seeing at high power (x150 plus, ortho eyepiece)

I think a good long focus achro will always outperform  short ones on doubles because:

a) the long focal length gives you much greater depth of focus (easier to slide into sharp focus ) - with a shorter F7.5 or less scope you really need a dual speed focuser to have a chance of finding the sharpest view your scope can deliver.

b) You can use the higher powers needed for good splits of close pairs much more easily and more comfortably. For example, an 80mm F15 has a focal length of 15x80mm =1200mm. A 6mm eyepiece will give you x200 in this scope. But an 80mm 7.5 whether ED or Achro will need an eyepiece of just 3mm focal length to give the same magnification: with a few premium price exceptions , a 3mm ep will have very tight eye relief and be very uncomfortable to use for any length of time.

A comparison of the ED103s vs a Vixen SP102 achro would be fun? .

Dave

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Interesting comparison but I do tend to feel that this was an 80mm vs 103mm comparison rather than an achro vs apo. In my experience a decent 4"  F/10 achromat (eg: Vixen 102, TAL 100) will split delta cygni quite well if the conditions are reasonable.

As Dave says, an ED vs Achro of the same aperture does make an interesting match :smiley:

 

 

 

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On 28/06/2018 at 09:10, Ben the Ignorant said:

Achros are good but they are an 18th century idea. They solved the problem of the single-lens refractor, okay, now low-dispersion glass solves the achro problem. Instead of endlessly debating achro versus apo, we should lobby glass and telescope makers to make low dispersion glass more affordable.

TS has two 102mm f/7 low-dispersion glass doublets with identical focusers and tubes, the FPL-51 semi-apo costs 600€, the FPL-53 apo is 1,000€. The price for the mating glass couldn't be that different, so the FPL-53 glass blank alone is worth 400€ more? And only in four-inch size? Is that possible?

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p4964_TS-Optics-ED-APO-102mm-f-7-Refraktor-mit-2-5--R-P-Okularauszug.html

https://www.teleskop-express.de/shop/product_info.php/info/p9868_TS-Optics-PhotoLine-102mm-f-7-FPL-53-Doublet-Apo-with-2-5--Focuser.html

As of 2017, based on Ohara's price list, FPL-53 costs around 70% more than FPL-51. As a yardstick, FPL-51 costs 11x more than Schott BK7 glass (crown glass). If they made more of it, the cost would be less for ED glass, but the number of melts per annum is low, presumably due to the low number of applications compared with the 200 other glass types that Ohara make.

 

 

 

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40 minutes ago, John said:

Interesting comparison but I do tend to feel that this was an 80mm vs 103mm comparison rather than an achro vs apo. In my experience a decent 4"  F/10 achromat (eg: Vixen 102, TAL 100) will split delta cygni quite well if the conditions are reasonable.

 

Yes, point taken and fair comment........

Just had a short session with my Vixen 80mm / 910mm and tried Delta Cygni yet again at high power.  Not absolutely certain, but at times I thought I noticed a brighter spot on the first diffraction ring at about the right position angle, so perhaps a hint of what may be possible under excellent conditions.

Ed.

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1 hour ago, NGC 1502 said:

 

Yes, point taken and fair comment........

Just had a short session with my Vixen 80mm / 910mm and tried Delta Cygni yet again at high power.  Not absolutely certain, but at times I thought I noticed a brighter spot on the first diffraction ring at about the right position angle, so perhaps a hint of what may be possible under excellent conditions.

Ed.

I've been trying the same target with my TV Ranger 70mm F/6.8 as well. Similar results. It's a tough nut to crack with less than 90mm I think.

Your Vixen will have a well figured objective lens. Vixen are good at making lenses :icon_biggrin:

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3 hours ago, John said:

Your Vixen will have a well figured objective lens. Vixen are good at making lenses :icon_biggrin:

 

Yes, the Vixen has surprised me lately at just how much detail on Jupiter can be spotted, especially given the low elevation, although at times the seeing has been rubbish, a few minutes later and I’m rewarded with a crisp view. The GRS has been easily seen, as have been shadow transits.

My Vixen is one of the very early models that had a collimatable cell, probably overkill, but a sign that they cared about getting it right.

But my clubmates 103 has me drooling..........?      Good luck with the charming Ranger.......?

Ed.

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13 hours ago, NGC 1502 said:

 

Yes, point taken and fair comment........

Just had a short session with my Vixen 80mm / 910mm and tried Delta Cygni yet again at high power.  Not absolutely certain, but at times I thought I noticed a brighter spot on the first diffraction ring at about the right position angle, so perhaps a hint of what may be possible under excellent conditions.

Ed.

It's worth adding that with an 80mm frac, the secondary of Delta Cyg lies almost on the first diffraction ring, whereas it is outside it with 103mm and upwards. Laws of physics, so no matter how "posh" the glass is (or expensive the scope) you cannot get round that....... In the simulations below, I've turned up the intensity so you can see the ring well.

Chris

deltacyg80mm.jpg

deltacyg103mm.jpg

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That's an interesting point you make Chris.

Would love to do a 102m v ED103, anyone wishing to lend out a classic 102m to myself an Ed?

I split Delta Cyg happily on 28th during a marathon for session, it split nice and looked nice.

The planets were my main target that night though.

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10 hours ago, 25585 said:

Ed has your 80mm got Japanese made lenses? 

The M has, MF has Chinese. Prices reflect that. 

 

Indeed, my Vixen has Japanese optics, pics attached showing the early style label, and the collimateable cell, with dew shield removed. The later non collimateable cell was much slimmer.  There was of course Celestron labled versions with either orange or black colour tube, early models also had the collimateable cell.  As already mentioned, later models didn’t allow collimation adjustments, and was probably an overkill design.

Ed.52172DDD-2EE0-4019-B126-B66908118A9F.thumb.jpeg.feba547245f754a26786ac82b3d00893.jpeg52172DDD-2EE0-4019-B126-B66908118A9F.thumb.jpeg.feba547245f754a26786ac82b3d00893.jpeg2BC803F7-24D6-4DC3-9504-8C5AE3EB6A5C.thumb.jpeg.9e251efb70a850e81c40a5b96d86d242.jpeg 

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5 hours ago, chiltonstar said:

It's worth adding that with an 80mm frac, the secondary of Delta Cyg lies almost on the first diffraction ring, whereas it is outside it with 103mm and upwards. Laws of physics, so no matter how "posh" the glass is (or expensive the scope) you cannot get round that....... In the simulations below, I've turned up the intensity so you can see the ring well.

Chris

deltacyg80mm.jpg

deltacyg103mm.jpg

 

Hi Chris, many thanks for this, makes me feel that the brightening on the first diffraction ring that I saw, could well be the secondary.

Yet again, it’s clear in SE Essex, no prizes for what I’ll be doing later on ?

Ed.

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If find that the diffraction ring can make spotting close star pairs challenging at times with my Vixen ED102SS. With my Tak FC100 though, the diffraction ring is finer / thinner so the same doubles split just a little easier.

When I was chasing Zeta Herculis a couple of years back with my ED120 I had a similar experience. In that case the dimmer componant was appearing as a thickened part of the diffraction ring of the brighter star:

 

zherc.jpg

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Tried again last night, but poor seeing, could see the first diffraction ring, but not sharply enough to spot any thickening or brighter spots.

Will report back if/when there’s anything relevant to say.

Thanks to everyone for their input, appreciated.

Ed.

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I looked again with my ED80 at Delta Cyg two nights ago. Visibly, it exactly matched the simulation - the secondary was visible as a bright point on the first diff ring, bulging it outwards slightly, whereas with larger scopes, I see it as a separate point of light. Fantastic as little fracs are, you can't buck the laws of physics unfortunately!

John: re the thinner/finer diff ring with your Tak, doesn't this mean that the optics in the Vixen 102mm are not "diffraction-limited", since with "diffraction limited" fracs, the shape of the waveform/disk and rings should always be identical? Put another way, maybe there is less residual colour with the Tak at peak observing wavelengths?

Chris

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1 hour ago, chiltonstar said:

I looked again with my ED80 at Delta Cyg two nights ago. Visibly, it exactly matched the simulation - the secondary was visible as a bright point on the first diff ring, bulging it outwards slightly, whereas with larger scopes, I see it as a separate point of light. Fantastic as little fracs are, you can't buck the laws of physics unfortunately!

John: re the thinner/finer diff ring with your Tak, doesn't this mean that the optics in the Vixen 102mm are not "diffraction-limited", since with "diffraction limited" fracs, the shape of the waveform/disk and rings should always be identical? Put another way, maybe there is less residual colour with the Tak at peak observing wavelengths?

Chris

The Tak does have better optics than the Vixen Chris. The Vixen performs entirely as I would expect a good 4" ED doublet should and I enjoy it immensely but there is no getting away from the Tak handling higher magnifications (ie: over 220x) better and the fainter, thinner diffraction ring making it slightly easier to split really tight double stars and no CA at all even either side of focus. The Tak cools down very quickly wheras the Vixen needs 20 mins or so to give it's best at high power.

Prior to the Vixen ED102SS I'd owned the Vixen SP102M and a Tal 100RT achromats which are very good achros but the ED102SS proved better than either of those so more than just diffraction limited I'd suggest.

Bear in mind that the Vixen is an F/6.5 ED doublet and the Tak an F/9 Fluorite doublet. I'm not surprised there is a difference, indeed I'd be a little dissapointed if there was not !.

What the Vixen also does though, which the Tak cannot, is to deliver up to 4 degree true fields and the views of objects such as the Veil Nebula (aided by an O-III filter) which are really lovely on a suitably dark night :smiley:

Plus I've owned the Vixen for over 10 years now so we are "old friends". It has a permanent home here :wub:

Edit: I'm not claiming that the Tak breaks the laws of physics BTW. :smiley:

 

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