Photo lens vs William Optics Z61

[mitchsweden]

Hello!

I am just beginning my astro photography journey and am wondering what advantages a WO Z61 (or similar) would give me over a 100-400 f5.6 telephoto lens? I have used this lens and gotton fairly good results, with post processing in Pixinsight. I have read and seen a lot of reviews of the Z61 but other than field flattening can’t really tell if it is worth the investment. 

Any help, or even suggested options would be greatly appreciated! 

Attached is my version of m42 taken with this lens and processed in PIS. I am also still trying to understand the processing workflow so be gentle with the criticism.

Thanks and clear skies from seemingly always cloudy Sweden!

[RT65CB-SWL]

I am not into astro-imaging at present, but my view is with the lens you mention (100-400mm) is it a telephoto zoom?

If “Yes!” then someone from my local club/society told me many years ago that I may get some unwanted reflections with a zoom due to the extra glass elements. At the time I was using an 80-200mm zoom. 

I now use a 70-300mm and I do get internal reflections when trying to image the Moon. I am thinking it maybe due to my UV/IR filter at the objective lens end. I also have an ‘fixed’ 400mm telephoto lens with an M42 thread (plus T-ring for my DSLR body) and not yet tried it.

Edited April 12, 2021 by Philip R

[vlaiv]

Lens are hardly ever diffraction limited.

Telescopes are diffraction limited and corrected for infinity. They will always outperform lens on astro images (except very cheap achromat vs lens costing many thousands of pounds).

By the way - that is very nice M42 image.

[Grant Fribbens]

I started out with a ZS61 as my pictures with a 55-210 Zoom and x2 teleconverter had reflections. I liked the fact that there is a non reducing flattener as well as a reducing flattener available which some telescopes just do not have the option of or you have to use a generic flattener and go through the process to get the correct distance dependant upon the focal ratio of the scope that you are using.

I use a one shot colour camera I have been finding on the histograms that even though the ZS61 has great FPL-53 glass that one colour was always a bit bloated as not all wavelengths meet at the same place and have got areas of CA like you have from your M42 image which are the blue colours around the stars. I have now moved on to a triplet (WO GT-81 IV) which I can see from the histograms has really sorted things out. I would expect this would not be a problem if imaging in mono as you would re focus or have offsets when you change filters to make sure that the wavelength you are after is always spot on focus.

The great thing is that since I have started other manufacturers have been producing some great triplets like the Sharpstar EDPH series. They are more tempting now that they have also got matching field flatteners rather than their generic field flattener but the are all reducers. The downside to a triplet is the extra weight and the fact that a 61 sized triplet has a lot less focal length than a 61 doublet and when you have reducing field flatteners then the focal length gets much smaller, great for wide field of view targets.

[mitchsweden]

Thank you for the information and advice, and even a compliment! I appreciate it very much. I will look into the WO rather than a new lens purchase. 

23 hours ago, Philip R said:

I am not into astro-imaging at present, but my view is with the lens you mention (100-400mm) is it a telephoto zoom?

If “Yes!” then someone from my local club/society told me many years ago that I may get some unwanted reflections with a zoom due to the extra glass elements. At the time I was using an 80-200mm zoom. 

I now use a 70-300mm and I do get internal reflections when trying to image the Moon. I am thinking it maybe due to my UV/IR filter at the objective lens end. I also have an ‘fixed’ 400mm telephoto lens with an M42 thread (plus T-ring for my DSLR body) and not yet tried it.

 

22 hours ago, vlaiv said:

Lens are hardly ever diffraction limited.

Telescopes are diffraction limited and corrected for infinity. They will always outperform lens on astro images (except very cheap achromat vs lens costing many thousands of pounds).

By the way - that is very nice M42 image.

 

22 hours ago, Grant Fribbens said:

I started out with a ZS61 as my pictures with a 55-210 Zoom and x2 teleconverter had reflections. I liked the fact that there is a non reducing flattener as well as a reducing flattener available which some telescopes just do not have the option of or you have to use a generic flattener and go through the process to get the correct distance dependant upon the focal ratio of the scope that you are using.

I use a one shot colour camera I have been finding on the histograms that even though the ZS61 has great FPL-53 glass that one colour was always a bit bloated as not all wavelengths meet at the same place and have got areas of CA like you have from your M42 image which are the blue colours around the stars. I have now moved on to a triplet (WO GT-81 IV) which I can see from the histograms has really sorted things out. I would expect this would not be a problem if imaging in mono as you would re focus or have offsets when you change filters to make sure that the wavelength you are after is always spot on focus.

The great thing is that since I have started other manufacturers have been producing some great triplets like the Sharpstar EDPH series. They are more tempting now that they have also got matching field flatteners rather than their generic field flattener but the are all reducers. The downside to a triplet is the extra weight and the fact that a 61 sized triplet has a lot less focal length than a 61 doublet and when you have reducing field flatteners then the focal length gets much smaller, great for wide field of view targets.