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M31 with a $60 lens

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#1 Alen K

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Posted 17 August 2019 - 11:44 PM

I recently got a chance to put in some "quality" time on M31 with an SMC Pentax-M 200mm f/4 lens operating at f/5.6. That lens in good condition sells for about $60 on ebay. (I bought mine used in a camera shop decades ago.) This image is the result. It does contain some residual chromatic aberration that more careful image processing could reduce but I don't think it detracts from the image too much. South is up in this view, which is my preferred orientation because the nature of the tilted disc of M31 is more obvious than when North is up. No flats, bias or dark frames but I dithered and used kappa-sigma clipping during stacking. Pentax K-3II on tripod only with tracking using Astrotracer (built in to camera). Total exposure was 87 minutes (260x20s @ ISO1600). Bortle ~3.5 sky with average transparency. 

 

gallery_96203_4345_231457.jpg


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#2 wrnchhead

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Posted 18 August 2019 - 12:29 AM

Love this shot Alen!

#3 james7ca

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Posted 18 August 2019 - 02:08 AM

Nice work.

 

One of the advantages of these old Pentax lenses is that they use a screw mount for attachment to the camera. It's an M42x1 thread which is close enough to the standard T-thread (M42x0.75) that some people actually use these lenses without an adapter on T-threaded astro cameras. However, I'd strongly advise to get an M42x1 to T-thread adapter so that you don't risk cross-threading or an insecure or tilted attachment to the T-threaded camera. Of course, if you already have a Pentax camera body that accepts screw-mount lenses then you don't need the adapter.



#4 moxican

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Posted 18 August 2019 - 10:34 AM

I have that very same lens. It performs very well when it is stopped down one step. I personally rather use step down rings on the front of the lens, gives more natural looking stars. 37mm ring stopping it down to F5.4 does the trick. The residual CA is so small that it can easily be removed in post processing.

Great image btw.



#5 whwang

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Posted 18 August 2019 - 10:53 AM

Nice shot.  I wouldn't guess it's a $60 lens if you didn't tell us.

 

Unfortunately, paying 10x more probably will not get us a 10x better picture.  It's always very expensive to go from good to very good.


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#6 jerobe

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Posted 18 August 2019 - 01:31 PM

Vrry good shot!  I guess I had better dig out my SMC Takumar screw mount 55mm f1.8 and 120 mm f2.8 lenses and give them a try.  They have been dormant since my Spotmatic F days in the 70s.



#7 Alen K

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Posted 18 August 2019 - 05:52 PM

I have that very same lens. It performs very well when it is stopped down one step. I personally rather use step down rings on the front of the lens, gives more natural looking stars. 37mm ring stopping it down to F5.4 does the trick. The residual CA is so small that it can easily be removed in post processing.

Great image btw.

Thanks. And thanks to everyone.

 

I do have the requisite rings to step down to 37mm and it does exactly what you say. I chose not to install them for this shot for two reasons. First., there are really only two stars in the FOV that are bright enough to show obvious diffraction spikes. If there were more bright stars or brighter stars period I would have used step-down rings. Second, I have found that fall-off (vignetting) is considerably worse when stopping down using rings. Whereas I can get away without flats at f/5.6 achieved with the internal iris, I definitely would need flats at f/5.4 achieved using step-down rings. I haven't got around to shooting flats for any of my lenses yet but that is high on my AP to-do list. (I realize that such long-after-the-fact flats would only correct for vignetting. But I don't yet have any obvious dust motes.)

 

Re the small residual CA, the reason it has not been entirely corrected here is that I did the CA corrections after stacking. With Astrotracer, the target moves in the field of view between shots. So every few shots (in this case four to six) I recenter the target. This means the target moves around in the FOV and hence the CA is a little different for any given star from shot to shot (because it will be in a different position in the frame from shot to shot). Properly, I should remove CA for each shot individually (can be done in a batch process since each frame will require the exact same settings) after raw development but before stacking. But that all takes a lot more effort than what I did here, which was to use Sequator to stack from the raw files and as a later step do CA correction.  


Edited by Alen K, 18 August 2019 - 09:45 PM.


#8 Alen K

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Posted 18 August 2019 - 06:23 PM

Vrry good shot!  I guess I had better dig out my SMC Takumar screw mount 55mm f1.8 and 120 mm f2.8 lenses and give them a try.  They have been dormant since my Spotmatic F days in the 70s.

Thanks.

 

I have heard very good things about the SMC Takumar 55mm f/1.8 for astrophotography. OTOH, I haven't heard anything about the 120mm f/2.8 for AP, but likely simply because not many APers have tried it. If you do, please let us know the results. The Takumars are generally very good for AP (the stringent star test) but some of them are less well suited because you have to stop them down too much before star images are acceptable. Realistically, you have to stop all of these old film-era lenses down at least one or one and a half stops to get acceptable star images just on APS-C. They aren't anywhere close to the performance of a Sigma Art lens or even a Rokinon/Samyang. But they are 10x to 15x less expensive to acquire.

 

In addition to the SMC Pentax-M 200mm f/4 I have the SMC Pentax-M 28mm f/2.8, SMC Pentax-A 50mm f/2, SMC Pentax-M 100mm f/2.8 and the SMC Pentax-M 135mm f/3.5. All of them are inexpensive. The 28mm f/2.8 is barely acceptable on APS-C at f/4. I am looking to replace it with something better. The 100mm f/2.8 is, OTOH, quite acceptable on APS-C at f/4. I have not tried the 135mm f/3.5 for AP yet. I don't use the 50mm f/2 anymore because stars are still terrible in the corners at f/5.6. To replace that one I have an SMC Pentax-A 50mm f/1.4 on order (supposedly the best of the Pentax/Takumar 5Xmm for AP). I also have an SMC Takumar 300mm f/4 on order. Both were less than $100US (before shipping).  



#9 james7ca

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Posted 18 August 2019 - 10:32 PM

Yes, when you displace the aperture stop from its designed position you affect the correction of the lens and the amount of vignetting. Wide angle lenses are particularly affected by this problem, but it is less of an issue with telephoto lenses. That said, I've used front mounted aperture stops on 50mm, 105mm, and 200mm lenses with good results and with shorter lenses (like the 50mm) you can reduce the vignetting by keeping the final step-down ring as close a possible to the front lens element -- you can mount the final ring(s) backwards to keep them close to the lens.

 

It's too bad that no one is offering these lenses (modified) with an internal, perfectly round aperture stop that would suppress the diffraction spikes without introducing vignetting and other aberrations. A 200mm camera lens stopped down to something around f/5.6 can make a very good astrograph with today's small pixel CMOS cameras.



#10 44maurer

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Posted 19 August 2019 - 01:26 AM

When I saw your heading, the first thing that popped into my head was “$60, it probably looks like it”, but when I clicked on it, wow, that’s really nice. Good purchase. waytogo.gif


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#11 CodeBlind

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Posted 21 August 2019 - 11:00 AM

This is really cool. I'm a huge fan of images that push the limits (real or perceived, doesn't matter) of the gear they were created on. I find it inspiring, and you did just that. Nice job!


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