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Equipment Discussions >> Eyepieces

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Sarkikos
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: mitaccio]
      #5650995 - 01/29/13 07:29 PM

I called Meade about an hour ago and asked about my order. The man called the vendor. He gave me a tracking number and sent me a "shipped" email. It shipped on Monday. So I should be getting my package soon.

Mike


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EdZ
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: Starman1]
      #5650998 - 01/29/13 07:30 PM

Let's use your estimated a maximum of about 6" length behind the back end. Even at that we get F= 1775, so f/11.8. Just refer to the graphic, it's the easiest way to find F

edz


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Sarkikos
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: EdZ]
      #5651014 - 01/29/13 07:39 PM

Edz,

Is there anything inherently wrong with increasing the f number of the C6 by using a 2" visual back and 2" diagonal? Shouldn't that make it easier to observe planets and the Moon, since higher magnification can be reached with eyepieces that have somewhat longer focal length, which tend to have longer eye relief?

I suppose there would be vignetting of the image if wide-field 2" eyepieces are used. But if the C6 is mostly used for medium to higher power viewing of the Moon and planets, and the field stop of the eyepieces are kept at or below the clear aperture of the C6, would there still be a problem with vignetting?

Mike


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JIMZ7
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: bob midiri]
      #5651034 - 01/29/13 07:49 PM

It figures-a great sale after I sold my Bosma 4" f/9.8 refractor.

Jim


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Starman1
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: JIMZ7]
      #5651116 - 01/29/13 08:36 PM Attachment (13 downloads)

Ed,
I attach a chart showing the change in focal length with back focus distance. On the 6", I see that a 6" back focus results in a focal length of just under 80", which is f/13.3


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EdZ
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: Sarkikos]
      #5651167 - 01/29/13 09:06 PM

Quote:

Edz,

Is there anything inherently wrong with increasing the f number of the C6 by using a 2" visual back and 2" diagonal? Shouldn't that make it easier to observe planets and the Moon, since higher magnification can be reached with eyepieces that have somewhat longer focal length, which tend to have longer eye relief?

I suppose there would be vignetting of the image if wide-field 2" eyepieces are used. But if the C6 is mostly used for medium to higher power viewing of the Moon and planets, and the field stop of the eyepieces are kept at or below the clear aperture of the C6, would there still be a problem with vignetting?

Mike




Two things

the mirror is not designed to operate outside a specific range. So the further you move from the norm, the higher the spherical abberation

The longer (or shorter) the focal lenght at which the mirror is forced to operate, the more the aperture becomes obstructed. The more the aperture becomes obstructed the higher the ratio of central obstruction to effective aperture, and therefore the more destructive the result to the contrast function.

BTW, I'll respond in this post to the graphic on effective focal length. If the focal length in that graphic is not correlating to what I've shown in my graphic, then it is wrong. I've seen numerous wrong graphics. I've actually tested the scope.


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EdZ
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: Starman1]
      #5651183 - 01/29/13 09:17 PM

Quote:

Ed,
I attach a chart showing the change in focal length with back focus distance. On the 6", I see that a 6" back focus results in a focal length of just under 80", which is f/13.3




This grapic is INCORRECT.

I've seen it before and commented on it before, in fact, here in these forums. I get tired of the same old *BLEEP*. That's part of the reason why I walked away from this site for a while, the proliferation of inaccurate data.

It assumes all (at least the C5, the C6 and the C8) SCTs have the stated nominal F precisely at the backplate. That is incorrect. That's why I took the trouble to ACTUALLY MEASURE the C5 the C6 and the C8 and posted those plots in the SCT forum.
Not only that but it presumes at 6" of backfocus the C6 is operating at 80" F=2032. That's just palin outright wrong. At near 6" of backfocus the C6 is found to be operating near F=1800. The differnce may not be due to the slope of the increase. More likely it's due to the fact that the C6 at the backplate is more like F=1300.

So, I'm going to ask you Don, Have you ever tested any of this? And if not, why then would you believe that chart is more accurate then my test results and post that chart and ignore my test results?

So, for all you other forum members, you can continue to rely on inaccurate assumed data plots, or you can rely on actual data. Your choice.

edz


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Sarkikos
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: EdZ]
      #5651260 - 01/29/13 10:05 PM

Edz,

Quote:

the mirror is not designed to operate outside a specific range. So the further you move from the norm, the higher the spherical abberation

The longer (or shorter) the focal lenght the more the aperture becomes obstructed. The more the aperture becomes obstructed the higher the ratio of central obstruction to effective aperture, and therefore the more destructive the result to the contrast function.




Thanks for the info. I was aware of bits and pieces of this here and there, but you put it together nicely in one easy-to-understand - though maybe hard to accept - package.

So, what would be the longest total length of accessories behind the visual back of a C6 that would neither appreciably increase spherical aberration nor appreciably decrease clear aperture? I'm looking for a good rule of thumb to keep in mind when considering accessories for the C6.

And is there any combination of accessories that would allow a Burgess Binoviewer (CA of about 22mm) to be used on the C6 without appreciable increase of SA and decrease of CA? I'd rather stay within these constraints with a 2" VB if possible, for the extra stability. But if that's not possible, so be it.

Mike


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EdZ
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: Sarkikos]
      #5651291 - 01/29/13 10:21 PM

All of that is addressed in the thread I linked to above. That same thread that has the plots of the effective focal length also has tests that determine the effective aperture for various configurations. Almost any configuration you can put together on the C6 that invloves a binoviewer is going to reduce the aperture, That thread has a lot more to read.

Quote:

is there any combination of accessories that would allow a Burgess Binoviewer (CA of about 22mm) to be used on the C6 without appreciable increase of SA and decrease of CA




No. even with the standard 1.25" VB and a standard 1.25" diagonal, it would be operating near F=2100, D=140, therefore f/15, with a CO of 41%.

edz


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Sarkikos
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: EdZ]
      #5651315 - 01/29/13 10:43 PM

So I take it that binoviewing planets or the Moon with the C6 would not be a good idea, if we're interested in important things like optimum detail and contrast? There would be an increase in SA, a decrease in effective aperture, and an increase in the CO ... all of which means a decrease in surface detail and perceived contrast. Correct?

Mike


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EdZ
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: Sarkikos]
      #5651317 - 01/29/13 10:45 PM

correct
try the C8 for much better performance


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Sarkikos
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: EdZ]
      #5651337 - 01/29/13 11:00 PM

Well, I don't think I'll be getting a C8 anytime soon. I'm full up with scopes as it is (look at my sig).

In any case, I recently bought the C6 as a grab-n-go scope, an upgrade from my little 90mm Mak. The C8 wouldn't exactly be grab-n-go for me here. But I'll just try to accept the C6 for what it is, with its abilities and contraints, and be happy with that.

Mike


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Starman1
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Reged: 06/24/03

Loc: Los Angeles
Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: EdZ]
      #5651510 - 01/30/13 01:27 AM

Quote:

Quote:

Ed,
I attach a chart showing the change in focal length with back focus distance. On the 6", I see that a 6" back focus results in a focal length of just under 80", which is f/13.3




This graphic is INCORRECT.

I've seen it before and commented on it before, in fact, here in these forums. I get tired of the same old *BLEEP*. That's part of the reason why I walked away from this site for a while, the proliferation of inaccurate data.

It assumes all (at least the C5, the C6 and the C8) SCTs have the stated nominal F precisely at the backplate. That is incorrect. That's why I took the trouble to ACTUALLY MEASURE the C5 the C6 and the C8 and posted those plots in the SCT forum.
Not only that but it presumes at 6" of backfocus the C6 is operating at 80" F=2032. That's just plain outright wrong. At near 6" of backfocus the C6 is found to be operating near F=1800. The difference may not be due to the slope of the increase. More likely it's due to the fact that the C6 at the backplate is more like F=1300.

So, I'm going to ask you Don, Have you ever tested any of this? And if not, why then would you believe that chart is more accurate then my test results and post that chart and ignore my test results?

So, for all you other forum members, you can continue to rely on inaccurate assumed data plots, or you can rely on actual data. Your choice.

edz



I've checked the math in the rest of the article, and the curves are correct. The mistaken assumption of the author, to which you alluded, is that the reference focal length position is the back plate of the scope. A Celestron engineer told me years ago that the 2032mm focal length of the C8 was actually at a distance of 4" (100mm) behind the back plate of the scope, i.e. with the visual back and 1.25" star diagonal in place. Backing that up was the fact that when I collimated the scope, straight through, the field size was slightly larger than the focal length of the eyepiece indicated, showing the shorter focal length.

I apologize for not immediately assuming that this was true for other sizes of SCT as well--that the reference focal length was at a set distance behind the back plate of the scope.

It would be valuable information to know the reference focal length position behind the cell for every SCT and MCT, as you have determined for some. I find it odd the manufacturers don't publish the information.
The graph should be revised to accommodate the data.


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EdZ
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: Starman1]
      #5651635 - 01/30/13 06:12 AM

Sorry I got snippy.

Quote:

A Celestron engineer told me years ago that the 2032mm focal length of the C8 was actually at a distance of 4" (100mm) behind the back plate of the scope, i.e. with the visual back and 1.25" star diagonal in place.




That's approximately correct for the C8.

For both the C5 and C6, they operate at the nominal f/10 at approx 60mm behind the back port.

The C5 with a standard 1.25"VB and 1.25" diagonal is operating at F=1370.
The C6 with a standard 1.25"VB and 1.25" diagonal is operating at F=1650.

edz


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dscarpa
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: Sarkikos]
      #5652008 - 01/30/13 10:40 AM

Mine came yesterday! The UWA is comically large. It gets stuck in the bottom of a giant bolt case. I ended up using the case upside down. I viewed M42-M43 with the UWA in my IM-715 mak back to back with my 24 Brandon and am impressed with it's performance. David

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Sarkikos
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: dscarpa]
      #5652057 - 01/30/13 11:06 AM

The ETA of mine is this Friday. I'll have to see how it compares to my ES 82 30. The weight is about the same. I will put the Meade on a diet. But then I'll have to see if I can find an eyecup to fit it.

The Meade 5k UWA 24 has better coatings and a wider AFOV than the Brandon 24. I'm not surprised that the Meade would impress by comparison.

On the other hand, the simpler coatings of the Brandon can allow more structure to be seen in bright nebulae. But you can approximate that by screwing on a DSO filter. So would you rather have your filtering built into the eyepiece or in a filter you can remove?

Mike


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jgraham
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: Sarkikos]
      #5652259 - 01/30/13 12:50 PM

Mine is still listed as 'Shipping soon'. If that doesn't change in the next day or so (a full week after placing my order) I'll probably cancel my order.

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csrlice12
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: jgraham]
      #5652270 - 01/30/13 12:57 PM

Truthfully, a week is not a long time for a response, I'd give it a few more days anyway.

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Sarkikos
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 new [Re: jgraham]
      #5652294 - 01/30/13 01:12 PM

If you're getting antsy, call the company and ask for the status of your order. Mine had already shipped when I called, but they hadn't caught up with sending out emails to notify customers that their packages had shipped. I received the email and tracking number soon after I called.

Mike


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jgraham
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Re: Meade UWA 24mm & 2" Dialectric Diagonal: $159 [Re: Sarkikos]
      #5652396 - 01/30/13 01:56 PM

Yeahhhh, I'll be patient, but it kinda sucks the fun out of an impulse buy. If you have to wait more than a week for something bought from a weekly special to even ship there's something of a mismatch there. A friend of mine once told me that the most efficient part of any business should be the part that takes the money.

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