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Eyepieces and Afocal

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#1 GOLGO13

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Posted 06 October 2021 - 10:51 PM

I was reading the article in S&T from 2018 and have some questions about eyepieces and Afocal.

 

Currently I have the 67mm (adapter inserted into the 55 TV Plossl) and a TV 40mm plossl for Afocal use. That's really the only a-focal options I have. Technically, I have my 24 pan setup for it (attaching a adapter to it), but supposedly it's not optimal for the setup.

 

When I want to look at Globulars I usually will use the 8 inch SCT which provides good image scale. I use my setup at prime focus and use a 2x barlow. This setup does very well, but is probably still a bit low in magnification.

 

If I use a 3x barlow it's usually too far and the view is better in a 2x barlow.

 

What would happen using something like a 7mm Delite for this situation (a-focal). 

 

Has anyone experimented with a-focal and many different focal length eyepieces, even high powered ones? I think I would be able to do planetary nebulas also with some extra magnification. Such as seeing some detail in the Blue Snowball (obviously not blue anymore, but seeing some detail inside of it).

 

 



#2 ButterFly

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Posted 08 October 2021 - 05:47 PM

I use a Baader zoom afocally.  I has internal reflections below a certain focal length, so I barlow at lower ones to get those.  Some planetaries do just fine with higher power.  It depends on your tube just how far you can go for extended objects without the noise getting in the way.  Point sources don't dim much with higher power until their extent becomes apparent.  I use them for fasteroids and moons to make motions more apparent.  All my Ethos up to 6 do fine with my tube for point sources with my 15".  APM's new zoom should have native Dioptrx support, when finally released.  Filters don't block too much of the good light (h-alpha being the most restrictive), so it really is just a matter of what your EBI is whether you can still use filters or not.  Extended objects' (such as planetaries and the sky) surface brightness decreases with higher power so they can get lost in the noise.  At some power, your sky background is less than your EBI and the object keeps dimming with higher power, whereas the background does not.  Some of the Abell planetaries are just too big and dim to benefit much from zooming in.  Protoplanetaries can have more going on than just h-alpha because they are still young, but usually small and bright.  Tiny elliptical galaxies in galaxy clusters can stay point-like for quite a while.

 

You can add extension tubes between your 2x barlow and the nosepiece in prime to get between 2 and 3x.

 

 

 

What would happen using something like a 7mm Delite for this situation (a-focal). 

 

Stop supposing and find out!  I suppose it will be similar to prime with the 3x barlow and will not cause forest fires.


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#3 GOLGO13

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Posted 08 October 2021 - 06:19 PM

Don't have any eyepieces lower than 24mm in the TeleVueline, so would need to know before purchasing

#4 ButterFly

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Posted 08 October 2021 - 07:30 PM

The APM zoom is looking quite good as it has native Dioptrx support.  That's still in the air.  Once you have prime though, its only real advantage over a barlow is that it's continuously variable.  Depending on your sensor size, prime is equivalent to around a 20-something eyepiece, without glass in the way reflecting things.  The barlow has some number of elements, and the eyepiece as well.  Let's say it's seven elements total, and in that part of the spectrum, the transmission is down to 90%.  So 0.9^7 gets through ~ less than half.  But it could still be 15K improvement with NV nonetheless.

 

If your barlows work fine for prime, barlow the 24 Pan.  That should give you 24, 12, 8, and 4 to play with.  With the 40 Plossl, that adds 20, 13.3, and 6.6.  Both take Dioptrx.  The 24, 20 and 12 vs 13.3 is good for the effects of the extra Pan elements over the Plossl.  That's more than enough to give you a sense of what happens, and if you like it, more than enough choices.  If your 2x barlow permits extension tubes, especially variable ones, it's less glass in the way than with an eyepiece, but can still be added to an eyepiece all the same.

 

If you can do unfiltered galaxies, try 891 out.  See whether the image scale helps you see more detail in the lane.  M15 is a good test glob.  As always, transparency matters, so give it time.

 

Bear in mind that point spread functions are twice as big in near-IR than in blue!  That's some pretty bad seeing to disrupt a decent view, but it's also half the power until it looks extended to the sensing elements.  Seeing "errors" add to the point spread function in quadrature.  SCTs start out pretty darn high in focal length.  My 15" dob at 1830mm has a shorter focal length.  For example, my 6 Ethos is about 300x in that scope.  It can only be used for point sources when I'm up on a mesa with great seeing.  I have tried to hold it up to the 3-6 Nagler when hunting Nereid, but I was unsuccessful.  The bloat was just hurting at that point.




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