APM 140mm APO doublet

[gregbradley]

I am considering this scope:

https://www.teleskop-express.de/shop/product_info.php/info/p9109_APM-Doublet-SD-APO-140-f-7-FPL53-OTA.html

For dual purpose visual and for astrophotography with a KAF16200 sensor and perhaps a Proline 16803 if it can cover it.

I am not sure about a doublet being colour free. Its FPL53 and the 2nd element is Lanthanum. I have used a Tak FS152 before which was fluorite and crown glass element doublet. It was great visually but a bit false colour in images.

What do you think?

The other choice is either a high end 107mm F6 triplet or a medium end same size.

4 inch scopes are a bit lacking in aperture for anything other than widefield star scenes in my experience though.

There seems to be a lot of air spaced FPL53 triplet scopes on the market these days. They seem to have good colour correction and appear to be from Kunming in China up to 150mm in aperture.

Greg.

[Marke]

Have you looked at the Skywatcher Esprits Greg ? they come in a few of the larger sizes .

[casstony]

In CN Vendors section there is a sale today only on the 140, shipped worldwide for US$2500. (around AU$4000 with gst)
https://www.cloudynights.com/topic/623719-flash-deal-for-americans-holiday-july-4th/

[gregbradley]

They seem more expensive. I guess I know Skywatcher from its earlier days and don't know anything about their more expensive offerings. Are they any good? They seem 2/3rds the price of a Tak.

Greg.

[gregbradley]

Thanks for the link Tony.

I am not ready to buy yet. I don't even know if its a good scope.

Its probably fine for visual but not being a triplet will it be any good for imaging or will it give blue ringed bright stars like most doublets do (Tak FS scopes).


Greg.

[Atmos]

From what I have read lanthanum does help with control in the blue end of the spectrum but it is still a F/7 doublet so I wouldn’t hold my breath for perfect colour correction.
I’d say it is probably good for visual but lacking in what you’re after.

[Slawomir]

I would not recommend a doublet for imaging.

[Peter Ward]

Ditto.

[LewisM]

Tritto

[Hans Tucker]

The Takahashi FS series refractors seem to produce good results.

[Slawomir]

A very good doublet at a slowish (for imaging) focal ratio will produce good results. However, a very good triplet will yield photographically better result at the same focal ratio.

Blue end of the spectrum usually suffers in a doublet. Attached is a neat chart from Takahashi comparing chromatic correction of a triplet and a doublet both with the same aperture and focal ratios.


@Greg - unless there are reasons not to go with CFF, I think their limited edition 135mm f/6.7 triplet with 3.5FTF might be just the right scope for both visual and imaging (it has a large dedicated corrector resulting in a 55mm corrected imaging circle).

[Marke]

They are very good and while they seem expensive they do come with their own FF , very good focuser which starlight make a motor for and a large lockable carry case.
A Tak 120 would be around $7k the SW 120 $4k nearly half .

[LewisM]

The FS series produce wonderful images but not up to the level of a good triplet (but leaps ahead of cheaper triplets). I have seen many great FS and FC images but all show a little blue.

A TSA 102 or 120 is essentially perfect. An AP 130 GT or an APM LZOS or LOMO triplet would be divine.

The Esprits are good...BUT you really need to work on them a little focuser wise. Once adjusted, the stock focuser is really quite good. I have though seen some as good as AP images taken with the 120 and 150 once replaced with an FT.

[troypiggo]

Quadrutto

[gregbradley]

Unless there were some graphs and images that prove otherwise it seems a doublet is risky. I do recall Roland Christen saying it was possible to make an APO doublet but I don't see any AP scopes using that formula.

Thanks for the advice. Also a bit suspect about Lanthanum and whether its entirely non radioactive or is there a tiny portion that is.

Greg.

[Atmos]

You can make a very well corrected APO doublet but I doubt it’ll be as fast as F/7 for good correction.

The alternative is to create matched flatteners like Borg; doublets with 4-element correctors. Not that I’ve been overly impressed with their performance from what I’ve seen.

[LewisM]

If you are concerned about lanthanum in glass, you'd better throw away your TV, fluorescent tubes, camera lenses...

"Lanthanum is one of the rare chemicals, that can be found in houses in equipment such as colour televisions, fluorescent lamps, energy-saving lamps and glasses. All rare chemicals have comparable properties. La2O2 is used to make special optical glasses (infrared adsorbing glass, camera and telescope lenses). If added in small amounts it improves the malleability and resistence of steel. Lanthanum is used as the core material in carbon arc electrodes. Lanthanum salts are included in the zeolite catalysts used in petroleum refining because they stabilize the zeolite at high temperatures.

Health effects of lanthanum
Lanthanum is mostly dangerous in the working environment, due to the fact that damps and gasses can be inhaled with air. This can cause lung embolisms, especially during long-term exposure. Lanthanum can also cause cancer with humans, as it enlarges the chances of lung cancer when it is inhaled. Finally, it can be a threat to the liver when it accumulates in the human body."

Read more: https://www.lenntech.com/periodic/elements/la.htm#ixzz5KL6YUg51


https://www.sciencedirect.com/topics/chemical-engineering/lanthanum-oxides

Of course, everything has a risk, and it seems at one point or another, EVERYTHING has been labelled carcinogenic (anyone else remember in the 80's when they tried telling us apples were?), so make your own informed decision. Seems Vixen is not concerned, nor many other manufacturers.

[LewisM]

I have seen more violet with Borg than I like.

[glend]

There is a recently released TS 125m f7.8 Doublet that uses FPL-53 and Lanthanum elements.

https://www.teleskop-express.de/shop/product_info.php/info/p10133_TS-Optics-PhotoLine-125mm-f-7-8-FPL53-Lanthanum-Apo---2-5--Focuser.html

They claim excellent correction and imaging suitability. And the price is pretty attractive (less than $2k AUD, before shipping and GST). It can be reduced to f6.16 using the 3" Photoline Reducer.

[gregbradley]

Thanks for the tip. The price is very attractive. They really need to show some example astrophotos I don't like taking marketing blurbs as my reference.

But it seems way better value than the 140.

Greg.

[rustigsmed]

For a bit more wouldn't you go for the ts 130 f7 triplet?

https://www.teleskop-express.de/shop/product_info.php/info/p6679_TS-Optics-PHOTOLINE-130-mm-f-7-Triplet-APO---FPL53---2-5-inch-RPA-focuser.html -
there is a more expensive one if you want a larger focuser.

[casstony]

I wonder if it's a mistake for a fussy imager to try to make one scope do two jobs? :)

The NP127is might work though?

[glend]

Well the question related to Doublets, not Triplets. It does not reflect my personal preference. As a personal preference, yes I would prefer the 130mm FPL-53 Triplet.

[gregbradley]

Sounds like a nice instrument. FPL53 air spaced triplets were a rare type of scope not that long ago.

How times have changed.

Greg.

[gregbradley]

Petsvals as I understand them generally are not good for visual.
My TEC180FL was good at both visual and imaging so not necessarily. But if its a Petsval then yes its more oriented towards imaging and not visual.

I think I am more interested in an APO triplet and F7 with a nice reducer/flattener available.

Greg.

Greg.

[casstony]

In terms of planetary viewing the NP's are only marginally behind the Tak doublets and triplets and the NP is designed to be easy to use for visual.

The most noticeable difference between my NP101is and FC100d is depth of focus due to the focal ratio's and the FC is a little more contrasty I think (need to get them side by side on Jupiter again).

An NP127 is a lot pricier than a TS triplet though.


Here's an interesting review: http://doctordreviews.com/4inch-shootout.html

[FlashDrive]

Get this one Greg .....

https://www.cloudynights.com/topic/6...ope/?p=8677450 (https://www.cloudynights.com/topic/621007-show-us-your-lzos-telescope/?p=8677450)

Credit to denis0007dl of CN

Col....

[gregbradley]

Thanks for that. I thought about a TV NP127i but to be honest I have not seen any spectacular images using that scope. Good ones but not top notch.
I suspect the correction is not as good as it needs to be these days to stand out. If China is putting out respectable air spaced FPL53 triplets for 1/3rd the APM LZOS price then the playing field has changed. Air spaced triplets were only done by TMB/LZOS up to a few years ago.

We also now have FPL 55 glass making its appearance. I am not sure how much better it is over FPL53, I suspect only a little.



Greg.

[gregbradley]

Here's an interesting review: http://doctordreviews.com/4inch-shootout.html[/QUOTE]



hehe.

Greg.

[Atmos]

From what I’ve read FPL-53 has a higher abbe number (closer to fluorite) but FPL-55 is easier to figure and possibly has better mating glasses.

[gregbradley]

Yes almost identical Abbe number. Better grinding capability may end up meaning a higher strehl number for the optician to achieve.

Greg.

[LewisM]

With hints at a possible Roland Christen retirement, the end of the GTX-130 line, and a SHORT run of the 92mm Stowaway, if you want anything touched by the hand of Rolando, I'd jump now (though you cannot get on EITHER waiting list as both are closed).

Otherwise, the Tak 120 is VERY hard to beat, except with the TOA 130 (which is, for all intents and purposes, perfect).

[gregbradley]

Yes I am on both of those lists. The AP 130GTX list I got on very early.

I can see those 2 Tak offerings are very nice. The TOA though is often said to be heavy and front heavy at that. Not sure if that is a real problem in use though. Too bad they don't do a TOA110. That would be nice.

The Tak 120 does sound nice. The offerings are odd though in that the TSA line is 102 and 120 and TOA is 130 and 150. I wonder if there is a reason for that other than marketing and a change of direction.

Greg.

[LewisM]

TSA line is ONLY 120 now - the 102 has been out of production a few years now, replaced - oddly - with the FC-100 (not a wise Tak choice to cancel the 102 IMHO, as it was THE best 4" scope I have ever owned or used). I expect the FC-100 sells better, being considerably cheaper than the 102 ever was.

The design of the TSA is pure triplet, whereas the TOA is well-spaced triplet. The new fluorite FOA shares the same design principle, and is one of the highest Strehl production scopes ever made (it's for all intents and purposes a poly-Strehl of 0.995)

[beren]

Its a shame Tak don't have a "F" version of the TSA-120 with the larger focuser like the TOA-130. For visual its fine but hate the TSA-120 focuser for imaging, tried a few electronic focusers like robofocus and nothing worked when I had the large STT-8300.

[Merlin66]

I use for visual an original TV Genesis.....amazing!
Certainly nothing wrong with the TV Petzval designs.

[gregbradley]

Thanks Lewis. I wondered why I could only find the TSA120 listed.
What's with the tiny focuser and no dual speed at US$4430?

First thing would be to throw out the focuser and put a Feathertouch on it.

FOA is only the 60mm right? Sounds good but very small aperture.

If they could make a 110mm FOA it would be attractive.

Greg.

[gregbradley]

Yes it was the first thing I noticed about the scope, the tiny focuser. It would only handle small sensored cameras. Plus no dual speed ?? uuhhh what? Every scope I have seen recently listed has a dual speed focuser - every single one.

But then a lot of scopes need their focusers replaced. Too bad Tak doesn't sell OTAs with no focuser!! haha.

FC100 I take it is a fluorite doublet. It replaces the FS102?


Greg.

[Atmos]

The main difference between the FS102 and the FC100 range is that the FS102 had the fluorite element at the front where as the FC100 has the fluorite element at the rear. The FS series may have also been a bit beefier.

[LewisM]

Stuart,

You can buy the 4" focuser directly from Tak and fit it to the TSA-120 directly - both share the 125mm tube.

Cost is...well... so FT may be a better option lol.

I personally find the Tak focuser just fine. Fitting the dual speed add on is so simple too, and not expensive (the FSQ-106ED's of course come dual speed as standard, unlike the old N model)

[Wavytone]

Having had both an APM 130mm f/7 triplet and a 102mm f/7 ED doublet with an FPL53 element, if I was after a refractor again I'd buy a slightly smaller triplet in preference to a slightly larger ED doublet.