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The Best Telephoto Lenses for Astrophotography


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The Best Telephoto Lenses for Astrophotography

Rudy E. Kokich

Over the last ten to fifteen years excellent apochromatic telescopes have become available for visual use and photography. Along with improvements in telescope mounts, camera technology, filters, and digital image processing, these have allowed amateurs to produce astrophotographs of nearly professional quality.

However, these APOs have a couple of drawbacks. One is the price, which starts around $800 for the smallest units, and rapidly climbs into thousands of dollars for larger apertures. Another drawback is the focal length. Most of these APOs have F ratios around 6.5, and are unable to comprehend in their field of view large celestial objects such as the Andromeda galaxy, the North America nebula, and comets. Some APOs can be fitted with pricey telecompressors, but those invariably result in vignetting and coma.

Especially for beginning astrophotographers, who should first invest most of their finances into a good telescope mount, telephoto lenses are an excellent and affordable solution. 135mm and 200mm lenses are suitable for wide angle star-field views, and comet and asteroid hunting, while 300mm lenses serve very well for the Andromeda galaxy, large emission nebulae, open clusters, and even larger globular clusters. Aside from being much more affordable, telephoto lenses are easier to transport, easier to mount and easier to guide, and are much more likely to produce encouraging results to a beginner.

Over the years, I have tried more than two dozen telephoto lenses, until I finally found three or four perfect solutions. But first, there are several general rules which must be understood.

Zoom lenses are entirely unsuitable for astrophotography due to prominent aberrations of every kind. They are by nature designed to compromise by magnification and distance, and are therefore not optically optimized at any single setting.

Because of chromatic aberration, no telephoto lens can be used at full aperture. The best ones listed below serve well with a one stop reduction, and some require two or even three stops.

Reducing aperture with the built-in aperture iris interferes with the light path, and results in eight diffraction spikes around bright star images. Some people like these, and consider them decorative. I do not presume to further decorate the universe, and perceive them for what they are: interference. I therefore reduce the aperture at the front end of the lens (as an aperture stop) by screwing in a series of step-down rings into the filter thread. These are affordably available on eBay, and result in perfectly round star images, the way nature intended them to be.

There is some controversy about the use of UV filters, but I found that a good UV filter significantly improves contrast, sharpens small star images, and reduces chromatic aberration. By far the best one is the Tiffen Haze 2 filter. Unfortunately it is not manufactured in a multicoated version, and produces prominent internal reflection artifacts on very bright stars. Nevertheless, it performs excellently on most star fields, and is too cheap not to acquire. The second best, is the Hoya Pro One Digital MC UV(0) filter. I use it routinely in preference to many other multicoated filters I tested, including the new Hoya MC UV©.

All lenses mentioned below are adaptable to Canon EOS cameras with slim EOS adapters which allow the lenses to focus just slightly past infinity. Focusing should be done on moderately bright stars using the 10x magnified Live View. Because of some residual chromatic aberration even with the aperture stop, the best focus lies not where the star image is the smallest, but rather just slightly away from infinity, at the point where the star image barely begins to enlarge. This way the focus will favor the red light which is more objectionable within a star image than a bit of blue.

No telephoto lens can be used with cameras modified by the removal of the internal UV/IR cut filter and anti-aliasing filter. Such "full spectrum" cameras are somewhat more sensitive in the ultraviolet, but much more sensitive in the deep red and infrared. No telephoto lens, and no apochromat, is sufficiently corrected to accomodate such a wide spectral range. Whereas quality apochromats can be corrected with broad band filters, such as the Astronomik UV/IR cut filter or the CLS-CCD filter, telephoto lenses can not. However, they can be perfectly corrected with narrow band H-alpha or OIII filters.

Finally, to prevent image shift during exposure, all telephoto lenses must be supported at two points: at the camera end, and at the far end with a large retaining ring. To prevent damage to the lens finish, apply nylon acorn nuts (or cap nuts) to the tips of the retaining ring's three alignment screws. The screws should be set sufficiently tightly to prevent shift, yet not so tightly as to interfere with fine focusing.

It may be superfluous to add, but it can't do any harm, that in astrophotography all shutter control must be done with a wired or wireless electrical shutter release swith. Touching the telescope, even ever so slightly, will introduce vibrations which will ruin the photograph. Also, accurate guiding is essential. If the telescope mount is precisely aligned to the celestial north pole, unguided exposures of one to two minutes are possible. A series of such images can be digitally stacked to produce excellent results. However, I find the process tedious, and prefer single, manually guided, long exposures which seem to have deeper colors. My guidescope is a 5in F5 Jaeger's achromat with a 2.3x Barlow, and a 9mm illuminated reticle eyepiece. This gives me the power of 162x, which is barely sufficient for my 420mm fl APO astrograph at full camera resolution. Extrapolating from this, minimum recommended guidescope power is 120x for the 300mm telephoto, 80x for the 200mm, and 55x for the 135mm.

The first telephoto lens of choice, especially recommended for beginners, is the 135mm F2.5 SMC Pentax. This lens has the Pentax K bayonet mount, and requires the K-EOS adapter for attachment to Canon EOS cameras. When the aperture is stopped down to 37mm using step-down filter rings, this lens produces incredibly tiny pinpoint star images from edge to edge. In this configuration, the lens is still a very fast F3.4. The lens is available on eBay for around $200. It must not be confused with the much cheaper SMC Takumar, often deceptively advertised as SMC Pentax Takumar, which has the M42 camera thread, and is plagued with unextinguishable blue chromatic aberration.

The best 200mm lens is precisely the older 200mm F4 SMC Takumar, which comes with the M42 camera thread, and requires the M42-EOS adapter. When stopped down to 37mm, at F5.4, it also produces perfect, small and round star images across the entire field. It has just a hint of chromatic aberration on very bright stars and, if highly enlarged by 400-800%, the stars in the very corners barely begin to show a touch of astigmatism. These lenses can be had on eBay in mint condition for around $70, and are probably the most price efficient optical instrument in the world. Take care not to confuse this lens with the 200mm F4 SMC Takumar 6x7 which has a different optical configuration, and which I have never tested. Also, the newer and much more expensive 200mm F4 SMC Pentax with the K mount is decisively inferior, showing small but annoying red chromatic aberration.

The next 200mm lens of excellent quality is the 200mm F4 Nikkor F which requires the Nikon F to EOS adapter. When stopped down to 37mm, F5.4, it is almost identical to the Takumar except that on highly enlarged images it shows a hint of coma in the distant corners. In excellent condition, this lens retails for around $200. Selecting between it and the 200mm Takumar was not an easy choice but, in the end, I chose the Takumar because it seemed to have slightly better contrast.

The one and only 300mm lens I tested is the Zeiss Tele-Tessar 300mm F4. There was no reason to test any other because, when stopped down to 49mm, F6.1, this lens is simply perfect, comparable to any APO on the market. It has no chromatic aberration, and no hint of star deformities in the corners. It requires the Contax-EOS adapter for attachment to the camera. I bought my lens in mint condition for $350 from Japan, but I see that some retailers are asking significantly more. This lens has only two drawbacks. One is its size and weight, which requires a sturdy support on the telescope. The other one is the inevitable and persistent regret that, because of chromatic aberration, the full 75mm aperture of this beautiful lens can not be used in full visible spectrum photography. However, I am convinced that its large aperture and fast F ratio would perform exceptionally well in three color or narrow band H-alpha and OIII photography.

M13, Hercules, Zeiss 49-300mm, exp 163 seconds, iso 800, Hoya MC UV(0) filter. Note the small galaxy NGC 6207 in
the 2 o'clock position from the great cluster. (Click the image to load full-sized version.)


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85 Comments

Very nice work, Peter. And, great photos too.

I still have a couple of unanswered questions.

What is the material and the source of your lens cell, holding the achromat lens?

What is the material and the source of the black tube in the front of the telescope holding the nested aluminum tube? That material looks like thick fiberglass or plastic.

Rudy

We're definitely off topic now. I'll PM that info to you. 

Photo
William Jin
Jul 23 2015 03:43 PM

Rudy,

 

Thank you so much for your article. I am a very beginner in astrophotography and your article has given me a different perspective to this hobby. 

 

After reading your article, I ordered from Ebay, 200mm F4 SMC Takumar M42 Lens (I hope I got the right one). I also ordered the M42-EOS ring. I have Canon 40D. I can wait until the items will arrive at my door.

 

I am also thinking about ordering the step-down rings (Is there a certain size that I need to be aware of?) You have mentioned 2 UV-filters. Which one should I get? I am also looking into a Zeiss Tele-Tessar 300mm F4. There are many similar kind under the same names. Confusing.

 

I am still confused about focusing "away-from- infinity" concept.

 

Anyway, thank you so much for your article.

 

Clear Skies,

 

William

William,

 

I think it is good to begin with simple optical equipment, then advance to longer focal lengths once you have learned the principles. No matter how experienced you become, a good 200mm lens will always serve you well for portable use to dark sites, for photographing large objects and comets, and for blinking for asteroids.

 

At this point, I would NOT yet buy the Zeiss 300/4 lens. That is a huge, heavy lens, and requires strong support and pretty accurate guiding.

 

On my Takumar 200/4 I have the following step-down rings: 58-52mm, 52-46mm, 46-43mm, and 43-37mm. Any other combination which brings the aperture down from 58mm to 37mm is just fine. When using step-down rings, you would keep the lens aperture wide open at f4.

 

The best UV filter is the Tiffen Haze 2, but it has no anti-reflection coating, and will produce internal reflection artifacts on bright stars. Of the many multicoated filters I have tried, the 58mm Hoya pro 1 digital MC UV(0) was the best.

 

Regarding focusing, in achromatic optical systems violet light will focus closer to the lens than red light. Camera sensors and "live view", like the human eye, is most sensitive to green light. If you focus to the smallest star image in the 10x live view, your green and blue will be in best focus, but red light will be slightly defocused resulting in tiny red circles around bright star images. To eliminate these, the lens needs to move ever so slightly AWAY from the camera. On the Takumar, that means turning the focusing ring a tiny bit COUNTERCLOCKWISE as seen from the camera from what appears to be perfect focus.

 

I can't attach any photos here, but I will send you some via PM to show how I mounted the lens on top of a guidescope and a small portable mount.

 

Best Regards,

Rudy

 

 

 

What is the role of the Hoya pro 1 digital MC UV(0) in AP?

Kevin,

In my humid atmosphere in Virginia, faint star images come out distictly sharper and better defined with a good UV filter. I presume this is due to the removal of violet light which is more scattered by the atmospheric particles than other colors. At the same time there is an obvious decrease in red chromatic aberration which remains on many telephoto lenses and achromats even after aperture reduction. This manifests itself as a reddish ring on the inside periphery of bright star images. I think the improvement is due to the narrowing of the spectral range reaching the sensor, leading the photographer to find "perfect focus" slightly more toward the RED. See my comments above on focusing toward the red, slightly away from infinity.

I noticed the most dramatic improvement with the Tiffen Haze 2 filter, which has a slight yellow tinge. Unfortunately it is uncoated, and produces prominent internal reflection artifacts on bright stars. I tried all major brand of multi-coated filters, and the second best is the Hoya MC UV (0).

Regards,

Rudy

I'll give it a try. I have that Hoya filter for the 200mm L (in 72mm) and step down rings for in front of it.
Though I think I will not use it with the CLS in tandem.
 
When I used this lens last week I noticed on BYEOS live view that my focus star toggled from red to blue as I electronically walked focus past minimum FWHM. I landed it on the blue side as it looked sharper on the screen. So I should have elected for the red side in that case?
 
I'm not sure how much FL travel a single step is on that USM. It's got to be microns, so either way I had a decent focus. I think a step down ring to 62mm will sharpen it up even more.
 
BTW previewing 200mm/APCs sensor combo in Stellarium shows a perfect fit for the Veil. It will be difficult for a non modded DSLR (which is half the appeal  :smirk: ) but it is the top of my Fall target list.

 

.

Is it better to focus somewhere between the red and blue focus positions rather than on red? Most of my lenses and my achromat seem to have a common focus for red and green. I have heard but have not verified claims that you can't focus on blue as tightly as on red/green.

Is it better to focus somewhere between the red and blue focus positions rather than on red? Most of my lenses and my achromat seem to have a common focus for red and green. I have heard but have not verified claims that you can't focus on blue as tightly as on red/green.

 

Some photographic lens are a bit different than the average achromat.

This one has nine elements in seven groups. Two of the elements are ED. I'm sure finding the prescription would involve espionage. It certainly isn't an achromat.

 

The difference between focusing blue/green vs red on this guy is less than one step of its USM motor, likely single digit microns. I'll be happy no matter which side it lands on. I'm really not sure if a "blue" star means its bloated in blue or sharpest in blue. 

Kevin,

I should add that my comments regarding "focusing toward the red" pertain to achromats and older achromatic telephoto lenses.

In my TS APO astrograph, the "best focus" on 10x magnified live view IS the best focus on the photograph. The same should hold true for truly apochromatic telephoto lenses.

As I can not add photographs here, I will eventually make a separate post comparing images with and without UV filters. Note that adding a UV filter will change the spectral range and the focal length ever so slightly and, if you are particular about your star images, the camera should be refocused. Also, a UV filter may be of less benefit in cool, dry, high altitude environmets.

Rudy

 

I drove out to a moderately dark site (for Atlanta) last night to image the Veil.

 

I did one set with step down rings to 62mm and one set using the internal iris @ F3.2. The set with step down rings showed noticeable vignetting in the corners compared to the other.

 

Moisture and moonlight conspired to impinge the results but I may be able to make an image out of the subs. 

I kept the UV on the front for the whole session. I also used the CLS throughout.

 

I didn't use dew heat as my field battery started out at 11.6V. It didn't seem wet enough to dew up in that temperature but it did cool off around 1:30AM (when my subs got darker). The moon set at that time so I didn't think anything amiss. 

 

On teardown, I noticed dew in the center of the UV filter. The UV filter protected my objective from dew so that's something.

Hmmmm,

With simple optical systems you get vignetting when the light beam is interrupted between the lens and the focal plane. With the old lenses I mentioned in the article there was no vignetting at all with aperture stops up to 35% diameter.

However, I do remember having significant vignetting with a new Canon kit lens when I used step down rings to attach a warming filter which was too small. It seems that aperture stops can not be used with lenses of newer design unless you plan to crop the image.

I am sending you a PM with images to show you the benefits of a good UV filter when it is not fogged up.

Rudy

Kevin,

See the following link for the effect the Hoya HMC UV(0) filter has on star images. This test was done with the SMC Takumar 200/4, but exactly the same effect is present with SMC Pentax 135/2.8 and Zeiss Tele-Tessar 300/4.

 

http://www.cloudynig...omatic-systems/

 

Rudy

    • KevinS likes this

Let's not forget the russian Tair-3 300mm f/4.5. This lens delivers a pretty flat field for the APS-C sensor and the quality is good from f:5.6. 

You can easily fit the California nebula , heart or m31 in the fob of $500 80mm Eon, which for imaging is better than most 500mm focal length lens and costs so much less. I also used zooms for astrophotography. I can also state that l Canon lens in zoom format are pretty capable of imaging the night skies. I understand where you are coming from but I honestly disagree with most of the recommendations made here.

In my experience a Nikkor 300mm EDIF f/4 resulted in quite a bit of red fringing on most stars using a Canon 450D. After the camera was modified (L2 removed) it no longer was able to focus using the Fotodiox adapter. I did not replace the filter with clear glass.

Ranger Tim,

 

I have no experience with a Nikkor 300mm f/4. I did have a Zeiss 300mm f/4 which, I had hoped, was as good as telephoto lenses can be. But, just like all the other telephoto lenses tested, it required a reduction in aperture to 48mm before it would produce images mostly free of chromatic aberration. I sold that lens because it was simply much too heavy, and because I had bought a TSAPO65Q with flawless optics. My most recent acquisition is the Canon EF 200mm f/2.8L II USM lens, which is excellent, but also requires an aperture stop of 52mm. By my standards, NO telephoto lens I tested could be used at full aperture with an UNMODIFIED camera

 

I think I mentioned in the article or in subsequent comments that NO telephoto lens I tested, even with the aperture stop,  was suitable for use with a MODIFIED camera due to very prominent red aberration. I tried the Astronomik UV/IR block filter and the CLS-CCD filter to limit the spectral range, but to no avail. So, don't feel too badly if you find that you can not achieve focus with your type of modification. But, before you give up on the lens, try the following: 1) use a deep red filter or an Ha filter if you have one, 2) add a 48mm or smaller aperture stop. Increased depth of field just might help you reach satisfactory focus.

 

The cheapest way to preserve the use of good old telephoto lenses might be to buy a new, unmodofied Canon 600D camera body. Even with an APO astrograph, I find that preferable for NON-Ha targets like galaxies, reflection nebulae, open and globular clusters, rather than messing for hours in post processing adjusting the color balance on images taken with the modified camera. The UV/IR block filter is not a substitute.

 

See the following link

http://www.cloudynig...r-modification/

 

Rudy

Photo
Kidastronomer
Jul 01 2017 06:09 PM
I have a Cannon 400mm f5.6 that I'm hoping to try out when I get a good tracker. I haven't pointed it to the sky yet but it's brilliant with daytime use (mostly birds)

The best of all telephoto lenses I had an opportunity to test is the Canon 200mm F2.8 L II US. Ideal correction was obtained when the aperture was reduced to 55mm with filter thread step-down rings, converting the lens to F3.4. With accurate guiding, the smallest star size on full resolution images is only 2x2 pixels, or about 9um in diameter. Any smaller, and the stars would be indistinguishable from hot pixels.

The Sigma APO 1.4x tele-extender is a perfect companion for this lens, converting it from F3.4 to 280mm F5. The image remains outstanding, and the focal length range is ideal for wide angle astrophotography of extended objects. I have used this tele-extender with equally good results on a Pentax SMC 135mm lens, and TSAPO65Q and TSAPO100Q astrographs.

The image attached to the following link is one of several first-light photos taken with a Canon 200mm F2.8 L II US lens, 55mm aperture stop, Sigma APO 1.4x tele-extender, and a full spectrum modified Canon T3i camera.

https://www.cloudyni...-tele-extender/

 
The image was obtained from a single 300sec exposure at ISO 400, minimally processed with gamma, stretching, and color balance in XnView and StarTools. No darks or flats were used. The exposure was autoguided on a Celestron AVX mount with the Orion StarShoot camera and a Jaeger's 3 inch F5 guidescope. Limiting magnitude on the original image is 15.5.

Rudy, why didn t you include any L lenses from canon? I had a 70-200 f/4 that i used unstopped at 200 with awesome results. you can see here a lot of photos mostly shot with the f/4 version. 

http://www.vladdumit...bove/stars.html

 

Now i have the f2.8 version, and while the resolution is better it s under no circumstance as good as the f/4 one. I also tested 200 f/2.8 tele and it is one of the most perfect lens in existence, as well as the 135. Then you should have tried the 180mm nikkor ED, the old one, which is the favorite tool of a lot of astrophotographers. i also have the 300mm f4.5 non ED nikkor which is quite nice .

 

I was expecting a lot more of an article that says "the best telephoto lenses for astrophotography". 

I have a manual focus 180mm F/2.8 Nikon lens, and now that I have an SCT and an equatorial mount, I think it would be a good way to put this lens to use. Any word on how Nikon's 105mm F/2.5 and 135mm F/2.8 telephoto lenses do for astrophotography?

 

Taras

Taras,

 

I have no experience with Nikon lenses except the one mentioned in the article. However, the very best lens with which to start astrophotography is the one you already have. And, Nikon has an excellent reputation for a reason.

 

Let us know how it works out.

 

Regards,

Rudy

Photo
jim@ping23.com
Dec 21 2017 08:59 AM

I am a newbie to the vintage lens thing (and in general) and I really appreciate this post and all the great discussion info - many thanks!

 

I obtained a 200mm f4 SMC Takumar m42 on ebay for next to nothing.  My first shots had big red bloated stars but then I came across this post and followed your advice (stopped down, slightly away from focus) and I got a first image that I'm really happy with!:

 

https://www.astrobin...6100/B/?nc=user

 

I've ordered some step-down rings to get rid of the diffraction spikes (though I do admit I think they're kind of pretty smile.gif )

 

Now I just need some clear weather...

 

Thanks again!

-Jim

    • rekokich and Star Stalker like this
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Star Stalker
Jul 30 2018 06:16 PM

The Milvus 135 f2.  @f2.  This is a Nikon model with aperture ring shot on a Hutech mod 6D with a Cls ccd clip filter.  No coma, no CA, flat out to the very corners. 

 

AutosaveProc_zpszlye1mdb.png

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Star Stalker
Jul 30 2018 06:27 PM

To be precise, clip-in filters for Canons fit in the space that is occupied by an EF-S lens but not an EF lens, as I understand it, and so cannot be used with an EF-S lens.  Canon makes two series of lenses, EF and EF-S.  The latter are only for smaller (APS-C) DSLRs.  The former are their regular lenses.

 

This is why I use Canon bodies with all F-mount fitted lens.  The 2mm lens adapter needed will clear all Nikon mount lens from the clip filter. 

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Astronomy4You
Aug 29 2018 12:25 PM

Rudy,

 

Sorry for reopening an old thread, but I found your article interesting enough that I picked up a Takumar 200 f/4 for my Pentax K-1 and am anxious to try  it out. I'm waiting for my M42 to K mount adapter to arrive along with some clear nights and time to get to a dark sky site.

 

I do have a question however that I didn't see answered in the article.

 

What's the difference between stopping down with the rings in the front of the lens vs stopping down via the lens aperture? Don't they both do the same thing, lessen the  light getting to the sensor?



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