Pentax 67 200mm F/4.0 Optical Results

[twade]

To all,

I finally had a clear night to see how well the lens performs using red, green, blue filters.  As you recall from an earlier post, I was pleasantly surprised on its performance while imaging with a h-alpha filter.  This is the latest version of the lens (i.e. SMC Pentax 67 200mm f/4).  It is not the older Takumar version.  Although the lens' focus of the red and green spectrum is very good, the focusing of the blue spectrum is terrible.  Even moderately "bright" blue stars have a bloated, out of focus ring around them.  I can't recommend this lens to anybody wanting to create RGB images; however, it is a stellar performer in h-alpha.

I intended to complete the testing of this lens back in November 2009; unfortunately, yesterday (2/20) was the first clear night I've seen.

I centered Mars in the field of view and took six 30s red, six 30s blue, and six 30s green images.  These were calibrated in Maxim DL, registered, integrated, and stretched in PixInsight.  The image below is a 100 percent crop of the center of the image.



Wade

[Carlos Milovic]

Hi Wade

Did you change the focus to correct chromatic aberration, or just leaved alone?

[mmirot]

  Many lenses will do a poor job in one shot color because the colors are not parafocal.
They do will do better if it is a mono camera with filters and you refocus each channel.

Max

[twade]

Carlos and Max,

I focused each filter separately using an excellent V-curve created in FocusMax.  At final focus, FWHM values for each filter ranged from 0.90-1.02 which for all practical purposes at this focal length is near "perfect" focus.  Unfortunately, Pentax, for some reason, did not put much thought into the blue channel.  What's really sad, is my blue filter has a sharp cutoff in the ultra-violet so even putting a minus violet filter on this lens won't help much, if any.  It still is an excellent h-alpha lens.  It may also do well with other narrow-band filters; however, I do not have any narrow-band filters, except h-alpha.  It would be interesting to see results using H-beta and O-III filters with this lens.  I would hope the lens could bring a 3nm wide spectrum into focus. 

Wade

[Carlos Milovic]

Ok Just checking

[mmirot]

Unfortunately , this typical for many camera lens even with ED glass.  Design goals are different for daylight imaging.
It is hit or miss 

I just got back my Borg F 2.8 125mm  ED astrograph from Japan.
It needed repair.
It is really a just a big camera lens. I have not used with it a CCD,  only film until 2002.
It is not as good with chromatic correction as a normal ED telescope with a field flattner.   
It did a good Job with film. CCDs images are more sensitive.
I am waiting to see how good it will be when a I can refocus with mono CCD.  I need a adaptor for the 16803
One shoot color tests with the QHY8 look OK but not perfect.
Perhaps stopping down to F3.5 or 4 will do even better.

Max

[xatamec]

Hi twade,

At what f-ratio did you shoot?
A couple of weeks ago I was testing the older version, the Pentax Takumar 6x7 200mm. At the moment of doing the tests I didn`t have a Bahtinov mask, which is the way I focus my scopes, so I did not get a perfect focus. I shooted at f7.1, at this f-ratio the corners were good and there was not much CA. The shots (14x5') were taken with a Canon 350D with an Omegal Optical NPB filter from my home, 20km to the NE of Barcelona, so it's a very light polluted site (that's why I used that filter). The processing was just neutralising the background and stretching the histogram:



Link to full size image: http://img297.imageshack.us/img297/8223/cintur.jpg

Now I've got the Bahtinov Mask for the lens. I'm waiting for the clouds to leave to do some testing at lower focal ratios, f4 (wide open) and f5.6.

Regards,
ÇSergi

[twade]

ÇSergi,

I was shooting the lens at its wide-open (f/4.0) setting.  The next time I get a clear sky, I'll test the lens at f/5.6.  I'm hoping this is as far as I have to stop it down; however, I'm willing to go to F/6.7 which is the f-stop I have to shoot with while shooting with the Pentax 67 105mm f/2.8 lens.  One more thing I forgot to mention.  I used a FLI Proline 16803 CCD camera which has a pretty large (36mmx36mm) sensor. 

Wade