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Old 04-07-2018, 03:12 PM
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APM 140mm APO doublet

I am considering this scope:

https://www.teleskop-express.de/shop...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.
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Old 04-07-2018, 03:30 PM
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Have you looked at the Skywatcher Esprits Greg ? they come in a few of the larger sizes .
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Old 04-07-2018, 03:54 PM
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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/6...iday-july-4th/
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Old 04-07-2018, 06:48 PM
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Quote:
Originally Posted by Marke View Post
Have you looked at the Skywatcher Esprits Greg ? they come in a few of the larger sizes .
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.
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Old 04-07-2018, 06:50 PM
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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/6...iday-july-4th/
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.
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Old 04-07-2018, 06:57 PM
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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.
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Old 04-07-2018, 07:04 PM
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Slawomir (Suavi)
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I would not recommend a doublet for imaging.
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Old 04-07-2018, 07:50 PM
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I would not recommend a doublet for imaging.
Ditto.
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Old 04-07-2018, 08:07 PM
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Tritto
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Old 04-07-2018, 08:25 PM
Hans Tucker (Hans)
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I would not recommend a doublet for imaging.

The Takahashi FS series refractors seem to produce good results.
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Old 05-07-2018, 05:47 AM
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The Takahashi FS series refractors seem to produce good results.
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).
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Old 05-07-2018, 08:38 AM
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Quote:
Originally Posted by gregbradley View Post
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.
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 .
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Old 05-07-2018, 08:54 AM
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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.
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Old 05-07-2018, 09:30 AM
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Tritto
Quadrutto
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Old 05-07-2018, 10:19 AM
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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.
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Old 05-07-2018, 10:32 AM
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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.
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Old 05-07-2018, 10:34 AM
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Quote:
Originally Posted by gregbradley View Post
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.
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/el...#ixzz5KL6YUg51


https://www.sciencedirect.com/topics...nthanum-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.
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Old 05-07-2018, 10:35 AM
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Quote:
Originally Posted by Atmos View Post
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.
I have seen more violet with Borg than I like.
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Old 05-07-2018, 10:51 AM
glend (Glen)
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There is a recently released TS 125m f7.8 Doublet that uses FPL-53 and Lanthanum elements.

https://www.teleskop-express.de/shop...--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.
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Old 05-07-2018, 03:14 PM
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Originally Posted by glend View Post
There is a recently released TS 125m f7.8 Doublet that uses FPL-53 and Lanthanum elements.

https://www.teleskop-express.de/shop...--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.
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.
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