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More experience with 40mms in Mark V

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#1 Eddgie

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Posted 21 January 2013 - 10:11 AM

Just a followup. I have been using the Televue 40mm Plossls more in the Mark V behind the C14.

I have mixed emotions about the combination, but still come down on the positive side for larger SCTs I think.

Last night, I spent a lot of time on larger clusters. M35 (didn't fit into the field of course), M36, M37, M38, M47, and a few lesser known clusters.

In all cases, whey I liked was that becuase of th low magnification, stars were brilliant pinpoints. When seeing is less than perfect, stars can flare and blur at higher powers in a big aperture, so keeping the power low simply makes it hard to see this activity. Stars look more pinpointy.

I used to use my 41mm Panoptic a lot even for smaller clusters because the low magnification simply produced a brilliant view filled with tiny stars.

The 24mm Hyperions give far more magnification, and I had found that much of the time, seeing was keeping the stars from appearing with such beautiful sharpness.

The 40mm Plossls restore all of that. The stars are lovely small pinpoints all the way to the edge of the field even when seeing is less that perfect. And really, for stars, it doesn't take a lot of magnificatoin. For example, the E and F stars in the Trap were easily visible at 99x.

Clusters also just appeared brighter. No doubt the bigger exit pupil takes the credit for this. Using binoviewers makes the field look dim for two reasons.. The first is that well, it is dimmer... But the second is that in the SCTs, a given eyepeice will always work at higher power in the binoviewer so the exit pupil is smaller. This is a double whammy.

The 40s have restored all of that. The clusters just jump out more.

And I spent a lot of time around Orion Nebula. Once again, the big exit pupil has really increased the extension of the Nebula. the down-side is that now it doesn't fit (It fits into the field of a 41mm Pan). Still, there is no doubt at all that the field was brighter and that the Nebula just jumped out more.

The downside... The view is not nearly as immersive. The quality of being "Immersive" has been discussed in conjunctoin with the T4 Naglers many times. There is something about the expansiveness of the field in the Naglers that is hard to describe, but that many observers report. And I think Binoviewers give that same feeling of immersion even with narrower AFOV eyepeices.

But not in the 40mm Plossls. The view is beautiful, but lacks the spacious feeling that one gets in the widefields.

But I like my stars sharp. I can't help it. A field filled with sharp stars has become far more important to me in recent years than a wide apparent field.

Some of this may be a function of the rather narrow 43 degree apparent field in the 40s.

Next step... Ok, maybe if I split the difference. I am going to try a pair of inexpensive 32mm Plossls. I was going for some 35mm Plossls, but new ones are stupidly priced and I could not find used ones.

Anyway, this will spit the difference between apparent field and magnification.

The desired result is to try to re-capture some of the immersiveness, but to maintain the super-bright and super fine pinpoint peformnace that the 40s are delievering.

I have to say this too. The more I use the 40s, the acclimated I have become to the narrow field.

Just as an expirement, I did go back and forth between the 24mm 68s, the 41mm Pan, and the 40s in a bino, and the more I just focused on the target, the less bothered I became with the narrow field.

The 32mm Plossls are in flight now and I hope to give then a run in the next couple of days. I will of course share my impressions.

For now though, I have to say that the 40s have been 100% successful at shapening the view (by reducing the blur caused by seeing) and brightnening the view. Clusters really popped using the 40s, especially if the cluster fit easily into the field (m36 for example). If the cluster was well framed, it really jumped out in the 40s.

#2 junomike

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Posted 21 January 2013 - 10:34 AM

Eddgie, I find the my 40mm TV Pl to be a little more easy to use then the 32mm Pl. This is dependent on what scope I'm using them in. For my SCT I usually grab my 24mm Pans, but all my TV Pl's (NJ's) from 17mm - 40mm see use.

In a friends 16" Dob, I found the 32mm NJ's put up an Image of M11 that had everyone in awe, even with the much needed 2X Amplifier in the Optical path.

I'm also not so sure It's the seeing that's hindering the Hyperions. I have a had a variety of eyepieces and find that although the TV Pl's can be bested for transmission and scatter control, they are tough to beat for star sharpness and color saturation.
The 24mm Pans come close, but I find that for sharpness the TV Pl's come first, then the 16T5's followed by the Pan.
Just my $0.02

Mike

#3 Eddgie

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Posted 21 January 2013 - 01:18 PM

Well, off axis the Hyperions I don't think are quite as good as Panoptics, but even on axis when you are using 165x (the power the 24mms give in the C14) even when seeing is decently good, stars start to fuzz up a bit. It is just a lot of magnification.

I think for faster scopes, 24mm would not be an issue even is seeing was not great.

But at amost 4000mm of focal lenght, unless seeing is pretty darn good, brighter stars will start to appear a bit bloated at 165x.

I bought a cheap pair of 32mm to see if I can still keep sharp stars with a bit higher magnification but a bit bigger apparent field.

So far though, I can't complain about the 40mms. As I get more accustomed to the narrower field, I am finding it less objectionable.

Also, I have to say that as I reported earlier, it does make it easier to take in the whole field. With 68 degree eyepeices, I think it is a bit harder to see things at the edge. I get blackouts sometimes if I roll my eyes to one side or the other. One eyepiece will black out. Likely hard to hold the exit pupil exactly on axis for both eye because the head has to shift a bit to one side as you roll your eye to the other.

Again, this is because of the very small exit pupil a 24mm eyepecie gives in a C14.

#4 junomike

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Posted 21 January 2013 - 03:29 PM

Eddgie, I can see your point on magnification, but I've had excellent results at high power with the TV Pl's.

The 4000mm F/L of your scope does limit your choices for eyepieces, but wouldn't you be running closer to 4800mm with the Bino's in the optical path?

In regards to the exit pupil, wouldn't your C14 be just a tad less than my C11 (F11 vs. F10)?
Also, with the Bino's in use, does the exit pupil further decease as the Bino's add F-ratio (say F14 instead of F11 for you and F10 for me)?

The reason I ask is one of my most used and preferred Bino pairs are the 18mm UO H.D.'s. Super sharp and the FS is easily seen. I've never worried about the Exit Pupil before.

Mike

#5 Eddgie

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Posted 21 January 2013 - 04:47 PM

Unlike the smaller Celestron SCTs, the C14 was designed for optimal focal length (3910mm) with a 2" diagonal (150mm back focus for optimal focal lenght, while the other Celestron SCTs are set up for 100mm for optimal focal length).

I have taken great pains to keep my focal lenght as short as possible. My current configuration measures about 200mm. This is obtained by using a Baader T2 diagonai and the Mark Vs.

At 200mm of back focus, the C14 is going to have a focal lenght of right at 4000mm.

Also, the C14 starts to loose aperture at any back focus past this point, so this is as long a light train as I will use for this scope. I am only loosing a couple of millimeters of aperture in this configuration.

For me, exit pupil = Brightness. I feel like the view is far brighter using the 40s over the 24mm or shorter eyepecices.

Lately I have started to feel like the loss of brightness is enough that it is indeed slightly dimming the view.

Oh, it is good, don't get me wrong, but I feel as if even the 24mm eyepecies just seem to make some targets a bit dimmer than with mono-vision, though I still prefer the view even though I am loosing a little brightness.

My plan is to try to compensate by using a slightly narrower AFOV eyepecice with slightly lower powers to compenstate for the bino induced dimming and increased focal length resulting in a smaller exit pupil with less light.

#6 junomike

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Posted 21 January 2013 - 05:31 PM

Eddgie, I know the C14 is shipped with a 2" XLT Diagonal whereas the others usually come with a cheapo 1.25" Diagonal, but the figures Celestron issues have all their SCT's with similar Back focus and the C11/C14 Identical with 5.475" when using the 3.25" - 2" adapter and 5.975" If going right from the 3.25" Threads. Is this correct?

If so, both the C11 and C14 would have almost 6" Back focus from the lock ring.

Mike

#7 Eddgie

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Posted 21 January 2013 - 07:56 PM

These I think are for imageing and are based on edge of field illumination for a 35mm film frame.

The C14 has focal ratio of f/11 and a focal lenght of 3910mm at about 150mm of back focus.

The C11 has a focal ratio of f/10 and reaches a focal lenght of 2800mm with about 100mm of back focus.

I have ray traces for the C8, C9, C11, and C14 done by Ken Hutchinson that has tables with the focal ratio, focal lentgh and field illumination with various amounts of back focus.

It also has aperture reduction figures for various amounts of back focus with and without focal redcuers.

It is an excellent paper.

If you would like a copy, just PM me your email address and I will send it along.






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