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Zoom telephoto lens for astro photography



Hi, I've been off-grid for quite some time due to CGEM problems. That now being fixed, I'm considering buying a Canon zoom telephoto lens of the 55-200mm. variety in order to capture images of deep-sky objects that don't quite fit my T1i's sensor as used with my 952mm focal length telescope.  Canon type EF or EFS zoom lenses with and without image stabilization are available, but since I want to use this lens for all purposes, and because I have hand tremors due to old age, I would go for the IS (image stabilization) option.  These lens offerings have a 58mm objective lens which is good for me because I already own filters of that size, but, are the 4.0-to-5.6 focal ratios adequate for deep sky imaging?  Should I, if I can afford it, go for the professional grade of Canon lens with a greater range of focal ratios, lower than 4.0 as a minimum? BTW, I don't want a fixed-focal length telephoto lens. Thank you for any suggestions.


Bob Z. 

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I would recommend the William Optics GT81 triplet apo refractor. It would have a 478mm focal length as is and a 382mm focal length with a 0.80X focal reducer. It is priced less that a premium Canon zoom telephoto but would not be ideal for terrestrial use.


Guylain also sells some Explore Scientific 80mm triplet refractors, as well!

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Unfortunately, if you are looking at ONLY the Tele-Zoom Canon Lenses, you have very few choices before the Prices quickly climb as High as a Good APO Refractor.


Consumer-level Tele-Zooms are designed for "Good-to-Very-Good Daytime Imaging at Most of the Zoom Span", but have significant Aberrations which make them less desirable for AP Imaging.  This is partly because of the large number of Optics Components it takes to make a "Good Lens" across such large Focal Ranges.


You need a Lens with rather Large Aperture / Fast Optics, as you'll really WANT to Stop-down the Lens a couple of Stops in order to avoid the worst of the Aberrations.


You also want one that has a very solid Manual Focus Mechanism, as AP Imaging will not be able to Auto-Focus.




The EF 55-200mm f4.5-f5.6 was discontinued in the 2005-2007 era - and was NOT that suitable for AP Imaging.


The replacement is the EF-S 55-250mm f4.0-f5.6 IS II and EF-S 55-250mm f4.0-f5.6 IS STM - apparently the Lens you are considering.  The various Daytime Imaging Reviews report that these Lenses are Quite Good.


The next-better Canon Lens is the EF 70-300mm f/4-5.6 IS USM - not as Wide at the short end but significantly Longer at the Telephoto end.  I've used this Lens for quite a while before moving on to Prime Focus Lenses for my Wide-Angle AP Imaging,


And further up the line are the 70-200 and 80-200 "L" series Lenses - first the F4s and better yet the f2.8s.  These, you can easily buy that APO Refractor for less coin...

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