Apochromatic vs Achromatic Refractor

People should think twice before purchasing an apochromatic refractor as opposed to an achromatic telescope. There is a huge difference in price and a small difference in performance according to an experiment I made recently.

In doing astrophotography, people use to spend a lot of money in equipment. It is important not to fall into the elitist apochromatic myth if you want to save some money.

I took a single 30 seconds shot to Pleiades open cluster (M45) using my 6-inch Sky-Watcher achromatic refractor that costs around 1,000 US$. Then I used a 6-inch apochromatic Takahashi TOA-150 that costs around 10,000 US$ to take an equivalent shot on the same field.

Resulting images speak by themselves. Obviously achromatic refractor shows a blue halo around stars, due to chromatic aberration. Nevertheless, a simple post-processing technique can remove the halo, obtaining a similar image to apochromatic’s.

Now, the question is: Is it worth paying the extra 9,000 US$?

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25 Responses to “Apochromatic vs Achromatic Refractor”

  1. Simon Says:

    Nice review !

    what about the image with a 6 inch reflector or bigger ?

    keep posting, always interesting,

    Thanks .

  2. More to the point, if you’re doing sequential color with a monochrome CCD and color filters you can focus each separately, so all that an apo adds is the ability to include a luminance channel. Further, if you do narrowband imaging (as is necessary for most of us due to light pollution), a good achromat is just as good as the apo and needs no additional processing.

  3. De Lorme Broussard Says:

    Hi, Thanks for the comparrison. Been thinking about a apo.{ I have C6″achro. right now} Would you say this holds true for planets also? Tired of wasting money just to find out. It’s hard to find the truth about equipment since sellers have an obvious bias. { to sell you the product]. Any help would be greatly appreciated.

    Thanks, De Lorme

    • computerphysicslab Says:

      Under big magnifications (planets and Moon) chromatic aberration (secondary spectrum) becomes more obvious. Achromatic refractor users get use to the false colors these telescopes yield. Probably, an apochromatic refractor will give a slightly better resolution but the cost multiplies by a factor of 5, at least.

  4. I’m looking at purchasing a used 6″ Sky-Watcher Achromatic Refractor (f8, 1200mm) with a EQ6 with SynScan [Both Good Condition]. They would like $900 for it and I’m wondering if its a good price/lens for astrophotography (my primary goal).

    I’m looking to do long exposures with a Guide Scope using a Nikon D90 (so hopefully less grain than the pictures above).

    What are your thoughts?

    • I should mention right now I’m shooting on an EQ1 and just using my regular camera lenses. Also the telescope is circa mid-2000’s

    • computerphysicslab Says:

      I bought my used 6″ Sky-Watcher Achromatic Refractor (f8, 1200mm) with a EQ6 (No SynScan) for 700 euros, more or less, a similar price to 900$. So it seems to me a very good price.

      Astrophotography through a 1200mm focal telescope is always difficult, very difficult. Nevertheless you will get nice results using short exposure times, around 30 seconds.

      • So price wise it seems okay, but how is it hard to get a photo from a larger aperture? I would of though a larger aperture that let’s in more light would be better.

      • computerphysicslab Says:

        More aperture is more light, yes, but when using large focal distance it is very difficult to track the stars with accuracy. Equatorial mounts like EQ6 have periodic error around 30 seconds. When using so large focal lengths 30 arcseconds of tracking error spoil any picture.

  5. I was planning to get a nextguide from celestron and a tracking scope, do you think that should adequately deal with the periodic error?

    • computerphysicslab Says:

      This will help, sure. Probably an autoguider will allow longer exposures with nice results.

  6. De Lorme Says:

    Hi, Again thanks for the review . What camera were you using to tke the pictures of the Pleades? I plan on buying the Atik Titan next month . Are there any specific filters that would take out the blue halo? I have the Baader semi-apo but have not used it yet. I know binning increases sharpness and decreases resolution but in real practical terms is binning good to achieve pixel to arc/secs thus matching telescope to camera? The Titan is a good match for my c6 refractor but with the Opticstar 142M { it has 1.4m, Titan 7.4um with 325,000} I would have to bin the 4.65 pixesl to get a good pixel to arc/sec match. What does your universe say? Thanks for being there. I’m really excited! can’t wait till spring! Thanks De Lorme

    • computerphysicslab Says:

      I used a DSLR camera Canon EOS 450d (Rebel XSi). I took out part of the blue halo using a Color-Fringe filter meant for achromatic telescopes. The binning is a good idea to match the seeing. Sometimes you get an under arcsecond seeing and others a 2 or more arcsecond seeing. It depends on weather mainly.

      Good luck!

  7. I know this if off topic but I’m looking into starting my own weblog and was curious what all is required to get setup? I’m assuming having a blog like yours would cost a pretty penny? I’m not very internet savvy so I’m not 100% sure. Any tips or advice would be greatly appreciated. Cheers

    • computerphysicslab Says:

      Making a blog like this is free. You just need to sign up at http://www.wordpress.com and create a new blog, using your preferred template and periodically adding new posts, and reviewing new comments. That’s all.

  8. Hi, very nice and informative blog. I recently ordered the Explore Scientific AR102 but am not really sure if I made the wrong decision because I was also considering the Orion ED80. Do you think the ED80 is a better scope than the AR102 because the AR102 doesn’t have an ED glass? I plan to use it to image although I am still very new to it. Would like to have something like the EQ6 also someday.
    TIA

    • computerphysicslab Says:

      Probably Orion ED80 delivers higher quality images, but AR102 gathers more light. So at deep sky AR102 would beat ED80, but for planets, maybe ED80 will give more detail.

  9. ED glass is supposed to eliminate CA right? But nowadays, CA can be corrected via PP. I think I’ll just stick with the AR102 for the FL and aperture. Thanks!

  10. Pinci Says:

    Hi
    I was thinking to get a short focus refractor (like 150mm f5 or f6) and using filter fringe and post process to see how much aber…i can eliminate…what is your opinion?
    Great site btw

    • computerphysicslab Says:

      Through a fringe killer filter you may enhance image a little bit, but never completely. F5 or F6 is too short focal relation and in these cases chromatic aberration is very strong. A great blue halo around stars and planet colors are different that real colors. In the Moon you will see a lot of chromatic aberration in the dark areas and the limb. A fringe killer will not cut that more than 50%.

  11. Andrew Says:

    Surley the difference between an achro and apo is obvious?
    People who have 5 or 6 inch apo’s rave about the sharpness and clarity of the view. I myself have mainly used achro’s but I am considering a 5 – 6 inch apo. I have never looked through an apo so am I going to be disappointed if I buy one?

  12. Rick V Says:

    What software did you use to remove the blue halos?

  13. well, you probably leave out the visual argument as you use astrophotography to make your point.. For anyone who doesnt wish to use astrophotography , let alone just take it out to gaze at the sky with own tired eyes… I suppose apochromatic is unbeatable. The reason is that aphochormatic pulls all color spectrum in one focus that will make stars brighter to the eyeballs than achromatic can. Even if achromatic can make up the difference through longer focal length beyond f/15 and at least 50% larger aperature, it will pull in more unwanted light pollution no differently than those light buckets with primary mirrors that requires use of filters. Apochromatic lens pulls in star light not the glow all over. Jupiter is a big glow ball and Venus could be worse if it is further from Sun and get higher in the sky.. Venus is usually no more than two hours away from the Sun at best.. Jupiter ruins everything around it for ten degrees away especialy those faint DSOs.. Apochromatic telescopes can be compared to LED televisions with blackest background as opposed to LCD televesions of yesteryears.. the beauty of astronomy is seeing heavenly objects in the blackest background money can buy. it is a contrast game ..
    To be fair … achromatic is fine for first time buyers and any star with higher than , say, mag 3 , which usually wont show much color abberation if any.. Sirius shows the most color abberation with its long bluish whipllike lashes around it that can make you so fearful of it…Someone ought to make a video clip of that !!

  14. If it requires high magnifications in addition to bright image and also
    good resolution to determine objects, it’s preferred that
    you opt for newer and high end telescopes.
    And should you not have a very lot of cash to shell
    out, it’s perfectly acceptable to begin with binoculars.
    The role as receiver of light can often be more essential compared to the
    magnification since it allows a glimpse of celestial objects visible or invisible hard spot with the
    naked eye.

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