Achro vs. Apo

[emadmoussa]

I recently had a quick comparison between my 102ED and 152mm achro. One thing is obvious, on DSOs both seemed to produce similar views - if you compensate for the difference in aperture.

Naturally, you can see that an apo is cleaner on planets on the moon, but the achro was more than satisfactory. Apart from a hint of CA, it was as sharp as the ED. 

DSOs...well...the same.

Just a thought....while I would love to get my hands on a 6" apo after I've sold my wife and kid, I still find the achro as effective on DSOs. Which makes me wonder whether it's worth  it   dreaming about a 6" apo  if you're a visual astronomer.

[Pig]

What do you have your starwave mounted on Imad ?

[emadmoussa]

What do you have your starwave mounted on Imad ?

both the Starwave and TS Individual are mounted on the ES Twilight II giro.

[swamp thing]

makes me wonder whether it's worth  it   dreaming about a 6" apo  if you're a visual astronomer.

Well I never do

[Space Dragon]

Agree, I think these Kunming 152mm are great, especially for the money, I'll certainly be hanging onto mine for a while and while I've never looked through a 6" Apo, I can't imagine the view on anything less than Mag 4 being any different, but I stand to be corrected.

I picked up a 102 ED as well, only had it out for a couple of hours but like what I see, nice flat field. I don't think it would qualifiy as an Apo though, at least with mine, I'm seeing flashes of blue around brighter objects but it is good on the Moon with only the very slightest hint of yellow/blue on the limb and even then you have to really look for it.

Of course, sensitivity to CA can vary from person to person and some are not even bothered by it if they can see it. Others cannot stand even a bit.

So if someone offered me a 6" AP for under £2000, I might have to think about it but other than that, I think the money would be better spent taking the family on holiday, as Neil English would say. :-)

[Naemeth]

Why spend all that money on a 6" apo when you can just get a big dob that will blow it out of the water for DSOs?

[ronin]

I suspect that when achros were the only realistic scope available that glass types available were also limited.

If you look at the selection of crown and flint glasses now available there is a greater choice.

So if you choose a crown glass that is still classed as crown but closer to the ED glasses then an older glass then you get better correction. Where does crown end and ED start ?

If you said FPL-51 was the first ED glass then if you pick the one just below you get a crown but with very good colour correction possibilities. Would be an "achro" but close to indistinguishable from an ED.

There are now many crown and flint glasses so more combinations and a greater chance of a good match. I would guess that the glass types selected and used are what seperates an average achro from a good achro.

[Alien 13]

I suspect that when achros were the only realistic scope available that glass types available were also limited.

If you look at the selection of crown and flint glasses now available there is a greater choice.

So if you choose a crown glass that is still classed as crown but closer to the ED glasses then an older glass then you get better correction. Where does crown end and ED start ?

If you said FPL-51 was the first ED glass then if you pick the one just below you get a crown but with very good colour correction possibilities. Would be an "achro" but close to indistinguishable from an ED.

There are now many crown and flint glasses so more combinations and a greater chance of a good match. I would guess that the glass types selected and used are what seperates an average achro from a good achro.

I have allways wondered about this exotic glass nothing new one of my camera lenses uses radioactive glass in it, ED glass seems to have allowed shorter focal lengths but a good early achro will still be using some sort of quality glass too.

[Pig]

both the Starwave and TS Individual are mounted on the ES Twilight II giro.

Thanks

[step_hen]

I had my ED80 set up alongside the 80mm F11 achro last night, both working at the same magnification. Not a lot between them on Jupiter or DSO, but the ED80 was a better colour on the moon and had better contrast. 

Stephen

[Space Dragon]

Why spend all that money on a 6" apo when you can just get a big dob that will blow it out of the water for DSOs?

I did own an 8" Dob for a while but just didn't get on with the form factor, it's very hilly round here and the one thing a Dob really needs is a flat surface.

Once I've built an observing platform I might give it another go with a 12" or so.

[nightfisher]

There has been times i considered selling my TAL 4" achro to get a 102 ED, i got talked out of it and now would never betray my russian wonder

[estwing]

Why spend all that money on a 6" apo when you can just get a big dob that will blow it out of the water for DSOs?

for me a 4" frac is the limit on many issues...any bigger and its a Dob issue!

[John]

You will be loosing some detail and contrast on the moon and planets from CA with an F/5.9 achromat. It's just the physics of achromat refractors crown and flint design. I've played around with devices that remove most of the CA from F/8 achromats and the difference is obvious when the CA is reduced by around 80% or so. 

On deep sky objects CA does not seem to reduce the quality of the images much though.

FWIW My ED102 and ED120 refractors show, to my eyes, no CA, at focus, around Jupiter, Saturn or the lunar limb and just a small touch of it around the brightest stars and Venus. 

But if you like the views through your scopes, what does any of that matter 

I like achro refractors but when they are F/10 or slower.

[emadmoussa]

My achro is f/5.9, but the amount of CA it produces is noticeably less than other achros I used. In fact, CA is too minimal that I stopped noticing it. That is why Im not bothered investing in a semi apo or fringe killer filters...

[lardy]

Chalk and Cheese  i'm afraid … The Acrho and APO, have a look through a high end (AP/APM) 152mm APO in comparison to your TS, I think you might be surprised at the difference. And size does not always win … my APM 130 beats my 14" SCT in many many ways.

[emadmoussa]

Chalk and Cheese  i'm afraid … The Acrho and APO, have a look through a high end (AP/APM) 152mm APO in comparison to your TS, I think you might be surprised at the difference. And size does not always win … my APM 130 beats my 14" SCT in many many ways.

Lend me yours and I will happily run a thorough comparison

[lardy]

2 1/2ish hour drive … your very welcome to come and try 

[emadmoussa]

2 1/2ish hour drive … your very welcome to come and try

What?! You dont have a teleportation machine!

[John]

I suspect that when achros were the only realistic scope available that glass types available were also limited.

If you look at the selection of crown and flint glasses now available there is a greater choice.

So if you choose a crown glass that is still classed as crown but closer to the ED glasses then an older glass then you get better correction. Where does crown end and ED start ?

If you said FPL-51 was the first ED glass then if you pick the one just below you get a crown but with very good colour correction possibilities. Would be an "achro" but close to indistinguishable from an ED.

There are now many crown and flint glasses so more combinations and a greater chance of a good match. I would guess that the glass types selected and used are what seperates an average achro from a good achro.

It's all to do with refractive indexes I believe. The glass lenses have to be figured and polished to high standards too and the mating elements have to be carefully selected as well.

My understanding is that an ED doublet that uses an FPL-51 element and an appropriate mating element should show 4 times less CA than a crown flint achromat of the same quality, aperture and focal length.

With FPL-53 the CA reduction is greater again.

There is nothing wrong with CA anyway just as Coma is inherent in the newtonian design.

[philhas]

A common theme on Achro versus APO is when you start chucking magnification at them. My Evostar 120 is a good scope however when I up the mag it slowly but surely loses definition and contrast, from reports on here the APO's just keep delivering at high magnifiction conditions permitting of course.

[emadmoussa]

A common theme on Achro versus APO is when you start chucking magnification at them. My Evostar 120 is a good scope however when I up the mag it slowly but surely loses definition and contrast, from reports on here the APO's just keep delivering at high magnifiction conditions permitting of course.

This is true. With an apo it's possible to push the scope beyond its limits and still get reasonable results.

[John]

Many of the chinese achromats I've used suffered from spherical aberration, usually being a bit under corrected. When I was using the Chromacor correctors they sorted out both most of the CA and the under correction and it was noticeable then how the scopes would handle much higher magnifications with aplomb. 

Unfortunately the Chromacor's are out of production now and are both expensive and rare on the used market. A couple of manufacturers having been talking about introducing correctors of this type but, as yet, nothing has materialized. I suspect the relatively affordability of ED doublets has reduced the potential market for them anyway.

[Tim]

I don't really do visual astronomy on anything faint, so I can't offer anything useful, but on my 6" APO at Kelling Heath a very experienced observer wanted to check the scope over, and to do so used a 4.xx mm televue eyepiece on Jupiter and Mars. Apparently there was no green or purple evident, which is a good thing by the sound of it.

As for APO vs Dob, I have yet to meet the dob which gives as nice a view of a star cluster as an APO, and I've had the chance to view through some pretty amazing dobs, including a 24" 1/4 wave.

Just my tuppence worth

[emadmoussa]

It's the everlasting question whether a Dob with its affordable massive aperture can beat and smaller yet astronomically expensive apo. I wrote a short speculative thread on hands-on comparison between my 10" Dob on one side and the 102mm ED and 6" Achro on the other. General conclusion is that there isn't such a thing as a perfect telescope, it turns out that every telescope serves a certain purpose. While I love the views my fracs show of star clusters, doubles and binaries, I still find the a Dob a better suited weapon for galaxies and faint nebulae.

I think an all-round telescope is a telescope that's ''good'' for everything, but won't excel at any. I found for example that the 6" achro produces only slightly more defined n views of  emission nebulae than a 10" reflector. The later however - given it's aperture - showed slightly more details - albeit not as defined (still satisfactory quality - perhaps my eyepieces fell short in  a fast Dob). The refractor fell short against the much bigger reflector on diffuse nebulae - which is natural I reckon. Comparing these results to my late 11" SCT (based on memory), the Schmidt Cassegrain fell in a middle category between both reflector and refractor. Big aperture that showed faint objects yet lacked some brightness and contrast as apposed to either a reflector or a refractor. Just my personal opinion, some might disagree.