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EP for 16" f5 truss dob


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Thinking of a widefield ep for the PADAS dob.

Televue Naglers are out due to Society budget constraints and we need to get a shroud as well.

Thinking of a second hand ES 28mm 68 degree, any others come to mind?

 

20170525_221508.jpg

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Nearly identical to the ES 68 are the Meade SWA and its identical twin, the Maxvision 68°. I have a 28 mm Maxvision and it is very good at f/5. (Never tried it on anything faster than that.)

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Sorry can't help you re make of dob as we bought it second had recently with no provenance apart from it having Oldham Optics mirrors.

Must admit its been put together very well and with what looks like ptfe sliding blocks its very easy to track objects like the ISS.

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The 28mm Explore Scientific Maxvision optically is very good quality.  It shows visible coma at f/5 at the at the outer 1/3rd but that's to be expected with any eyepiece at this focal length.

Bresser have some ex-demo available of various eyepieces that may be looking at.  I've bought from there before with no problem (although the packaging could have been a little better!).  The 28mm Maxvision is 75 Euros plus postage (7 euros ish if I remember rightly....)

https://www.bresser.de/Sale/Ausstellungsware/Explore-Scientific-Maxvision-68-Okular-28mm.html

They also have the 34mm ES68 for a good price (no idea what this one is like though...).

Alternatively, FLO have a new 28mm ES68 available for £129.  I really like my ES68 (24mm) it's very comfortable to use and great performance for the money.

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I found my 27mm Panoptic worked very well in my 15" f5 dob when I was regularly using it.  I would agree with the 28mm ES/Meade/Maxvision recommendations above.  I'd also invest in a GSO coma corrector for it as well; although I have yet to find a European dealer for it.  There are multiple US retailers offering it.

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  • 4 weeks later...

You've probably already got your answer at this point, but I'll throw in my two cents since I'm going through something similar. I have a 12" f/4.9 truss dob. I want something that will give me a wide enough field of view and low magnification to capture the entirety of the largest DSO's like Andromeda and the Orion Nebula. I also don't want something with an exit pupil larger than 7mm. That's a pupil size that's generally referred to be about as large as the human eye is capable of. If I'm not mistaken, you also risk a darkening in the center of the image due to the obstruction from the secondary mirror if you go too much larger. So doing some quick math, I have a focal length of 1500mm and an aperture of 305mm. Using an 7mm exit pupil, means my lowest useful magnification is about 43-44x which means I want an eyepiece of about 34mm focal length. The tricky bit is finding one with an appropriate apparent field of view and decent eye relief. There are some 35mm eyepieces, but their field of views are too small. I need something that's at least 68 degrees to capture the largest objects. Tele Vue makes a Panoptic 35mm with 68deg field that puts me just above 7mm and a magnification of 43x. Explore Scientific makes a 30mm with an 82deg field that puts me at about a 6mm exit pupil and a magnification of just over 50x. Both give me almost exactly the same true field of view with the Explore Scientific being just a bit wider thanks to the wider apparent field. Either would be an excellent option for my telescope, but I've decided on the Explore Scientific for the slightly wider field and to keep the exit pupil below 7mm. I doubt my old eyes could get that big anyway.

It really depends on what sort of objects you want to view. For you, at an aperture of 16" (~400mm) and a focal length of about 2000mm, you'd be looking at about a 35mm eyepiece to give you that 7mm exit pupil. So definitely don't go any larger than that. Then you need to start looking at what's going to give you a wide enough field of view to capture what you want to see. If you want to capture the full Andromeda Galaxy or the Orion Nebula and the nearby NGC's, you're probably going to need to go larger than 68mm. Again, it all depends on what you're after. First Light Optics (forum sponsor, link at the top of the page) has an excellent Field of View Simulator tool for comparing different eyepieces and what you can expect to see through different scopes. There happens to be a custom 16" f/5 dob in the drop down list for telescopes. You can change your target to any celestial body you want to see and just add and delete equipment as necessary until you find an eyepiece that might work for you.

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55 minutes ago, Buzzard75 said:

The tricky bit is finding one with an appropriate apparent field of view and decent eye relief.  Explore Scientific makes a 30mm with an 82deg field that puts me at about a 6mm exit pupil and a magnification of just over 50x. I've decided on the Explore Scientific for the slightly wider field and to keep the exit pupil below 7mm.

Just be aware that the eye lens is massively recessed as seen in this image.  This vastly reduces usable eye relief.  I have the older mushroom version with plenty of eye relief but not waterproofed.  The 31mm Celestron Axiom and 30mm Meade UWA are optically identical.  All of the older versions can be decloaked to reduce weight and make them narrower, but eye relief remains about the same (except for the Celeston which improves).

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