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Equipment Discussions >> Eyepieces

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BillP
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Reged: 11/26/06

Loc: Vienna, VA
Re: Eyepiece Performance using a Prism Diagonal new [Re: junomike]
      #6228016 - 12/01/13 09:36 PM

And the fringing on the lunar limb was when it was positioned dead center of the FOV or not. If not, then can also be lateral color from the eyepice...although an XO would not have that. But if only lateral color, then that has no impact for an on-axis planetary view.

Anyway, thanks for the details. I will be sure to run some lateral color compares also as maybe the prism induces this. I did no lateral checks as was just doing on-axis planetary observing.

Edited by BillP (12/01/13 09:37 PM)


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Tank
Carpal Tunnel
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Reged: 07/27/09

Loc: Stoney Creek, Ontario, CANADA
Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6228119 - 12/01/13 10:48 PM

Bill
I will back Mikes result from what i recall the prism showed me CA that was not in the 99% mirror diagonals
I felt that there a significant difference and liked the mirror diagonals for a few reasons.
In the APO triplets/ mirrored scopes i felt the prism made them a doublet scope which had a small amount of CA.
The Diagonals that were tested were the AP,WO 99%, Denk 99% from my results i would recommend a mirror over a prism.


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Tank
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Reged: 07/27/09

Loc: Stoney Creek, Ontario, CANADA
Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6228120 - 12/01/13 10:48 PM

Bill
I will back Mikes result from what i recall the prism showed me CA that was not in the 99% mirror diagonals
I felt that there a significant difference and liked the mirror diagonals for a few reasons.
In the APO triplets/ mirrored scopes i felt the prism made them a doublet scope which had a small amount of CA.
The Diagonals that were tested were the AP,WO 99%, Denk 99% from my results i would recommend a mirror over a prism.


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jrbarnett
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Reged: 02/28/06

Loc: Petaluma, CA
Re: Eyepiece Performance using a Prism Diagonal new [Re: Sarkikos]
      #6228276 - 12/02/13 12:56 AM

"Now how will a good prism diagonal work vs a mirror diagonal in an SCT or Mak?"

Similarly, it will depend on the scope, but you have so many other things going on with a Cat that it may be harder to get the scope in optimal condition to be able to isolate the rather subtle diagonal differences.

Regards,

Jim


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Fomalhaut
scholastic sledgehammer


Reged: 08/16/08

Loc: Switzerland
Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6228387 - 12/02/13 04:29 AM

Bill,

a) When used without diagonal OR with the Baader Maxbright (140 layers of dielectric coatings) my Tak FCT100 f/6.4 doesn't show any false colour neither inside nor outside of focus. And what might be even more comprehensive, in these two configurations it shows its most perfect startest i.e. absolutely indistinguishable identity of defocused airy disks on either side of focus.

b) The Tak 44mm-prism diagonal introduces substantial overcorrection resulting in visible spherochromatism, less than perfect star-test and slightly reduced planetary performance. Main reason for all of this => presumably the telescopes' short f/6.4 focal ratio.

c) Compared to b), the smaller Tak 1.25-inch prism diagonal just shows a tiny little bit of spherochromatism as well as clearly better low-contrast Jovian surface detail. Compared to a), it shows only slightly reduced perfection of star-test (quite better than b: the 44mm-prism).
Surprisingly, the 1.25-inch Tak prism diagonal tends to show slightly better (!) Jovian surface detail even as when compared to either of the two seemingly perfect a)-configurations. Since there seems to be no difference in stray-light between a) and c), I do not have an explanation for this last comparison's diverging results...

Chris


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Sarkikos
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Loc: Scotophobe Maryland, USA
Re: Eyepiece Performance using a Prism Diagonal new [Re: Fomalhaut]
      #6228481 - 12/02/13 06:58 AM

Quote:

c) Compared to b), the smaller Tak 1.25-inch prism diagonal just shows a tiny little bit of spherochromatism as well as clearly better low-contrast Jovian surface detail. Compared to a), it shows only slightly reduced perfection of star-test (quite better than b: the 44mm-prism).
Surprisingly, the 1.25-inch Tak prism diagonal tends to show slightly better (!) Jovian surface detail even as when compared to either of the two seemingly perfect a)-configurations. Since there seems to be no difference in stray-light between a) and c), I do not have an explanation for this last comparison's diverging results...

Chris




Maybe the tiny bit of induced spherochromatism from the Tak 1.25 prism diagonal enhances contrast for Jovian surface detail in the same way that a good contrast filter works, by subtly shifting the color balance in the image? If so, it'd probably be better to just use a Baader Moon & Sky Glow, and keep the mirror diagonal in the focuser.

Mike


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BillP
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Reged: 11/26/06

Loc: Vienna, VA
Re: Eyepiece Performance using a Prism Diagonal new [Re: Fomalhaut]
      #6228595 - 12/02/13 08:48 AM

Quote:

Surprisingly, the 1.25-inch Tak prism diagonal tends to show slightly better (!) Jovian surface detail even as when compared to either of the two seemingly perfect a)-configurations. Since there seems to be no difference in stray-light between a) and c), I do not have an explanation for this last comparison's diverging results...




Chris,

Perhaps a possibility is what Jim B. suggested, that if we know nothing of the main optic then really difficult if not impossible to figure out root cause. With spherochromatism in particular, most designs have some, as does the prism, so question becomes what is the exact spherochromatism plot for each wavelength of light in both. Knowing that is the only way to know how the prism may be impacting the overall image. So this then necessarily means it will vary by scope as well, and target we are observing. Jupiter is a fairly neutral colored object with more colors in the blue range than anything else. So performance on Jupiter may differ when compared to performance on Saturn, vs Mars, vs Luna. All depends on the the predominent colors involved with each target and how the prims's offset if compared to the main objective's offset with color performance.

While the root causes are indeed interesting, for me this is a system test by target. So how do these diagonals offer up the view of Jupiter differently in a TSA-102. As scopes are changed and targets are changed, I imagine results will change also. If we want to get down to the science of it so we can have a predictive model, that is well beyond anyone here as we would first need the bench tested results for each component relative to its spherochromatism and other factors.

I will definitely test the impacts by focal ratio as you pointed out as much as I can. Will be interesting to take my f/6 APO and run a test, then mask the objective to make it a longer focal ratio and repeat to see how things may have changed. Better to use same scope IMO than two different ones since that just adds more unknowns.

Thanks also for the comment on false color when just out of focus...another test I did not do


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jjack's
super member


Reged: 04/01/12

Loc: normandy
Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6229156 - 12/02/13 01:31 PM

You must know that the speed of light is reduced when traveling into theese thick pieces of glass !
after a lot of time transiting into space, photons could be annoyed if they are slowed down just before there destination


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jjack's
super member


Reged: 04/01/12

Loc: normandy
Re: Eyepiece Performance using a Prism Diagonal new [Re: jjack's]
      #6229168 - 12/02/13 01:39 PM

Seriously : my SC5 react the same way as an apo refractor. I can see some red or blue fringes around the external circle when i defocus slightly a bright star with a Mirror diagonal. This effect is reduced with a 1"1/4 prism diagonal, and disapear totaly with a 1"1/4 45° amici prism.
Images does not seem affected, but i keep the 90° prism diag.


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JustaBoy
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Reged: 06/19/12

Re: Eyepiece Performance using a Prism Diagonal new [Re: jjack's]
      #6229176 - 12/02/13 01:43 PM

What's that you've been drinking

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junomike
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Reged: 09/07/09

Loc: Ontario
Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6229298 - 12/02/13 02:36 PM

Quote:

And the fringing on the lunar limb was when it was positioned dead center of the FOV or not.




Bill, I placed the Lunar Edge in the vertical middle of the eyepiece and then slewed the scope so that the middle of the arc (Lunar edge was smack dab in the middle. It didn't seem to matter as the induced CA was visible all over the FOV. I can't explain It, but It's the only time I've ever seen CA in that scope aside from some very miniscule CA on Sirius @ 300X + (which most people wouldn't probably notice).

I was so disappointed (for the second time) that I had Tony offer an opinion. He was equally surprised.

Mike


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george tatsis
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Reged: 11/20/08

Loc: Flushing, NY - Europe
Re: Eyepiece Performance using a Prism Diagonal new [Re: junomike]
      #6229405 - 12/02/13 03:14 PM

Just wonder if it's worth buying a 1.25" prism diagonal for my SW 120ED

Has anybody ever tried this set up?

George


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BillP
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Reged: 11/26/06

Loc: Vienna, VA
Re: Eyepiece Performance using a Prism Diagonal new [Re: george tatsis]
      #6229534 - 12/02/13 04:04 PM

Quote:

Just wonder if it's worth buying a 1.25" prism diagonal for my SW 120ED

Has anybody ever tried this set up?

George




Well if anyone reading this chimes in that they have a SW120ED and they live close to Wanshington, D.C. (i.e., less than 1 hr away), then let me know and might take a trip out to them with the diagonals to experiment on that scope

Edited by BillP (12/02/13 04:05 PM)


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John Harold
member


Reged: 11/29/06

Loc: Seattle
Re: Eyepiece Performance using a Prism Diagonal new [Re: george tatsis]
      #6230285 - 12/02/13 09:21 PM

I use an old Celestron 1.25" prism in my 120, but I'm not qualified to say whether it has an advantage over the dielectric that came with it: I've never attached it to the scope! I do this for weight reasons. That scope is my dedicated backyard planetary cruiser, and the 2" diagonal and adapter would make it that much heavier. I will say that in good seeing I can jack the power to 225 (4mm ortho)no problem, and I'm pleased to say that the detail is just as good or better than what I've seen in comparable scopes. Looking at stars I see no CA. I use a 2" prism in my 127 at star parties. When I bought it the consensus was that prisms pass more light, however that was before dielectrics came on the scene.

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Roy McCoy
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Reged: 10/13/05

Loc: Glendale, AZ
Re: Eyepiece Performance using a Prism Diagonal new [Re: george tatsis]
      #6230461 - 12/02/13 10:34 PM

Quote:

Just wonder if it's worth buying a 1.25" prism diagonal for my SW 120ED

Has anybody ever tried this set up?

George




Yes I have with a friend who was borrowing a SW120ED to evaluate.

We noticed that the color shift was not as pronounced as with the Orion 80ED, there was less fringing either side of focus. Mainly we saw a noticeable change in contrast and detail in nebula as has been mentioned several times in this thread. My astro buddy said that if he bought this scope, his very next purchase was going to be the T2 Prism diagonal. We tested it with the 2” prism and the contrast and color fringing was better than the mirror that came with the SW 120, but the color fringing was worse than with the T2 with contrast being about the same.

There were five of us comparing the diagonals that night. Each person easily noticed the difference straight away.

Unfortunately sky conditions deteriorated before we were able to put in a known good mirror diagonal to compare directly with the supplied mirror. So this is the only unknown unfortunately because perhaps the included SW diagonal is not up to comparison.

Off the topic of the SW120, in the Orion 80 ED 7.5, color fringing almost completely disappears in or out of focus with the T2 Prism. And of course the details are at another level. This was compared on different nights as from the SW. This comparison was against a AstroTech Quartz dialectric 2” diagonal.


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george tatsis
professor emeritus
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Reged: 11/20/08

Loc: Flushing, NY - Europe
Re: Eyepiece Performance using a Prism Diagonal new [Re: Roy McCoy]
      #6230537 - 12/02/13 11:26 PM

John, Roy,

Your reports have been very informative! Thank you guys.

What I need now is a good Samaritan to email Bill to conduct his experiments for the final verdict!

George

Edited by george tatsis (12/02/13 11:31 PM)


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Ava
sage


Reged: 11/30/11

Loc: Sweden
Re: Eyepiece Performance using a Prism Diagonal new [Re: george tatsis]
      #6230651 - 12/03/13 01:38 AM

I did a similar comparison about a year back (or maybe two?), compared my Tak 1.25" prism with my Baader ClickLock 2" diagonal (or it might have been the T2 Maxbright mirror, I don't remember) and with the stock 1.25" prism that came with the 8" EdgeHD. I compared them in the EdgeHD only, on stars only (no planets or moon as far as I can remember). The differences were subtle but repeatable:

1) The mirror showed no detectable color in focus.
2) The Tak did show some color around the stars.
3) The Celestron showed even more color than the Tak diagonal.

I don't remember about differences in scatter. A detail, my Tak prism diagonal has a fine hairline scratch off center, about 5-7mm long, I don't think it has any impact but just to let you know about that.

I was a little disappointed with the Tak so have not used it much since.

I've been meaning to redo the comparison on my 80mm apo and the 120mm Equinox if I ever get a clear night around here, George I'll let you know about the results with the Equinox when I've had the chance to try it out. I can see very little color in the Equinox (much much less than I was expecting) so it will be interesting to see if the prism makes it even better, or worse, in that regard.

One more thing, Bills last point about different results for different parts of the color spectrum I think may be the key to interpreting the different results some report here. I will try to compare Jupiter as well as stars the next time.


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Roy McCoy
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Ava]
      #6230665 - 12/03/13 01:52 AM

Hi Anders,

Try something in the 50X-70X on M42 with the Equinox.
And as low power as you can go in your diagonal on M31.

Regards


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george tatsis
professor emeritus
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Reged: 11/20/08

Loc: Flushing, NY - Europe
Re: Eyepiece Performance using a Prism Diagonal new [Re: Ava]
      #6230784 - 12/03/13 05:02 AM

Thanks Anders,

Your results with the Equinox + the 1.25" Tak, and hopefully Bill's shoot-out will determine whether or not I'll go for it.

I keep my fingers crossed for some good results!

George


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george tatsis
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Reged: 11/20/08

Loc: Flushing, NY - Europe
Re: Eyepiece Performance using a Prism Diagonal new [Re: george tatsis]
      #6232726 - 12/04/13 05:20 AM

I remember someone mentioning ,about two years ago, that the Vixen prism diagonal was pretty good in his scopes.

I think it was Jim, but not 100% sure about it.

Anyway, has anybody ever had any experience with this particular diagonal?

http://www.vixenoptics.com/acc/indiv/3675.html

George


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