Why no slow APOs?

[West End Wendy]

Televue have recently discontinued their 102 telescope and their website suggests the NP101 as an alternative. The 101 is a much faster (not to mention more expensive) scope. So where are the slow APO refractors for visual observers?

The 102 is/was a doublet but the literature described it as an APO. My own ED100 is a slow doublet. Virtually everything else on the market (frac-wise) is either a fast APO or a slow achromat.

I appreciate that slower scopes show less CA intrinsically, and I also appreciate that visual observers may tend towards larger aperture scopes with designs that don't produce CA, but surely there is a market still for users who want a frac, don't want to see CA, and aren't into astrophotography?

Tim

[John]

I think it's because the main market for apo's is perceived to be imagers Tim. They tend to want faster scopes.

I'm a visual person and would appreciate a slower apochromat but I would find an F/12 or F/15 achromat (the sort of focal ratio you need to really reduce CA) unweildy to mount.

I guess there are just not enough of us around to grab a share of the market

Hang on to your ED100 - I certainly regret selling mine.

[Trull]

You can always barlow up a f/8 APO... or get creative and build a f/15 achromat which isnt as hard as many would make you believe. But I would think that a barlowed apo would outperform an achromat in real life (wind shake etc...)

If you have deep pockets there are some f/45 refractors around the country...

[West End Wendy]

I would think that a barlowed apo would outperform an achromat in real life

Yes, and I guess that's effectively what happens, what with modern EPs with barlow elements in them.

I suppose a short focal length eyepiece no longer presents the same problems it used to.

[ollypenrice]

The big ones tend to be slower, around F8. Start saving!!! You make a good point but I guess sales figures do the talking, which is a pity.

Olly

[spaceboy]

I may be wrong but I think it's because it costs more to make slower scopes. APO refractors are already expensive as it is to make so making a long FL APO would make the price tag less desireable to the bulk of astronomers. I did read some thing about longer FL costing more so I will try to find the article and post the details later.

[Alma]

I may be wrong but I think it's because it costs more to make slower scopes. .

Not really, if anything it's the other way round. Most people who use refractors for visual use though, prefer an f/6 to f/7 focal ratio in order to achieve a very broad field of view range.

With a 31mm Nagler in an f/6 refractor you should obtain something like a 4 degree true field and a very low magnitude of something like 20x. If it's a really good apo, you could then go all the way up to a 3.5mm Nagler and get 190x mag and a 0.4 degree field with still great views. No other scope can offer that level of flexibility.

Long focal length refractors provide great planetary & lunar views but can't show wide fields, plus their physical length tends to make them more awkward to observe through. With very good SCT's at much the same focal length these days and with wider aperture, there just isn't a huge consumer demand for long fl refractors, I'm afraid.

Alan

[spaceboy]

Not really, if anything it's the other way round.

Alan

You are correct Alan. Terrible memory I have. I found what I had read in Philip's "Stargazing with a telescope" P38 QUOTE: To make a lens with a shorter focal length, you must increase the curvature of the lense. This is a demanding task, involving skilled work, and it increases the risk of false colour and no doubt cost.

[ollypenrice]

You are correct Alan. Terrible memory I have. I found what I had read in Philip's "Stargazing with a telescope" P38 QUOTE: To make a lens with a shorter focal length, you must increase the curvature of the lense. This is a demanding task, involving skilled work, and it increases the risk of false colour and no doubt cost.

Sad but true. A Tak FSQ 85 capable of working perfectly at F3.9 comes in at the price of three ED120s...

Also fast systems need better mechanical construction because the depth of field is so shallow that you need a really fine focus control and one that remains dead square to the light path.

Olly

[spaceboy]

For what it's worth it also goes on to say that APO refractors should not be stored in conditions similar to an observatory (damp/out door) as this reduces the life of the optics. While I'm sure there are not many that would leave several thousand pounds worth of APO in an obsy there may be plenty who store it in a similar environment to allow for shorter cool down times.

[John]

For what it's worth it also goes on to say that APO refractors should not be stored in conditions similar to an observatory (damp/out door) as this reduces the life of the optics. While I'm sure there are not many that would leave several thousand pounds worth of APO in an obsy there may be plenty who store it in a similar environment to allow for shorter cool down times.

I've heard that Flourite crystal can be rather fragile but I'm not sure if that applies to FPL-51, FPL-53 and similar ED glasses

Interesting point though.

[Peter Drew]

I have a 5" triplet APO which is F15 so they do exist. It's a self built telescope based on a Ross triplet refigured by the late Horace Dall in the '70's.

[ollypenrice]

For what it's worth it also goes on to say that APO refractors should not be stored in conditions similar to an observatory (damp/out door) as this reduces the life of the optics. While I'm sure there are not many that would leave several thousand pounds worth of APO in an obsy there may be plenty who store it in a similar environment to allow for shorter cool down times.

Do you have a source for this? I'm not saying I don't agree, I just don't know about it. The only thing I'd heard , like John, was about fluorite and, in my case, its dislike of damp. Mind you many apos are fluorite free nowadays.

Olly

[Damo636]

Other than the obvious lack of CA in an Apo, what other advantages (optically) have they over a long focal length achro? My f11 achro produces a lovely clean sharp image and i have to really look for the CA to notice it.

[John]

They are easier to mount (shorter tubes) and can deliver wide views as well as handling high powers.

I'm not going to get into a short apo v's long achro debate though - that's Neil English's territory !

[spaceboy]

Do you have a source for this? I'm not saying I don't agree, I just don't know about it. The only thing I'd heard , like John, was about fluorite and, in my case, its dislike of damp. Mind you many apos are fluorite free nowadays.

Olly

It's quoted on the same page in Philip's stargazing with a telescope. It only refers to APO refractors as being at risk. No similar coments for ED refractors but I was under the impression these also contained fluorite in one of the lens. Sorry for the poor quality of the scan.