Jump to content

  •  

* * * * *

SharpStar Askar ACL200 200-mm f/4 astrographic telephoto lens


Discuss this article in our forums

SharpStar Askar ACL200 200-mm f/4 astrographic telephoto lens

 

Reviewed by Michael Covington

 

Michael Covington is the author of Digital SLR Astrophotography and other books. By day, he develops artificial intelligence software in Athens, Georgia.

 


DISCLOSURE: The lens reviewed here was lent by the manufacturer for review, and the reviewer was then allowed to purchase it for 30% below list price. Apart from that, the reviewer has no financial or business relationship with the manufacturer and was unacquainted with them until shortly before requesting to review this lens.

 

All photos here are by Michael A. Covington, including technical illustrations. Larger versions of the sample astrophotos can be displayed in a new tab/window by clicking on the review image.


 

A telephoto lens just for us

 

Telephoto lenses — can't live with 'em, can't live without 'em. They are basic tools of wide-field astrophotography, but even the best ones often disappoint us. Too many of them aren't sharp all the way to the corners when shooting star fields, and much of their cost comes from features we don't need, such as autofocus, zoom, and electronic aperture control.

 

Wouldn't it be nice if someone made a good telephoto lens just for astrophotography? It's being done. You've probably heard of the first major product of this type, the William Optics RedCat 51, a 4-element, 250-mm, f/4.9 lens specifically for astrophotography.

 

Now China sends in a formidable competitor. The Askar ACL200 is a 200-mm f/4 lens with 6 elements, two of them made of ED glass, and is very ruggedly built. Using a 48-mm T-ring, it fits any DSLR and, with extension tubes, dedicated astrocameras. Figure 1 shows its features, and Figure 2 is an example of what it delivers. The manufacturer is SharpStar, officially known as Jiaxing Ruixing Optical Company, of Jiaxing City, near Shanghai. See http://www.askarlens.com/.

 

Picture
Figure 1. Askar ACL200 has coarse and fine focus and aperture rings, all lockable, and provision for 2 sizes of filters.

 

Picture
Figure 2. Mu Cephei nebula. Askar 200-mm lens at f/4, stack of 16 2-minute exposures with H-alpha-modified Nikon D5500 at the Deerlick Astronomy Village.

 

Using it

 

The ACL200 is strikingly easy to use for astrophotography. Focusing is purely manual, of course. There are two focusers, coarse and fine, both lockable, and the lens will focus appreciably past infinity (to accommodate filters) as well as down to 10 meters (for nature photography).

 

Unlike the RedCat, the ACL200 does not include a Bahtinov mask. Of course, you can easily add one. But I found it best to focus for maximum visibility of faint stars on the camera screen in Live View.

 

The tripod shoe below the lens has 1/4" and 3/8" tripod sockets, and, more importantly, the whole shoe is a Vixen-size dovetail (Figure 3). Many equatorial mounts will take it directly, with no additional hardware. In Figure 4 you see it on my Celestron AVX mount.

 

A newer version of the lens, just now coming on the market, has a Vixen saddle on top as well as a Vixen dovetail below. That way you can mount a small guidescope on top of the lens. The new lens collar, with shoe and saddle, is available as an upgrade to go in place of the one supplied on the original-model lens.

 

Picture
Figure 3. The built-in tripod shoe is also a Vixen dovetail that fits directly into your equatorial mount.

 

Picture
Figure 4. The dovetail fits directly into many telescope mounts with no additional hardware.

 

If you put the ACL200 on an equatorial mount, you may need a lighter-than-normal counterweight. With my AVX, no Celestron counterweight was light enough, so I used a 2-kilogram counterweight made for the iOptron CEM40, whose shaft is about the same diameter.

 

Note that the ACL200 may be too heavy for lightweight camera trackers. It weighs 1.8 kilograms (4.0 pounds), not counting the weight of the camera. That's more than twice as much as a Nikon 180-mm f/2.8 ED AI lens, for example, or a Canon EF 200-mm f/2.8 L, even though those lenses are one stop larger in aperture.

 

You can install a 48-mm filter inside the rear section of the lens (Figure 5). I have not tried this. That is the filter size used with 2-inch eyepieces and many astrophotographic systems.

 

Picture
Figure 5. You can install a 48-mm filter internally inside the rear section of the lens mount.

 

Finally, the lens attaches to your DSLR camera with a 48-mm T-ring — not the standard camera-store 42-mm T-ring, but just like it except for the diameter. As with conventional T-rings, the back focus is 55 mm. If you wish, you can use 48-mm threaded adapters to fit the lens to a dedicated astrocamera.

 

Optical design and performance

 

The lens design is not quite like anything I've seen elsewhere. It's inspired by classic telephoto designs with a positive achromat in front and a negative achromat in the rear, like a refractor with a Barlow lens. However, the ACL200 has two big positive groups in front, each of which has an ED-glass element, and a negative group at the rear. Obviously the designers were thinking for themselves, and they have achieved a good result.

 

This lens is apochromatic, more highly corrected for chromatic aberration than ordinary lenses. That was obvious to me as soon as I focused on a star in Live View mode with high magnification. Other lenses, even very good ones, show red or green fringes around star images that are slightly out of focus. Not this one — the star is always a white disk or point. I found that all the more impressive because I was using a camera with extended hydrogen-alpha response, so it picked up light outside the region of the spectrum that lenses are designed for.

 

I saw not a hint of decentering or miscollimation. Telephoto lenses are often poorly collimated by astronomical standards, for two reasons. First, manufacturing tolerances are considerably looser than for telescopes, and used to be looser yet. Second, the collimation of a lens suffers when it spends several years hanging around a busy journalist's neck and occasionally banging into objects. And those are the lenses we commonly buy secondhand. Compared to many other telephoto lenses I've tested, this ACL200 is unusually precisely made and assembled. Of course, that's just one; I can't tell how much variation is possible.

 

I took short exposures of star fields to compare the ACL200 at f/4 with my vintage Nikon 180/2.8 ED AI, also at f/4. Image analysis with PixInsight showed the two lenses to be equally sharp. The difference is that the ACL200 gave star images that were basically round all the way to the corners of an APS-C frame. (In the corners, they did have slight, faint coma.) Star images with the Nikon lens were compact but irregularly shaped, becoming triangular at the corners. Bear in mind that this is an old lens, but the shape of the star images reflects its optical design. Overall, the ACL200 wins.

 

I did not have a chance to test with a full-frame sensor, but Cloudy Nights user "Astrojedi" did — click here for his results, which also show almost perfect star images all the way to the corners, certainly much better than any conventional telephoto lens.

 

Curiously, the ACL200 did not become sharper at f/5.6 or f/8. It really is optimized for f/4. At f/4, there are no diffraction effects from diaphragm blades because the opening is circular; stars have no spikes. Diffraction effects kick in as soon as you stop it down.

 

Conclusions

 

My ACL200 has become a workhorse. It is one of the few astronomical instruments I've owned that can be relied on to do a truly excellent job of what it's designed for. Below are a couple more pictures to show what I've done with it. (The Pleiades picture shows that it's free of internal reflections.) Like Figure 2, Figures 5 and 6 were taken with the lens and camera riding on a Celestron AVX mount with PEC but no autoguiding — a very portable setup.

 

Picture
Figure 5. North America nebula. Askar 200-mm lens at f/4, stack of 20 2-minute exposures with H-alpha-modified Nikon D5500 at the Deerlick Astronomy Village.

 

Picture
Figure 6. Pleiades, strongly stretched to show scattered nebulosity. Askar 200-mm lens at f/4, stack of 16 2-minute exposures with H-alpha-modified Nikon D5500 at the Deerlick Astronomy Village.

 

I strongly recommend the ACL200 for wide-field astrophotography. It also has some uses for (fully manual) nature and landscape photography. The optical performance is exceptional, and the physical construction is unusually well suited for astronomical use. Its main drawback is its heavy weight (too heavy for lightweight star trackers). It costs appreciably more than the secondhand telephoto lenses we often use, but less than a good 180- or 200-mm lens new. Further, you do not need to buy a tripod collar or dovetail, nor large-diameter filters.


  • bill w, dswtan, Jim Waters and 20 others like this


58 Comments

Photo
Michael Covington
Sep 03 2021 08:55 AM

Michael, when you move the fine focus ring do you hear any noises?

Please listen in below short video:

https://youtube.com/...k?feature=share


How about when you lightly shake the lens. On the first unit I hear the strange noise of the ring, second unit I hear the same noise and notice like a loose lens sound rattling when I lightly shake it. I cannot have had bad luck twice in a row right?

I don't have the lens right where I can grab it at the moment, but I don't recall noises.  Note that T-rings normally contain a part that is loose when not attached to a camera -- I assume you're leaving that off.  If no one else pops up with answers, prod me later this weekend.

I was just looking at this lens again. It looks like the DSLR adapter can be removed and that leaves a female thread at the back of the lens, is that correct? That would allow for all threaded connections that are not M48 but larger. It also seems that I could get additional back focus that way, meaning I would have more space for cameras filter wheels etc...

Am I reading this correctly?

 

I also wonder if a motorized focuser is really needed. Has anyone determined if refocusing is really necessary between filters? (probably yes but I hope not). This keeping in mind that this is a super wide field lens and it's more about the large field than pixel peeping.

 

Lastly, how impervious is it to temperature change? I suppose given how short the tube is that refocusing with temperature change isn't really a must.

 

Any thoughts on all of the above?

Photo
Michael Covington
Sep 04 2021 09:42 PM

Mine is packed up and I can't look at how it comes apart right now... Concerning whether you refocus when changing filters, that depends entirely on how well the filters match each other, doesn't it?  Chromatic aberration in the lens is apparently very, very small, but filters can also be mismatched due to thickness or material.

That is true but my assumption is of course that my Chroma filters are par focal as advertised. Truth be told I do refocus between filters with all my telescopes, even the reflectors. It's just that I am hoping to be able to get away without an auto focus system, not because I wouldn't know how to implement/use it but for simplicity sake of a highly portable setup. 

However it may just be wishful thinking.

Photo
Michael Covington
Sep 05 2021 08:10 AM

I can tell you that I saw no chromatic aberration when viewing star images highly magnified on the screen of the DSLR with this Askar 200/4 lens.  That was quite a surprise, since all the other lenses I've used show at least a tiny bit of CA.

    • B 26354 likes this

I agree with Michael, I have not noticed any CA on the two copies I have used. I am noting a not so flat field on an APS-C sensor though, which seems to not be the case with the images shared here.

Thank you for the feedback! I am going to do a bit more research on this lens. They also make a 50mm f5.5 which looks interesting.

I wish I had the same experience with my ASKAR lens.  I had terrible results with a full frame sensor, and have now reduced down to an APS-C sensor (ASI 2600MC Pro) but still tilt and/or flexure remains and moves around when slewing.

 

I have invested in far more precise camera tilt units, used the lens with/without things like a filter drawer, just straight adapters, but still the problem persists. While the build seems solid, I think there is weakness in the M48 adapter on the lens.....held by 4 really tiny screws.  I removed that interface which reveals another adapter that contains 2 more sets of threaded parts, the innermost one being the built in M48 filter thread.  It looks as if that would also uncscrew but I cant move that part at all.

 

I am wondering just how much coma this lens has.

 

So far for me, this is a very disappointing lens.

Photo
Jesus Magdalena
Sep 26 2021 03:12 PM

I was just looking at this lens again. It looks like the DSLR adapter can be removed and that leaves a female thread at the back of the lens, is that correct? That would allow for all threaded connections that are not M48 but larger. It also seems that I could get additional back focus that way, meaning I would have more space for cameras filter wheels etc...

Am I reading this correctly?

 

I also wonder if a motorized focuser is really needed. Has anyone determined if refocusing is really necessary between filters? (probably yes but I hope not). This keeping in mind that this is a super wide field lens and it's more about the large field than pixel peeping.

 

Lastly, how impervious is it to temperature change? I suppose given how short the tube is that refocusing with temperature change isn't really a must.

 

Any thoughts on all of the above?

Yes, indeed there is no problem in space for the wheel and other accessories if they are necessary, it is capable of focusing beyond infinity if you extend the distance to more than 55mm.
I have tried whole nights focusing only at the beginning without appreciating blur at the end.
Even so I have attached a motor to be more sure, focusing every hour, but it is not necessary.
I always do the RGB shots alternating each filter, with the sequence R, G, B, R, G, B ... and when focusing every hour, it does so in the filter that is in that mome

 

51521392749_d9626f8d39_c.jpg

Photo
Michael Covington
Sep 26 2021 06:54 PM

I wish I had the same experience with my ASKAR lens.  I had terrible results with a full frame sensor, and have now reduced down to an APS-C sensor (ASI 2600MC Pro) but still tilt and/or flexure remains and moves around when slewing.

 

I have invested in far more precise camera tilt units, used the lens with/without things like a filter drawer, just straight adapters, but still the problem persists. While the build seems solid, I think there is weakness in the M48 adapter on the lens.....held by 4 really tiny screws.  I removed that interface which reveals another adapter that contains 2 more sets of threaded parts, the innermost one being the built in M48 filter thread.  It looks as if that would also uncscrew but I cant move that part at all.

 

I am wondering just how much coma this lens has.

 

So far for me, this is a very disappointing lens.

I would suggest contacting SharpStar or your dealer.  I think you have one with an element loose or out of place.

One thing I have learned with this lens is that accurate, and I mean very accurate, focus is necessary to get good stars across the frame of the camera.  If it is just very slightly off, then focus falls away very quickly as  you move out from the centre.  This is even more evident with things like dual-band filters.  I have a replacement lens now and things are better.

One thing I have learned with this lens is that accurate, and I mean very accurate, focus is necessary to get good stars across the frame of the camera.  If it is just very slightly off, then focus falls away very quickly as  you move out from the centre.  This is even more evident with things like dual-band filters.  I have a replacement lens now and things are better.

Good to learn that it was a faulty lens. I love my ACL200.

    • Michael Covington likes this
I just got a notice from Agena Astroproducts that they’d gotten a few of the new version in stock, so I jumped on it and ordered one.
Coincidentally, yesterday I had been looking at the TPO 180 and the Redcat, both were close to what I’ve been looking for but not exact.

I’d say ‘expect lousy weather in North Georgia next week’ but it’s always lousy here anyway.
    • B 26354 likes this

aorion314 here, this lens is now in stock at Agena astro so order now if willing to spend$$$$, am looking forward to using once the weather is more cooperative in N. Ca. New model comes with handle attachment which is great, just does not fit into provided container, I will figure out a solution for that issue. Additionally unsure of stock inventory for this lens at Agena astro. Respectfully submitted

aorion314 here, this lens is now in stock at Agena astro so order now....

 

....Additionally unsure of stock inventory for this lens at Agena astro. Respectfully submitted

????  So... which is it?

 

I got mine from Agena in January, and recently got the upgraded handle from them as well.

 

Regarding their inventory of the new version of the lens, their website says:

 

IN STOCK (4 Available)
Stock listed above is accurate & shown in real time

 

grin.gif

aorion314 here, B26354, at the time of my comments, I had previously spoke with Agena A., and inquired about depth of stock, reply was we have new stock, that was the extent of his response, hence, my unsure of stock, was as accurate as information I had obtained from the business.  Respectfully submitted. 

    • B 26354 likes this

Even though the new version of the Askar ACL200 200-mm f/4 is 1.3 Kg (vs 1.8 Kg for the original version), I'm guessing even with a ball head and small DSLR (1 lb) the weight might be too much for a budget tracker like the Move-Shoot-Move (MSM), which has a max payload of 6.6 lbs.  (Estimate of my ball head + arca plate + DSLR + Askar ACL200 = 5.2 lbs, which is about 79% of the rated payload for the MSM tracker.) 

 

Anyone have any experience with the MSM tracker with that much of a payload (80% of max) on it?  

Photo
Horst Ziegler
Oct 31 2021 10:39 AM

Here my last 4 images with my GEM28/ACL200/EOS77Da-Setup. Click on the link for details in the gallery.

 

https://www.astrovis...Widefields_Neu/

 

 

Widefields_Neu.jpg

widefield_neu.jpg

Something is wrong with what you have between the lens (either Askar or Redcat) and the camera body.  What exactly are you using?  How far off is it from correct infinity focus, and in which direction?

I'm not familiar with the Z6, but I would expect that you would use a 48mm Nikon F T-ring followed by a Nikon F to Z lens adapter.  At least, that should be one way to do it.

If the failure to reach focus is slight, you might try the fine as well as coarse focus.  But since you are having problems with two lenses, I think the problem is in the adapters.

The Z mount flange to sensor distance is only 16 mm. 

The lens is expecting 55 mm back focus spacing. 
You need to add a 39 mm spacer to put the  sensor at the focal plan of the lens.

Michael,
Niko Carver (Nebula Photos) did a review and comparison of the 'new version' with the Canon EF 200mm f2.8/L II USM   He did test photos of stars with a Full Frame Canon EOS R-A 

Link is below. 

Interesting spot in the focal length spectrum. 
At what point do you shift gears to a fast compact refractor? 

https://youtu.be/dVEbOdn0qh0

That link didn't work for me.

 

https://www.youtube....h?v=dVEbOdn0qh0

 

grin.gif

 

Edit:

 

Great review. Clearly demonstrates the Askar's total lack of CA, and its excellent edge-sharpness.

Photo
UKalwayscloudy
Dec 09 2021 04:05 PM

Great review. I am wondering if this will finally tempt me away from my Nikon 180mm that I bought second hand after reading a review by Jerry Lodriguss. I am pleased with the image quality I get from the Nikon with an aperture mask overlaid, but the focusing is always time consuming. 

 

One question (hope I did not miss it above). When using a lens with anything from no filter to a thick filter I find it helpful to adjust the backfocus for the filter thickness in order to get the edge to edge image quality spot on, especially with larger sensors. Is such a mechanism built into the lens and can it be applied with an ordinary camera, or is it something one could only manage with an astro cam and eg some T2 shims? 

Photo
Michael Covington
Dec 09 2021 04:34 PM

Great review. I am wondering if this will finally tempt me away from my Nikon 180mm that I bought second hand after reading a review by Jerry Lodriguss. I am pleased with the image quality I get from the Nikon with an aperture mask overlaid, but the focusing is always time consuming. 

 

One question (hope I did not miss it above). When using a lens with anything from no filter to a thick filter I find it helpful to adjust the backfocus for the filter thickness in order to get the edge to edge image quality spot on, especially with larger sensors. Is such a mechanism built into the lens and can it be applied with an ordinary camera, or is it something one could only manage with an astro cam and eg some T2 shims? 

(1) My Askar 200/4 is much better than the Nikon 180/2.8 (like Jerry's) at f/4 that I used to use.  That lens had a fair bit of residual chromatic aberration; the Askar does not.

(2) I'm not sure what your question is.  The Askar lens focuses by moving the entire system together.  There is therefore no back focus requirement -- you just focus it so that you get a sharp image.  I've used it with and without filters.  It focuses with the focusing ring in a different position, but that's all there is to it.

    • UKalwayscloudy likes this
Photo
UKalwayscloudy
Dec 09 2021 05:53 PM
That’s very helpful thanks. If that’s how the focus works then it simplifies matters massively. I’m used to camera lenses that have almost no tolerance to deviations from the flange distance for which they were designed. Your comparison with the Nikon is useful too, though I hadn’t noticed much CA at least with my aps-c astrocam.
Photo
Michael Covington
Dec 09 2021 07:28 PM

This one does move over a much larger focus range than a normal camera lens (including moving "past infinity") to accommodate filters or slightly nonstandard adapters.

    • UKalwayscloudy likes this


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics