80ED got beat by 80mm achromat on Saturn?

[nightfisher]

Having owned a Vixen 80m and an 80mm ED, i can honestly say i think the Vixen would be the leader when viewing Saturn, and yes to this day i still regret selling the 80M. A good achro scope is still a super instrument to view Saturn, but not so good for Jupiter, my best ever views of Saturn was with my old Tal100rs, plus a friend had a Carton 100 f13 and another had a Lyra 102 f11

[John]

7 hours ago, SIDO said:

Achromats are a hard to beat design from the standpoint of a refractor, just look at all the technology expense and centuries it took not to mention the broad cost comparison that still exists even today. A quality well tuned achromat at or above the Sidgwick standard is a way understated scope due to the higher quality of much more expensive and current designs even though in some cases they only barely succeed to best some better achromatic examples. Love to here this truth come out in the wash occasionally, they are so still relevant ?

The author Neil English championed the cause of the traditional long achromat over the more expensive apo "poodles" (his term) for a long time and got involved in some quite heated debates on another forum on the topic.

I believe he is a big fan and advocate of the simple newtonian design now though.

I've owned and used some nice achromats over the years but all my refractors are ED doublets (one triplet) now mainly because of the versatility of the design and relative ease of mounting.

There is a lot to be said for a long traditional achromat though and it's a magnet at a star party

[johninderby]

A long fl achromat is a specialist indeed so you wouldn’t want one for your only scope but nice to own one even if it doesn’t see that much use. 

[Ben the Ignorant]

My humble 80/600 achro becomes a razor when it's stopped down to 45mm (f/14) but is has to take in daylight to remain bright. A very large achro could stick to low power for the deep sky, and be stopped down to clean up planetary views; that would be a nice bargain. Given how rare observing opportunities are for some observers, that could make more sense then paying thousands for a fully apo tube.

[michael.h.f.wilkinson]

One thing to take into account is spherical aberration (SA). Whereas a fast ED doublet might correct better for chromatic aberration (CA), it might perform worse on SA. The hallmark of SA is that stars appear sharp, but the have a faint, hard to spot halo around them that can kill contrast. As an ED doublet has the same number of surfaces as the achromat, I do not think it could control SA better than the achromat. SA is reduced very fast as the focal ratio increases, so a slow achromat should show much less SA than a fast ED doublet. With a triplet APO things are different, as you have two extra glass-air interfaces to work with, so it might be possible to do something smart to correct for SA.  Aspherical surfaces would also help, of course, but that rather seriously adds cost.

In my own experience, with quite a range of fracs (starting with a nice 70mm F/10 achromat at school, up to a TEC 140, and now mainly an APM 80mm F/6 triplet), I haven't ever found views beyond 2x the aperture in mm satisfactory. I do not push my APM beyond about 140x, as a rule. I have a visual acuity of 1.75, at the last eye test, so that might be why I do not tolerate high magnification, because the image seems mushy to me quite quickly.

[John]

27 minutes ago, michael.h.f.wilkinson said:

One thing to take into account is spherical aberration (SA). Whereas a fast ED doublet might correct better for chromatic aberration (CA), it might perform worse on SA. The hallmark of SA is that stars appear sharp, but the have a faint, hard to spot halo around them that can kill contrast. As an ED doublet has the same number of surfaces as the achromat, I do not think it could control SA better than the achromat. SA is reduced very fast as the focal ratio increases, so a slow achromat should show much less SA than a fast ED doublet. With a triplet APO things are different, as you have two extra glass-air interfaces to work with, so it might be possible to do something smart to correct for SA.  Aspherical surfaces would also help, of course, but that rather seriously adds cost.

 

I agree with this up to a point with regard to really good quality achromats but most of the chinese achromats that I have used (both fast and slow) have had quite a bit of SA, as well as the CA. They do offer good value because they are low cost scopes but the SA does hamper their ultimate performance at high power. Of the achromats that I've owned the exceptions to this were the Istar 150mm F/12, the Vixen SP102M and the TAL100, which did seem to have better quality objectives than many.

The Chromacor corrector was designed to correct both CA and SA in the F/8 chinese achromats. You could get Chromacors with a null SA correction but these were the least numerous because demand was low - most of the scopes they were used in had a degree of SA. Seeing the difference in performance when the SA in an achromat is corrected by a Chromacor is quite startling.

FWIW I believe that the Skywatcher ED120 objective does have aspherical surfaces. This was an expensive and time consuming process for Synta but they wanted to get the 120mm ED doublet objective "right" so they delayed it's launch and put the effort in.

Maybe I've been lucky but the ED doublets that I have owned and used seemed to be well corrected for SA.

[nightfisher]

1 hour ago, michael.h.f.wilkinson said:

One thing to take into account is spherical aberration (SA). Whereas a fast ED doublet might correct better for chromatic aberration (CA), it might perform worse on SA. The hallmark of SA is that stars appear sharp, but the have a faint, hard to spot halo around them that can kill contrast. As an ED doublet has the same number of surfaces as the achromat, I do not think it could control SA better than the achromat. SA is reduced very fast as the focal ratio increases, so a slow achromat should show much less SA than a fast ED doublet. With a triplet APO things are different, as you have two extra glass-air interfaces to work with, so it might be possible to do something smart to correct for SA.  Aspherical surfaces would also help, of course, but that rather seriously adds cost.

In my own experience, with quite a range of fracs (starting with a nice 70mm F/10 achromat at school, up to a TEC 140, and now mainly an APM 80mm F/6 triplet), I haven't ever found views beyond 2x the aperture in mm satisfactory. I do not push my APM beyond about 140x, as a rule. I have a visual acuity of 1.75, at the last eye test, so that might be why I do not tolerate high magnification, because the image seems mushy to me quite quickly.

I think this hits the nail firmly on the head, and really brings a reason to why a good achro can provide sharp impressive image at the eyepiece.........very well said Michael

[F15Rules]

My previous 5" scope was a D&G F15 achromat and was fully 2 metres long. As such it was hard to mount, which was eventually why I sold it (I had a double hernia, though not caused by the scope!)..

It's shown below, now properly mounted on an EQ6 and permanent pier by it's (sorry, "her" name is Andromeda), new owner, Steve.

But optically this is a wonderful scope, with great figure and polish, which delivers jet black backgrounds and amazing detail on Jupiter on good night's. Airy disks are beautiful,and CA is minimal. At F15 very modest eyepieces will deliver great views, so no need to bankrupt oneself on high end glass..

My main scope now is a Tak FS128 operating at F8.1, so around half the length of the D&G. The Fluorite doublet of the Tak is also first class optically, many would say world class, but on excellent nights,  I can't say it beats the D&G on moon, planets or doubles to my (aging) eyes. It can, obviously, give wider field lie power views, and is much easier to handle and mount.

It's horses for courses IMO. And what your eyes show you I frankly more important than all the techie stuff, interesting though that is to read?.

One of these days Steve and I will have a side by side comparison of the two scopes. But it will just be for fun, and I will always be one apo user that will respect the capabilities of a good achromat, and that's why I will keep my existing SGL handle @F15Rules !?

Dave

 

[Ben the Ignorant]

1 hour ago, F15Rules said:

My previous 5" scope was a D&G F15 achromat and was fully 2 metres long. As such it was hard to mount, which was eventually why I sold it

What is not convenient will end up not being used.

[F15Rules]

8 hours ago, Ben the Ignorant said:

What is not convenient will end up not being used.

You missed the point completely.

The original poster was suggesting that an achromatic 80mm had bested or matched an ED80 on optical performance, specifically on Saturn in this case.

I have simply agreed that that is quite possible: it just depends on lens quality of both scopes. You can have a well made achromat and a poorly made ED, and vice versa.

The mounting issues I had weren't the fault of the scope, and even though I had those issues, I still had amazing views.

You have no idea how often I used this scope, or any other for that matter, so please don't make such blanket statements.

Now that the scope is mounted as it was always designed to be, it delivers outstanding performance.

Dave

[Saganite]

On 04/10/2018 at 03:11, SIDO said:

Achromats are a hard to beat design from the standpoint of a refractor, just look at all the technology expense and centuries it took not to mention the broad cost comparison that still exists even today. A quality well tuned achromat at or above the Sidgwick standard is a way understated scope due to the higher quality of much more expensive and current designs even though in some cases they only barely succeed to best some better achromatic examples. Love to here this truth come out in the wash occasionally, they are so still relevant ?

Absolutely true they are still relevant !

I have the 5" f15 D&G, bought from Dave (F15 Rules) 3 years ago, and it is a superb instrument. It has shown me breathtaking detail particularly on Jupiter, which I will never forget, as well as of Saturn, and razor sharp Lunar views. It is excellent for deep sky also. It is  a long scope which is why I made the pier tall and extremely solid, and whilst some may regard this as an inconvenient set up it most certainly does not render it underused. It is the scope I have wanted for 4+ decades. The ownership of a 'top flight' 5"Apo is well within my means, but I do not desire one, and why would I, when the huge extra expense would give me very little in return.

 

[John]

I really liked the performance of my Istar 150mm F/12. The problem was that it really needed a permanent mounting, which was not something that was practical in my current circumstances. By the time the scope was mounted on an EQ6 which in turn was on the taller Meade Giant Field tripod (3 inch steel tube legs !) the outfit weighed around 130 lbs. Setup too quite a bit of time and real care was required lifting the tube onto the mount head / rings, which was at head height.

So it was not the qualities of the scope that led me towards something more compact but simple practicalities. If you are determined to own and use such an instrument these can be overcome but I have to say that my 130mm F/9.25 triplet shows truly superb views and is realatively simple and quick (and safe !) to set up and tear down. With the mounting requirements taken into account, it has not cost much more than the 150mm F/12 Istar did to own and operate.

If I had an observatory with a permanent mounting facility I would certainly consider a traditional, long, achromat of good quality again though. I'm glad to have had the experience of trying one for a while.

We seem to have strayed a little from the 80mm aperture of the original topic though !

 

[SIDO]

58 minutes ago, John said:

I really liked the performance of my Istar 150mm F/12. The problem was that it really needed a permanent mounting, which was not something that was practical in my current circumstances. By the time the scope was mounted on an EQ6 which in turn was on the taller Meade Giant Field tripod (3 inch steel tube legs !) the outfit weighed around 130 lbs. Setup too quite a bit of time and real care was required lifting the tube onto the mount head / rings, which was at head height.

So it was not the qualities of the scope that led me towards something more compact but simple practicalities. If you are determined to own and use such an instrument these can be overcome but I have to say that my 130mm F/9.25 triplet shows truly superb views and is realatively simple and quick (and safe !) to set up and tear down. With the mounting requirements taken into account, it has not cost much more than the 150mm F/12 Istar did to own and operate.

If I had an observatory with a permanent mounting facility I would certainly consider a traditional, long, achromat of good quality again though. I'm glad to have had the experience of trying one for a while.

We seem to have strayed a little from the 80mm aperture of the original topic though !

 

I think that the age old lure of the traditional refractor is creeping in, for a minute there it almost sounded like you were defending using an apo lol...

 

[John]

1 minute ago, SIDO said:

I think that the age old lure of the traditional refractor is creeping in, for a minute there it almost sounded like you were defending using an apo lol...

 

I love my ED doublets and my 130 triplet. I really enjoyed the achromats that I've owned as well though.

At F/9.25 the 130 is not exactly a short scope:

 

[SIDO]

1 minute ago, John said:

I love my ED doublets and my 130 triplet. I really enjoyed the achromats that I've owned as well though.

At F/9.25 the 130 is not exactly a short scope:

 

Not by any means John, Very Nice Too...

[Paul67]

How would the Vixen perform on doubles?  In comparison to the kunming 102 F11.

[John]

1 hour ago, Paul67 said:

How would the Vixen perform on doubles?  In comparison to the kunming 102 F11.

80mm vs 102mm at around the same focal ratio and equivalent optical quality I would think so the maths suggests that the 102 should resolve a closer pair ?

 

 

[chiltonstar]

My 102mm Vixen objective f13 achro just out resolves my ED80, again roughly according to the maths. I would say the contrast is slightly better on a target like Saturn with the ED80, even if the resolution is a bit less than the 102mm. 

Chris