Imaging with a 101 dollar lens

[gorann]

On Sunday I had given up on a clear sky for the evening, but when I took my last walk with the dog the sky was suddenly crystal clear. I got the roof of the obsy but realized it was too late for setting up telescopes and guiding (had to work next day). So I decided to do it simple and try out the Pentax SMC Takumar 200mm f4 lens that I recently bought on e-bay from a Japanese guy for 101 USD. With a 5 dollar adapter I connected it to my Canon EOS 60Da on the NEQ6 and took five 2 min unguided exposures at 2000 ISO. It did not turn out to bad. The Super  or SMC Takumars have  had some good reports on AP fora for being bargains from the 70ties so I have also bought a 135 mm f2.5 (189 USD) and a 50 mm f 1.4 (80 USD) to try when skies finally clear. It is apparently important to get the Super or SMC versions of the Takumars as they have the best coatings. My 50mm Super Takumar will be extra exciting to test since it is known for being radioactive due to thorium oxide being mixed into the glass for superior performance (although the hourly dose is apparently not worse than taking an flight to New York) ......

Maybe the experts can tell me if the red rings around some brighter stars are chromatic aberration or just bad focusing. I noticed that the depth of focus was smaller than what I am used to on my refractors an focusing was tricky.

[gorann]

PS. I stopped down the lens to f 5.6 as advised on some fora discussing these lenses.

[michael.h.f.wilkinson]

Nice result. These old fixed focal length lenses are really great for this kind of work, especially for the money.

[pyrasanth]

There are so many stars.....I started counting in the top left hand corner......but gave up when I reached 10.........really nice wide field image.

[Stardust]

I'd say that  was a success, yet to try mine

[Rihard]

Very nice result gorann! 

In regards to focus, wouldn't it focus properly at infinity? 

I have collected quite a few old school lenses (mainly Carl Zeiss, Meyer-Optik and Pentacon) that I'm planning to use for DSO work too! 

In regards to the radioactivity of the thorium in the fast 50mm takumar, it appears to be no more radioactive than bananas are (yep, bananas are radioactive too, but people never talk about it ) 

Thanks for sharing! 

[gorann]

I have been reading up a bit more on this Takumar and the chromatic aberration at the edges that can be seen particularly at the top and bottom of my pic is well known. For some reason modern digital chips are more sensitive to this than the old films, so older lenses did not have to be so well corrected.

Still, it was an inspiration to try this wide field lens but I have realized that if I want to go seriously into wide field AP with telephoto lenses, the more modern ED / Apo lenses are clearly superior. Some are very competitive in price with small apo refractors. That include some Canon lenses if you stay away from getting the models with image stabilization (no use for AP and doubles the price, but great if you also want to use them at daytime). From what I have read on various fora on the net, one of the best Canon deals for a 200 mm lens that apparently is perfect optically is the Canon 200 mm f/2.8L II. It costs 569 punds on Amazon but then you are getting a perfectly corrected 70 mm aperture f/2.8 apo with flat field. With a 200 pound telecompressor,  such a lens can be turned into a FL 280 mm (f/4) or FL 400 mm (f/5.6) telescope, apparently still without much CA and field curvature.

Even more interesting, some modern zoom lenses are said to be virtually as good as fixed FL lenses, and I am very tempted by the Sigma 70-200mm f2.8 EX DG OS HSM. It is an apo with 70 mm aperture and contains two FLD (about the same as fluorite) elements within its 22 element / 17 group construction, alongside three elements made from the more conventional Super-Low Dispersion (SLD) glass. Like the Canon lens it can even be used for full frame DSLRs. It is now on Amazon for around 700 pounds (used to be nearly twice that price). What 70 mm aperture apo refractor can beat that? I am seriously thinking of investing in this Sigma lens since it will give great freedom in focal length (= framing) and would be fantastic for daytime use too because it has an image stabilizer.

[gorann]

By the way

Here is another one from the same night now using the smaller 135 mm (f2.5) Pentax SMC Takumar (five 2 min subs unguided at 2000 ISO). Not bad but the CA in red is quite apparent on the brighter stars (seen as red rings around the stars). I read that this is also experienced by others using the Takumars for AP. It could be made worse by the high H alfa sensitivity of the Canon 60Da, but without that the North American Nebula would be quite pale, I assume.

[Knight of Clear Skies]

Zooming in on your images I can see the red haloes on the brighter stars. I don't get this with my Takumar lenses, have you tried the thirds-focussing trick? You could try focussing on a star a third of the way into the frame rather than in the center, this gives better focus across the whole frame.

[gorann]

Thanks Knight!

That is a great tip that I will keep in mind when the b...y moon is gone and the sky is clear, but I may then try it with the Sigma 70-200mm f2.8 that I just received, mentioned in my reply above. Since it is 10 x the price of the 200 Takumar I hope to see a difference.

[michaelmorris]

My guess is that the red rings around stars is due to the fact that it's a modified camera.  The chip on the camera is quite sensitive to IR light, but lenses aren't usually made to correct for IR as well as visible light . This lack of complete correction usually isn't a problem because the camera's bulit-in IR cut filter blocks most of the IR light. However, removal/replacement of the camera's built-in IR cut filter means that there you will often get a small red halo around bright stars because the IR isn't in as sharp focus as the visible light. 

'Mystic Bob' has produced a great guide on one technique for removing these red haloes

http://budgetastro.com/micro/articals/red_halos/red_halos.html

[happy-kat]

My little 50mm f2 takumar doesn't show red halos either. I stop it down to f2.8 when using with a standard 1100d.

Mine is also old enough to be hot and gamma rays don't get stopped by walls but then I grew up in a granite cottage so probably the same dose.

[Knight of Clear Skies]

My guess is that the red rings around stars is due to the fact that it's a modified camera.  The chip on the camera is quite sensitive to IR light, but lenses aren't usually made to correct for IR as well as visible light .

I think this is unlikely. Recent DSLRs typically have two IR filters in them and only one is removed when modding cameras, my modded 1100D still has a UV/IR blocker. I don't know how the 60Da compares but if it's letting a significant amount of IR through then its performance with refractors would be poor, giving bloated stars, which would defeat the point of using it as an astrocam.

I suspect it's simply a focus issue, I can sometimes see red haloes on liveview but I adjust the focus until they vanish. Possibly I'm compromising the focus of the blue light slightly by doing this, I don't know, I'd have to look at the sizes of stars in different channels to really see how the lens is performing. Checking a couple of my less successful images more closely it looks like I have had problems with haloes, I'd previously put this down to losing the colour balance somewhere in the processing. I'll have to watch out for this in future.

[gorann]

I also have a feeling it is partly a focusing issue, combined with a CA issue. I read somewhere that these oldish lenses give these red halos when they are perfectly focused (to get a good focus with the 60Da is quite easy since I can get it to magnify the live view screen 10x). One trick is apparently to just defocus it a bit to get the red to dissolve into the star. The 60Da still has a IR filter so that cannot be the issue (H alfa (656 nm) is in the visible red and not IR part of the spectrum). However, it may be that the old colour film was less sensitive even in the red part of the spectrum and it had usually a lower resolution and the red halos are only apparent when zooming into the stars in the picture. Finally, AP has always been more challenging for lenses, particularly with pin-point stars. The Takumars were not designed with AP in mind and like most camera lenses they are a compromise not optimized for infinity focus like telescopes. I will get the new Sigma APO 70-200 telephoto lens this week so hopefully I soon find out how modern telephoto lenses behave. In any case it should be excellent for birds.

[gorann]

Well,

i have now tried the Sigma 70-200 mm at f/2.8 for AP, and my first impression is that it is probably best for daytime photography, loosing the first round to the Takumars, see:

http://stargazerslounge.com/topic/258252-first-try-with-sigma-70-200-mm-f28-ex-dg-apo-os-hsm/