TheSkySearchers

Bowlerhat wrote: ↑Mon May 31, 2021 2:26 am Other than scientific "APO", in practicality there are other definitions. Like "Marketing APO" - which brands just abused the term "Apo" for selling scopes which isn't..and there's also "Practically APO" which an experience of CA-free in non-APO scopes. I mean I use my classic scopes a lot and tbh I don't see CA (because of long focal length). There's also "Lens APO" where people start to define "apo" based on lens numbers..that doublets can't be APO, or the opposite. This is muddled up further by types of glass. FPL 51,53, 55, BK71, FCD 100, etc..

As a person who has used mirrors, achromats, EDs, fluorites..and eventually long achros, I'd say the term is rather flexible in the field. Personally, I'd just stick to Roland Christen's definition: "bringing 3 wavelengths to a common focus and be corrected for spherical aberration at two wavelengths."

notFritzArgelander wrote: ↑Mon May 31, 2021 2:44 am

Bowlerhat wrote: ↑Mon May 31, 2021 2:26 am Other than scientific "APO", in practicality there are other definitions. Like "Marketing APO" - which brands just abused the term "Apo" for selling scopes which isn't..and there's also "Practically APO" which an experience of CA-free in non-APO scopes. I mean I use my classic scopes a lot and tbh I don't see CA (because of long focal length). There's also "Lens APO" where people start to define "apo" based on lens numbers..that doublets can't be APO, or the opposite. This is muddled up further by types of glass. FPL 51,53, 55, BK71, FCD 100, etc..

As a person who has used mirrors, achromats, EDs, fluorites..and eventually long achros, I'd say the term is rather flexible in the field. Personally, I'd just stick to Roland Christen's definition: "bringing 3 wavelengths to a common focus and be corrected for spherical aberration at two wavelengths."

While it's true that Roland Christen's use of the definition is correct, he was not the first to use the principle in designing lenses. So the definition isn't "his". The history of who authored the definition first is murky because optics companies in the 19th century kept a lot of data proprietary. The first practical example of an apochromatic lens used for telescopes (AFAIK) is the Cooke triplet in 1893 invented by Harold Dennis Taylor. But Zeiss's Ernst Abbe probably invented the concept earlier and applied it to microscopes in 1886, the current definition is from Abbe. So Christen is using Abbe's ideas.

https://en.wikipedia.org/wiki/Harold_Dennis_Taylor

https://en.wikipedia.org/wiki/Ernst_Abbe

Bowlerhat wrote: ↑Mon May 31, 2021 2:54 am

notFritzArgelander wrote: ↑Mon May 31, 2021 2:44 am

Bowlerhat wrote: ↑Mon May 31, 2021 2:26 am Other than scientific "APO", in practicality there are other definitions. Like "Marketing APO" - which brands just abused the term "Apo" for selling scopes which isn't..and there's also "Practically APO" which an experience of CA-free in non-APO scopes. I mean I use my classic scopes a lot and tbh I don't see CA (because of long focal length). There's also "Lens APO" where people start to define "apo" based on lens numbers..that doublets can't be APO, or the opposite. This is muddled up further by types of glass. FPL 51,53, 55, BK71, FCD 100, etc..

As a person who has used mirrors, achromats, EDs, fluorites..and eventually long achros, I'd say the term is rather flexible in the field. Personally, I'd just stick to Roland Christen's definition: "bringing 3 wavelengths to a common focus and be corrected for spherical aberration at two wavelengths."

While it's true that Roland Christen's use of the definition is correct, he was not the first to use the principle in designing lenses. So the definition isn't "his". The history of who authored the definition first is murky because optics companies in the 19th century kept a lot of data proprietary. The first practical example of an apochromatic lens used for telescopes (AFAIK) is the Cooke triplet in 1893 invented by Harold Dennis Taylor. But Zeiss's Ernst Abbe probably invented the concept earlier and applied it to microscopes in 1886, the current definition is from Abbe. So Christen is using Abbe's ideas.

https://en.wikipedia.org/wiki/Harold_Dennis_Taylor

https://en.wikipedia.org/wiki/Ernst_Abbe

Yeah, the debate starts to pick up again when modern commercial scopes are starting to be more available and affordable. Of course there are examples prior.
I just think technical debates about it won't end because there are so many factors about it. Funnily it also birthed to "Mirror APOs" which circlejerks from mirror users that "well just use reflectors because then it'd be "CA-free!"

I've gone behind debating about it since I use classic scopes, it doens't matter much to me anymore, hahaha

Bigzmey wrote: ↑Wed Jun 02, 2021 10:21 pm The topic could be as simple or as confusing as people make it. It often comes down to "my scope is more APO than yours because it has FPL 53 glass", there is no hope there.

However, the way I see it is that if you can't detect CA visually or by AP then the scope is APO for your purpose. It could be slow achro, or well made ED doublet or (obviously) reflector or SCT.

Easy to decide/test for scopes you already have, but purchasing the next one could be complicated. I have been thinking about 6" APO/ED scope for awhile. ED doublets are more affordable, lighter and easier to balance, but my guts tell that I will see CA in them no matter what people say. 6" APO triplets... they are just too rich for my wallet. At the end I went with 9.25" Edge SCT for about half of the price of new 6" ED doublet.

Refractordude wrote: ↑Wed Jun 02, 2021 10:32 pm

Bigzmey wrote: ↑Wed Jun 02, 2021 10:21 pm The topic could be as simple or as confusing as people make it. It often comes down to "my scope is more APO than yours because it has FPL 53 glass", there is no hope there.

However, the way I see it is that if you can't detect CA visually or by AP then the scope is APO for your purpose. It could be slow achro, or well made ED doublet or (obviously) reflector or SCT.

Easy to decide/test for scopes you already have, but purchasing the next one could be complicated. I have been thinking about 6" APO/ED scope for awhile. ED doublets are more affordable, lighter and easier to balance, but my guts tell that I will see CA in them no matter what people say. 6" APO triplets... they are just too rich for my wallet. At the end I went with 9.25" Edge SCT for about half of the price of new 6" ED doublet.

It only cost five cents to turn my 150mm f/8 achro into a 150mm f/12 APO with a 4 inch cardboard mask. Nice

Refractordude wrote: ↑Wed Jun 02, 2021 10:32 pm It only cost five cents to turn my 150mm f/8 achro into a 150mm f/12 APO with a 4 inch cardboard mask. Nice

Lady Fraktor wrote: ↑Thu Jun 03, 2021 4:34 am

Refractordude wrote: ↑Wed Jun 02, 2021 10:32 pm It only cost five cents to turn my 150mm f/8 achro into a 150mm f/12 APO with a 4 inch cardboard mask. Nice

It will not be a apochromatic, the colour blur will be lessened slightly though.

Lady Fraktor wrote: ↑Thu Jun 03, 2021 8:59 pm Have you checked focuser alignment and lens collimation?
Granted these are large achromats with simple magnesium coatings but they do a good job.

Refractordude wrote: ↑Thu Jun 03, 2021 9:09 pm

Lady Fraktor wrote: ↑Thu Jun 03, 2021 8:59 pm Have you checked focuser alignment and lens collimation?
Granted these are large achromats with simple magnesium coatings but they do a good job.

The Meade can not be collimated.

Refractordude wrote: ↑Thu Jun 03, 2021 9:09 pm

Lady Fraktor wrote: ↑Thu Jun 03, 2021 8:59 pm Have you checked focuser alignment and lens collimation?
Granted these are large achromats with simple magnesium coatings but they do a good job.

The Meade can not be collimated.

Lady Fraktor wrote: ↑Thu Jun 03, 2021 10:01 pm I looked at some images online and it appears the Meade does not have a push/ pull system

notFritzArgelander wrote: ↑Thu Jun 03, 2021 10:09 pm

Lady Fraktor wrote: ↑Thu Jun 03, 2021 10:01 pm I looked at some images online and it appears the Meade does not have a push/ pull system

Well that’s disappointing. If you could collimate the optics would be fine. I sometimes think about reacquiring the scope. Since the Celestron version seems to have a push-pull cell, it can still be considered.

Bigzmey wrote: ↑Thu Jun 03, 2021 10:51 pm

notFritzArgelander wrote: ↑Thu Jun 03, 2021 10:09 pm

Lady Fraktor wrote: ↑Thu Jun 03, 2021 10:01 pm I looked at some images online and it appears the Meade does not have a push/ pull system

Well that’s disappointing. If you could collimate the optics would be fine. I sometimes think about reacquiring the scope. Since the Celestron version seems to have a push-pull cell, it can still be considered.

Orion/Celestron/SW achros are all made by Synta. I believe Meade outsources to a different company.

Bigzmey wrote: ↑Wed Jun 02, 2021 10:21 pm The topic could be as simple or as confusing as people make it. It often comes down to "my scope is more APO than yours because it has FPL 53 glass", there is no hope there.

However, the way I see it is that if you can't detect CA visually or by AP then the scope is APO for your purpose. It could be slow achro, or well made ED doublet or (obviously) reflector or SCT.

Easy to decide/test for scopes you already have, but purchasing the next one could be complicated. I have been thinking about 6" APO/ED scope for awhile. ED doublets are more affordable, lighter and easier to balance, but my guts tell that I will see CA in them no matter what people say. 6" APO triplets... they are just too rich for my wallet. At the end I went with 9.25" Edge SCT for about half of the price of new 6" ED doublet.

dagadget wrote: ↑Fri Jun 25, 2021 5:32 pm

Bigzmey wrote: ↑Wed Jun 02, 2021 10:21 pm The topic could be as simple or as confusing as people make it. It often comes down to "my scope is more APO than yours because it has FPL 53 glass", there is no hope there.

However, the way I see it is that if you can't detect CA visually or by AP then the scope is APO for your purpose. It could be slow achro, or well made ED doublet or (obviously) reflector or SCT.

Easy to decide/test for scopes you already have, but purchasing the next one could be complicated. I have been thinking about 6" APO/ED scope for awhile. ED doublets are more affordable, lighter and easier to balance, but my guts tell that I will see CA in them no matter what people say. 6" APO triplets... they are just too rich for my wallet. At the end I went with 9.25" Edge SCT for about half of the price of new 6" ED doublet.

Going to have to let you try my AT 152EDT sometime. Can you spell road trip? Or not but my AT 152 does not have FPL 53 but I have not seen color in it either. FK-61 Glass with Lanthanum and at F/8 I have pushed it very hard indeed. Very nose heavy but that I can deal with.

Bigzmey wrote: ↑Fri Jun 25, 2021 5:54 pm but I love to hear about other people experiences with 6" fracs.

John Baars wrote: ↑Sat Jun 26, 2021 11:10 pm

Bigzmey wrote: ↑Fri Jun 25, 2021 5:54 pm but I love to hear about other people experiences with 6" fracs.

Some economic priced Skywatcher refractors have no push-pull screws. I had a discussion with a another Dutch owner of a 150mm F5 Startravel refractor like me. He wanted a better red-focus and did this by decreasing the spacing between the two lenses.

More important was the fact that he noticed the collimation changed every time he screwed the lens-cell on the tube. Even if the lens cell ended in the same position as before. A strong indication of the coarse ( maybe even slightly skewed? ) thread on the tube. He thought of placing a spacer in the lens cel underneath the lens, but that was not the right solution. The final configuration kept on being unpredictable.

He solved the problem by winding some gas fitter tape around the threaded end of the tube and than screwing the lens cell on. Very near to the final quarter turn he started looking at his collimating device and stopped when collimation seemed ok. Result is a well collimated instrument, even though the lens cell has no push-pull screws.

I thought you might want to know.