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Doublet vs Triplet

peleks
19 Aug, 2019 11:05
Hello good people, i have a thought to buy a refractor telescope, in specifications it is said that triplets have better color correction than doublets, i.e. they are all more close to one point, but i was thinking, if  i am using Mono camera with filters, in theory i should achieve same results with doublet that are with triplet and color camera, just that i need to refocus after swapping filters, only thing that must be is an ED glass, so i wanted to know if anyone have an expirience with that. its just that doublets are cheaper than triplets, so i prefer bigger aperture and less color correction if its not a big issue with mono camera
whwang
19 Aug, 2019 11:49
Triplets do not only have better color correction, but also have better correction for spherical aberration. So even if you use a filter, even a narrow-band filter, you will find that triplets give you sharper stars than doublets, provided that they have similar aperture, F-ratio, and glass material.

That being said, other issues are whether you can notice the difference, and whether your skill levels are good enough to make that difference apparent (i.e., making all other factors under control, such as focusing, guiding, processing, etc). Under this consideration, I would suggest you to start with the cheaper option. Once you become experienced enough, you will know whether the optics is what limits you.

Cheers,
Wei-Hao
peleks
19 Aug, 2019 16:16
Thanks Wei-Hao, i was thinking about that, i have experience with guides/focusing just that i want a bit smaller scope so i can use it in wind, now i have 250/1250 newton on HEQ5, in windless nights guiding is not an issue, but if wind is greater than 2 m/s  + gusts, then that big sail start to move smile
astropical
19 Aug, 2019 23:08
Hello peleks,

Regarding wind, I appear to be a bit better off with my 6" Newtonian than you with your 10" smile
Nothing against the reflector, its is a fine scope, but for the same reason also including size and weight considerations,
and further trying to be gentle to my mount, I decided to get the small TS-Optics 71SDQ which, being a quadruplet, saves
me notable expenses for optional flattener and distance rings.
The aperture is merely Ø71mm, but it is for AP and provides brilliant optics. Its focuser is specified to handle up to 4kg,
more than the 3.2kg weight of the OTA.

The only cons I can see are a loosely sitting lens cap and a short dovetail, but this can be helped. smile

All the very best,
Robert (still a novice though)
XE1ZLG
20 Aug, 2019 03:05
If you plan to shot almost Nebula targets  with broadband  nebula filters a 6" achromat doublet is hard to beat ( see may M27 dumpbell pic here on Astrobin ) and it will be far less expensive than a 4" or 5 " Apochromat.   If you plant to shot globular clusters and Galaxies… A triplet is better option.  My brother uses its Orion 130 Triplet and very happy with it.

My personal recommendation will be APM 140 doublet… It is an ED buth in real life performs more like an Apochromat  ( VERY WELL CORRECTED Lanthanum Glass and not too heavy ) ! Also APM sells a Riccardi Reducer Flattener that makes the refractor very appealling.

Clear Skies.. Luis
Edited 20 Aug, 2019 22:31
Starstarter86
21 Aug, 2019 13:02
Hi Peleks,

I agree with astropical: Modern doublets have become very good, especially the ones using FPL53 glass elements have a very good colour correction.
I have been shooting with my TS-Optics Photoline 72mm f/6 FPL53 und Lanthanum Apo - 2.5" R&P focuser for about a year now and never noticed the slightest coulour fringing. If you buy it with the TSFLAT72 it's even cheaper than the 71SDQ, plus you get the option of using a TSRED279 Reducer to get a 340mm f4.8
https://www.teleskop-express.de/shop/product_info.php/language/en/info/p8866_TS-Optics-Photoline-72mm-f-6-FPL53-Apo---2-5--Zahntrieb-Okularauszug.html

Of course, high end Triplets will be better, producing smaller stars, but also at least twice the price.

Clear Skies, Marc
peleks
21 Aug, 2019 14:21
Marc Agostini
Hi Peleks,I agree with astropical: Modern doublets have become very good, especially the ones using FPL53 glass elements have a very good colour correction.
I have been shooting with my TS-Optics Photoline 72mm f/6 FPL53 und Lanthanum Apo - 2.5" R&P focuser for about a year now and never noticed the slightest coulour fringing. If you buy it with the TSFLAT72 it's even cheaper than the 71SDQ, plus you get the option of using a TSRED279 Reducer to get a 340mm f4.8
https://www.teleskop-express.de/shop/product_info.php/language/en/info/p8866_TS-Optics-Photoline-72mm-f-6-FPL53-Apo---2-5--Zahntrieb-Okularauszug.html

Of course, high end Triplets will be better, producing smaller stars, but also at least twice the price.

Clear Skies, Marc
i was looking at https://www.teleskop-express.de/shop/product_info.php/info/p7224_TS-Optics-Imaging-Star-80-mm-f-4-4---6-element-Flatfield-APO-Telescope.html this one, i like focal length so it will provide me with quite large fov smile + scope resolution / camera pixel scale and scope size will allow me to image even in stormy weather, of course i would go for 100mm apos, but 100mm apo with F5 are quite costly other ones if course i can buy and add reducers, but the size  of tube will still be quite long
Starstarter86
21 Aug, 2019 16:56
peleks
i was looking at https://www.teleskop-express.de/shop/product_info.php/info/p7224_TS-Optics-Imaging-Star-80-mm-f-4-4---6-element-Flatfield-APO-Telescope.html this one, i like focal length so it will provide me with quite large fov smile + scope resolution / camera pixel scale and scope size

Looks very good as well, if you look at the examples you can find on astrobin. However, on a 4/3 sensor like the ASI1600 you can get some distortions in the edges: https://www.astrobin.com/313054/0/?nc=user

But If you're shooting with small sensors, this should not be an issue.
peleks
21 Aug, 2019 16:58
Marc Agostini
However, on a 4/3 sensor like the ASI1600 you can get some distortions in the edges
  Well i have 16mm diognal sensor, so i do not think i will have any trouble with that smile
 
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