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Is upgrading from a doublet to triplet refractor worth it

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CrossoverManiac's picture
Is upgrading from a doublet to triplet refractor worth it

I have a 66mm f/6 doublet refractor that I was going to use for the brighter stars.  I got my scope in focus for my B and V filter but it was out of focus with my Ic filter.  Being used to a reflector, I forgot about chromatic abberation.  I was thinking about forking over $1K for a triplet and a 0.8x field flattener so I could use all three of my photometric filters, but I'm not sure if a triplet would be colored corrected in near infrared.  Does anyone have any experience with using a triplet?  Also, I know some people here do use an achromatic refractor as well and I would like to hear from them.

HQA's picture
doublet vs triplet

I've used both, and find that the filters limit the bandwidth sufficiently that it really doesn't make much difference in image quality.  What you will find is that any refractor will have different focus over the wide range of wavelengths available with a silicon sensor, so what is needed is an electronic focuser.  Investing in that is probably more important, and cheaper, than investing in a high-end, multi-element refractor.  If you want to use the system for visual observing, then of course go for the best telescope.  My $0.02


I second to that!

I second to that!

When I did set up "BSM Tartu" (consisting of a ED refractor William Optics Zenithstar 61, ASI183MM, and Astrodon B,V,R,I,u2,g2, and SA200)  for commissioning, I found that B,V,R share reasonably the same focus for photometry. Stellar images in Sloan g are much softer, Sloan u and especially Johnson-Cousins I are way-way out of focus.I was somewhat surprised that Ic is so macroscopically off, even if it is clearly out of nominal chromatism compensation wavelength region - it really depends on the exact receipt of the lens.

CrossoverManiac, the positive side is, that when stellar images are "soft" and there is a lot of signal, you may even win from that situation - more photons are collected, maybe with somewhat longer exposures that suppress scintillation. If you don't care about pretty pictures, just find a sweet spot where all the filters you're interested in are giving similarly out-of-focus images and fix it. If you take flats, defocusing is the same and everything is going to be fine. But of course, with that small telescope, that really works OK for bright(er) stars.

Best wishes,

Bernhard's picture
Skywatcher 102/500 mm


i've wanted to start out photometry my 8 inch Newtonian and i've bought a used Skywatcher 102/500 mm achomat (130€) for looking at Messier objects, while my Newtonian is collecting photons. Guess what, my Newtonian has an old 1,25 " OAZ, where my Canon could not be fixed well... So as a last ressort i swiched to the 4 inch achromat and i use it for photometry since then. ( I am installing a new OAZ for the Newton soon...)

With iso400 and 60 seconds exposure lights i can reach until magnitude 13,3.

Althoug around 12,5 to 13 is the end for the Muniwin photoemtry software.


With high magnitude stars e.g. mag 6,5,4,3,2,1... The achromat has a bright purple light around the stars, which may affect the sky anulus. But so far i have observed stars wirh max 7 th magnitude. And i had no problems with them. Regarding filters, a DSLR uses the Bayer Mask with the 4, R G G B channels. I have heard that ubvri filters for ccds are reducing the V magnitude by 2 times. While thus is not affecting the bayer matrix..

Regarding focus: i focus ..ähm de focus once, fix it and use it for months! I never change the focus, even if i have to put the scope away.. (-:


If i would buy a telescope now, i would go for the skywatcher 150/750 mm achromat for around 650 bucks. In light polluted cities, the 6 inch has advantages over a 10 or 12 inch mirror, because of the lower magnified sky..

What i have learned, you do not need an expensive scope! Every scope has their stars, but if one likes to spend a lot of money for sports cars, it is ok to invest money into a telescope. Mmhhh maybe you can invest into a semi robotic telescope, which can be controlled via a pc. My semi automated balcony observatory with 4 inch and 8 inch telescope, is now more efficient, than a big 10 or 12 inch telescope, which i would have to set up for each observation somewhere at a far dark site..

kindly WBEA

Bernhard's picture
Plejades with Achromat

Here i have a picture of the plejades, taken with the 102/500mm achromat.

I don't know how the purple color is affecting photometry, but usually i measure stars at magnitude 7. So the purple color is bigger at V=1...4 mag

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