Best 200mm Lens for Astrophotos

[scott_tucker]

<p>I am looking for a medium-format lens to use with a CCD camera for astronomical imaging. Because of the format of the CCD sensor (36x36mm) and the required flange focal distance (>52mm), I'm looking at medium format lenses. For my application, I'd like a 10-degree square field, which requires a 200mm focal length. I'm hunting for suggestions on the best quality lens. I'd like an f/2.8 or faster lens, but f/4 is fine if the quality is better. For the sake of discussion, let's say price is no object. My choice in 35mm format would be the Canon 200mm f/1.8, for reference, but obviously this lens doesn't meet my mechanical requirements. Basically, I need something very sharp wide open, or one stop down at most. I saw a recommendation of the Mamiya 200mm f/2.8 APO in the forums, so that's the leading contender, but I wondered if there were opinions on other lenses as well. Thanks for any suggestions!<br>

Scott</p>

[bob_sunley]

<p>If you have the budget, you will get way better results with this: http://www.landseaskyco.com/catalog/product_info.php?cPath=21_26_31_37&products_id=36</p>

 

[Ed_Ingold]

<p>It's really hard to get sharp astronomical images with a compound camera lens. Aberations and internal reflections play havoc with the results. The best results at reasonable price are probably from a good refracting telescope with a 4" achromatic objective (or an 8" or larger catadioptic reflector). The larger the diameter, the greater the light gathering capability. The relative aperture is immaterial. The "aperture" of an astronomical refractor (typically f/8) affects its magnification, not light gathering.</p>

<p>You also need a solid tripod, an equitorial mount and a tracking drive.</p>

[alpenglow]

<p>Scott,</p>

<p>The Mamiya 200mm APO f/2.8 would be perfect for that application. Not exactly cheap though.</p>

<p>Mike</p>

 

[alpenglow]

<p>Also... because the Mamiya's image circle will have much larger coverage than the 36mm CCD, you won't have any issues with coma or abberation... you'll be able to shoot wide open no problem.</p>

<p>Mike</p>

 

[ondebanks]

<p>Scott,</p>

<p>ME! ME! ME! (That's me jumping up and down, waving my arms, shouting to get your attention!)</p>

<p>You see, I'm the man you want to hear from: I own the aforementioned Mamiya 645 200/2.8 APO - and I use it for Astrophotography! (and visual astronomy too!)</p>

<p>First, some housekeeping: Bob is somewhat off-track, and Edward is talking pure nonsense.</p>

<p>Bob's suggestion of the $$$$ Tak APO refractor makes sense at first glance, but he seemingly didn't read Scott's specification: "I'd like a 10-degree square field, which requires a 200mm focal length". The Tak Sky 90 is 500mm focal length - way too long! Plus, it is not a flat-field design (it's just a doublet), so it needs a separate field flattener accessory. The flattener-reducer brings it down to 400mm focal length (still 2x too much) - and then the 45mm image circle is too small for Scott's 36x36mm CCD (51mm diagonal). So, wrong focal length, not enough image circle, and the ensemble is much more expensive than the Mamiya 200/2.8 as well.</p>

<p>(BTW, If I were to suggest a Tak APO for large-sensor imaging, it wouldn't be the Sky 90; it would be the one of the Q series: the FSQ-106, which is a 4-element flatfield design with a massive 88mm image circle; or its little sister, the "BABY Q" FSQ-85ED, with a 60mm image circle:<strong></strong> <strong></strong> <br>

http://www.landseaskyco.com/catalog/product_info.php?cPath=21_26_31_36&products_id=656<br>

http://www.landseaskyco.com/catalog/product_info.php?cPath=21_26_31_36&products_id=1313<br>

- but although both have the image circle for Scott, neither is short enough in focal length or quite fast enough either).</p>

<p>(BTW BTW, I strongly suspect that if Scott is using an astronomy-grade 36x36mm CCD, he already has a large high quality imaging telescope of medium to long focal length, perhaps something like those Taks, and is now posting this question because he is looking to _downsize_ in focal length in order to get this unusually large 10x10 degree field of view).</p>

<p>(BTW BTW BTW, yes it's true: only an astronomical telescope manufacturer would label an optic weighing 9 pounds as a tiny, cute little "BABY").</p>

<p>I'll come back to Edward later...</p>

<p>Scott, the Mamiya 200/2.8 APO will be perfect for your needs. 63mm flange focal distance. 70mm nominal image circle (645 diagonal). I've posted here before about its optical performance:<br>

http://www.photo.net/medium-format-photography-forum/00MYXw .<br>

That was relatively soon after I had got it. I have since shot it on a guided equatorial mount under dark rural Irish skies, and said some more about it:<br>

http://www.photo.net/medium-format-photography-forum/00TiMA<br>

I spoke of my solution for a tripod collar mount here:<br>

http://www.photo.net/medium-format-photography-forum/00Tzs9<br>

And some more ramblings, where i compared it to the CZJ 180/2.8 here:<br>

http://www.photo.net/medium-format-photography-forum/00Tqlr</p>

<p>Looking at centred 36x36mm crops (ah, Xnview!) of my 2400-dpi (10-micron "pixels") scanned deep-sky 645 photos on Kodak E200 and Konica Centuria 400, I see this performance:<br>

* f2.8: sharp over central region, slight longitudinal chromatic aberration, no lateral colour, no coma, slight corner astigmatism, moderate corner vignetting. The field is very flat: the corner stars are also sharp, but just a little tangentially stretched by the astigmatism. Not perfect, but very good for shots with your CCD where time is of the essence - e.g. a low or fast-moving comet.<br>

* f4: very sharp over central region, very slight longitudinal chromatic aberration, no lateral colour, no coma, perhaps a hint of corner astigmatism but you have to look hard for it, slight corner vignetting. I would consider f4 suitable for general imaging with your CCD. I normally use mine at f4 for deep sky on film, unless I'm being really picky.<br>

* f5.6: what Sky and Telescope would describe as "sensibly perfect". Very sharp over entire field, no astigmatism/vignetting/coma/lateral colour. There is still minor longitudinal chromatic aberration, but this might be due to scattering within the E200 emulsion or its extra-panchromatic sensitivity. It doesn't show up visually (more on that later), and on expired Konica Centuria 400 colour negative film, the chromatics are hard to see, the fainter stars are as pinpoint and tiny as the grain, and the bright stars show nice 8-point diffraction from the iris.</p>

<p>Image link - 645 full frame at f4, E200, 2400 dpi:<br>

https://dartagnan.nuigalway.ie/quixplorer/index.php?action=download&dir=Picz&item=E200_Z1_0002.jpg&order=name&srt=yes</p>

<p>Image link - 645 full frame at f2.8, Centuria 400, 2400 dpi [pardon the satellite trail]:<br>

https://dartagnan.nuigalway.ie/quixplorer/index.php?action=download&dir=Picz&item=vs64.jpg&order=name&srt=yes</p>

 

[ondebanks]

<p>Oops - just found that those image links don't currently work from outside my institution. I'll find another way...</p>