How Much Does Size Matter?
Posted 29 January 2013 - 01:30 PM
Posted 29 January 2013 - 02:37 PM
I use the AT111EDT for binoviewing most often. Occasionally the 8"SCT.
An 80mm scope with binoviewer is like 56mm binoculars at high power.
Posted 29 January 2013 - 02:42 PM
Posted 29 January 2013 - 03:11 PM
This may also be important in your choice of binoviewer systems.
I have repeated this many times now, but it always bears repeating... Binoviewer purchases are one of the most complex, so do your homework!
Back to the refractors... I have a 6" APO, and I am not about to cut the tube on it.
Because of this, when I used a Baader Maxbright with a T2 diagonal, I had to use a 1.5x configuration to reach focus. Suddenly, the scope was no longer 1216mm, but more like 1800mm. This means that with the Maxbrights, the biggest field I could get was about .66 degree.
For comparison, I can get a .67 degree out of my C14 (in monovision of course).
Now, let's go to my current binviewr, the Mark V. Even though I use the same diagonal, the longer light path of the Mark Vs means it no longer reaches focus with the 1.5x configuratoin.
This means that I would have to go to a 2x configuration, and now suddenly, my 1200mm refractor can't even give the same true field of my EdgeHD 8"!
Also, the binoviewers do dim the view. Even when I got the binoviewers to work with the 6" APO, the veiw never seemed bright enough.
Some of this is due to the binoviewer dimming, but much of it is that the high powers tended to lock me into a 2mm exit pupil (or smaller with widefield eyepeices).
After trying it, I personally was totally unsatisfied with binoviewing with the 6" APO. No thanks..
On the other hand, the 8" EdgeHD binoviews quite well. The focal lenght is similar to the 6" APO 2x configuratoin, but the extra aperture offsets the brightnes loss of the binoviewers some.
And surprisingly, the C5 was more interesting to use to me than the 6" APO. Once again, with the Mark Vs, I can get a bigger true field, and actually, a brighter true field!!!. I can get a much larger exit pupil in the C5 than I could with the 6" APO.
I also have started using 40mm Plossls for the ability to generate much bigger exit pupils, but this may not work with a smaller prism unit.
Anyway, I like them in the C5, C8, and C14, but they put me in a very high power box in the refractor, and won't use them again in that scope.
A great deal will depend on how much back focus you have in your refractors (unless you don't need low powers).
Again, do your homework.
The scope you will likely like most is the one that will give you the ability to generate a decent exit pupil. Measure your back focus on each scope and calculate exit pupils for the lowest power eyepeices you can use in a given scope.
Posted 29 January 2013 - 03:30 PM
Posted 29 January 2013 - 04:08 PM
My outlier eyepieces are a 32mm pair for low powers 46x and 53x, again offering 1°. If I want lower power than that I go to my 22x85 binoculars. My high power range eyepieces are 14mm,12mm,10mm, all 60° eyepieces, although I almost always us the 12mm for 120x and 140x. But take note, i can fit the entire moon into the field of view at 100x to 120x. I can get up to 170x with the 10mm, but seldom go there. At 100x to 140x, the image is crisp with high contrast. Views of Jupiter from 120x to 140x range are my favorite range.
Unfortunately the lowest power starsweeper setting on my Denks will not come to focus in my refractor, so in the refractor I get only 1.88x and 2.2x. the lowest power starsweeper would be 1.4x
IF you have a starsweeper low power switch on your binoviewer you can get somewhat lower powers on the C6 than with the AT111, but you cannot get any lower on the C8, even without the OCS.
BUT for the following reason you would not want too use the starsweeper on a C6.
detailed data on testing the C6 when using my Denk binoviewer with powerswitch set to reducer mode. This turns out to be the most severely reduced / vignetted system. With the reducer I'm operating at 0.60x the F which nets F=1100. However aperture is reduced to 118mm. And field of view is limited to about a 23-24mm field stop.
This reduced aperture results in D=118mm, Obstruction =49%, effective light gathering = 102mm
The C6 focal length gets extremely long when using a binoviewer on it, in fact, potentially outside the design range of the mirror. With no powerswitch, but with a 1.25" diagonal plus Denk binoviewer on a C6 results in an operating focal length of about F=2000, f/13.3. Not only that but the aperture gets reduced to 140mm. With the reduced aperture and the central obstruction now 41% of the diameter, effective light gathering is reduced to a 127mm scope. Put a 2" diagonal on the back end of that C6 and everything gets worse. Operating focal length gets longer and aperture gets reduced even more, therfore CO takes up more percent.
Aperture and Focal Length for Various Configurations of C5, C6 and C8
The C5 is far worse than the C6. My same Denk binoviewer in a 1.25" diagonal on my C5 results in F=1700, and aperture clipped to about 4", so I'm operating at f/17. The central obstruction is now 47% of the efffective operating diameter, which destroys contrast and the total light gathering in now the equivalent of a 90mm clear aperture.
Small SCTs are not a good choice for binoviewers.
The smallest SCT that I would recommend for binoviewing is the C8. The C8 works just fine, but operates at F=2400
Posted 29 January 2013 - 04:54 PM
I believe that if I went to the Baader 15mm SCT to T2 prism, I could get very close to the full apertuer, however, I would not be able to easily rotate the diagonal (bummer), so it is a tradoff.
It really helps to trim heavily on the light path length.
This is why I dumped the Supersystem in favor of the Baader system. I gave up the Powerswitchs (no big loss because of the aperture loss in low power and the huge jump in power on the high power arm...)
Posted 29 January 2013 - 05:04 PM
And if you attempt to use an OCS, my guess is that it is going to stick way down into the light path. The Denk OCS mounts on a tube that is about 4" long. MNs tend to have pretty short focuser tubes. Short focuser tube + long OCS tube is not a good match..
For bigger Newts, the lenght of the OCS tube is not as big a deal, but the MNs are very tight with their spacings. This is how they get away with small secondary obstructions.
So, unlikley that you are going to be able to binoview an MN, and for sure, you will need a 2X kind of system, so you are looking at 1800mm.
I could be wrong though, but I have owned two MNs, and based on my memory of them, I think a Binoviewer is not going to give a great result.
Posted 29 January 2013 - 05:42 PM
Posted 29 January 2013 - 06:20 PM
I have thought about buying a used 6" or 8" reflector and moving the mirror up so an OCS would not be needed. care has to be taken not to reduce the aperture, but I could go to a slightly bigger secondary if necessary.
For example, a 6" f/8 reflector would give a very decent 50x using a 2468s, and a very nice 5mm exit pupil using 4043s.
Another option (and one that I keep hoping for on Craigslist) is to buy a 120mm 1000mm achromat and cut the tube down so it would reach focus. This is actually quite appealing to me because with 2468s, you could get a very enjoyable 1.5 degree true field. I think this is big enough to make it worth it.
But it really does depend on how one defines ultimate.
I have been using my Mark Vs in my C14. I have tried them in all of my scopes, but the fact is, my C14 is my own ultimate scopes (from the ones I own) and gets used 95% of the time.
I would rather use my C14 with the binoviewers and a pair of 40mm Plossls than I would use the binoviewers in my C8 with a pair of 24mm Panoptics for things that would fit in the field of the C14.. Both setups give about 100x, but in one case you are looking though a 14" scope, and the other case you are looking though an 8" scope.
So difficult to know what "Ultimate" would be.
For me, bigger is better when it comes to telescopes. And if it doesn't fit into the field of the binoviewer, I will just look at something else.
Wide true fields are indeed great, but of the 1000 General catalog objects, only about 5 percent won't fit into the field of my C14 using binoviewers.
With the NGC catalog, it is like 1%.
It's all up to you....
Posted 29 January 2013 - 06:38 PM
Posted 29 January 2013 - 11:33 PM
And that is what I was saying... A 40mm plossl pair in my C14 gives a brigher view than a 24mm Hyperion pair in my EdgeHD.
Both have the same magnification, but the exit pupil in the C14 is a lot bigger.
And that was my point... I don't know what the ultimate scope for the OP is. Only he knows that.
But for me, it is a bigger scope. As compared to my 6" APO, I can get a bigger image at the same exit pupil, or a brigrher image at the same magnification.
Or I can get both by using a slightly bigger exit pupil with a slighly higher magnification.
All good. No substitute for aperture in my book, and my own ultimate telescope is my big one.
Posted 30 January 2013 - 06:08 AM
You have REALLY nice scopes. I'd get a bv and try it out on every scope, only real way to find out how it works for you.
Posted 30 January 2013 - 06:12 AM
Back to the refractors... I have a 6" APO, and I am not about to cut the tube on it.
Not sure if it will fit your older 6" but AP makes a shorter collar for those who need more infocus in the 155, part A1025-B.
Posted 30 January 2013 - 08:59 AM
The other issue is that there is a large steel tube weight just inside the end of the tube that also acts as a baffle. This would limit the in-travel of the focuser tube.
This would require a shorter focuser tube, but now, when I wanted to use mono-vision, I would have to use extensions on the focuser.
I don't use the scope much anyway because the aperture is so limiting, but when I do, I generally use it for the fantastic wide field viewing it offers and binoviewing, even with a shorter tube, simply can't provide the 2 degrees that I want out of it.
And I am more than content binoviewing with my C14. As I said earlier, from the scope I have, it is my own personal ultimate scope, so stepping down to the 6" APO isn't at all appealing to me.
Posted 30 January 2013 - 09:07 AM
The ultimate combo probably changes with the object to be viewed and seeing conditions. The view of the moon through a Traveler or small APQ is probably spectacular through binoviewers, and of course Jupiter through a larger aperture is optimal for seeing the lower contrasted details. The right tool for the right job. The beauty and advantage of having quality optics at your disposal is that you can be sure you are maximizing your view under the current conditions - as long as the scopes are collimated and cooled properly. I will report back what I find.
Posted 30 January 2013 - 09:13 AM
I use it for solar in the C5 mostly, but I posted a week or two ago about the moon in the C5 using 40mm Plossls. Was amazing.
So, yes, that is the best approach... That is why so many of us have so many differnt scopes.