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2" visual backs for Celestron 9.25

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#1 lsfinn

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Posted 27 July 2013 - 12:45 PM

Hi,

I've a non-Edge, CGEM 9.25 SCT, which is equipped with a 1.25" visual back. I've come into the possession of some very nice 2" eyepieces; so, I want to change-out the visual back and diagonal. I'm hoping you can help me with recommendations.

I'm particularly attracted by the Baader 2" click-lock clamp for the visual back. For the diagonal I've been trying to decide between the TeleVue Everbrite and the Baader Dielectric.

That having been said, I'm keen to get the benefit of your advice and experience. Should I be looking at other options? Are there things I should be aware of in the move from 1.25" to 2": e.g., focusing issues? Are there things I should be aware of with regards to any of the options I have investigated?

As always, thanks very much for all your help!

Best,

Sam

#2 Eddgie

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Posted 27 July 2013 - 04:52 PM

My answer is highy biased because of my belief that controlling back focus in an SCT is very beneficial, and if you are going to spend the money, buy something with as short a light path as you can.

Here is the solution I recommend:

Shortest possible 2: SCT configuration for C8 and C9.



The Baader Clicklock is very nice. I owned two, but just sold both of them.

Why? Long lighht path.

The Baader has 42mm of light path. This does not sound like a big deal, but every millimeter of light path you add adds about 3mm to the focal lenght of the system.

The other option I would suggest is the Televue Short SCT to 2" adpater.

This unit will have around 30mm of light path.

Not a bid deal but every millimeter counts.

When you chage from the 1.25" diagonal to a 2" diagonal, the diagonal will add about 50mm to the light path by itself (over a 1.25" visual back and typical 1.25" diagonal."

This makes your focal lenght go to from about 2350 to about 2500mm (about f/10.6). Another 10mm on top of that puts you at about 2530 or so (all figures are approximate).

Also, in theory, your system has the best optical correction when the focal lenght is 2350mm (1.25" visual back). The more back focus you have, the more spherical abberation you add to the system (though again, it is a very small amount).

To me, the CLicklock was OK, but I became more and more concerned about back focus and just decided taht if I could go shorter, I would go shorter.

Now if you go with the TV Short SCT to 2", be aware that you will also need to use the Televue Short 2" diagoanl, or some other diagoanl with a 30mm nose lenght or the diagonal may not fully seat into the visual back.

And if you ever decide to binoview later on, you will be very happy that you kept it as short as possible because with binoviewers, not only does the focal lenght get crazy long, but the system also starts to loos aperture once it gets past about 175mm (and this is important to me becuase I now think that binoviewers are the best accessory you can buy to improve planetary observing).

Now, if you want the real BMW solution, skip the visual back completely and get the Baader Clicklock Diagonal and Baader Lock Ring.

With this setup, the diagonal body threads directly on to the SCT thread. This allows you to keep your light path to about the same as you would get using the 1.25" visual back and diagonal.

You also need the SCT Lock Ring.

Expensive, but it includes the excellent Clicklock Eyepeice holder, and remember, you don't have to buy any visual back at all!

And if you go wtih binoviewers, you can buy an adapter that allows you to connect binoviewrs with a T2 Quick Connector right to the top of the diagonal body, which saves about 20mm of lightt path (though this is still much longer than the Baader T2 Prism).

If you were thinking of buying the Televue Everbright, then the Short version doesn't cost any more and will work perfecty in longer visual backs or in refractors, so why not go that route if you are going to buy the Televue anyway.

But by the time you buy the Televue and the CLicklock visual back, you can buy the Baader Clicklock diagonal and locking ring for the same money and get a 2350mm focal lenght scope rater than a 2500mm focal length scope..

#3 Eddgie

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Posted 27 July 2013 - 04:55 PM

Just to re-enforce... Light path lenght seems unimportant, but it costs you image brightness, true field, and at long back focuses, image quality (apeture loss).

For the larger SCTs that can use the AP 3.25" to 2" adapter, this diagonal will not save you much (a little, but not enough that it might be worth it to most people)

For smaller SCTs though, back focus is a small evil that is unnecessary to accept.

We all make compromises, and everyong gets to choose their own, but if you don't want to make compromises and want to have the lowest power, brightest field possible in a C9 while using a 2" dagonal, it reallly doesn't get any better than this.

For someone looking at spending $275 for a Televue diagonal and another $100 for the Baader Clicklock, I would propose the Maxbright diagonal with CLicklock eyepiece holder and SCT lock ring as the ultimate 2" solution.

#4 Eddgie

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Posted 27 July 2013 - 05:13 PM

And this argument will come up, so I will attack it now..

"But Ed" someone will say, "this is a semi-permanent setup because if I want to put it in my little refractor, I have to unscrew it and put the nose back on!'

To which I say this. It doesn't really matter what diagoanl you use in the little toy scopes. Buy any cheap used 2" diagonal that you can find for your little toy refractor.

But you want your primary scope to work at its full potential. Go "Right" here and make due with whatever you can scrounge up for the toy scope.

I know I sound harsh, but I have learned over the decades that you have one scope that is a serious scope and everthing else.

Put the money you invest in giving your serious scope the best possible equipment you can afford for it.

So, to me personallyy, I would not at all worry about this being "Semi-permanent." If you ever get a toy telescope, you will often finding yourself wanting to use it at the same time as your SCT so you will need a second diagonal anyway.

I have four telescopes that use diagonals, and I have a diagonal for each one of them. I use all four of them at the same time every now and then... LOL...

But mostly, I just like having the holes plugged and not having to go take things apart and move them around.

I'm full of it.. I know.

If readers want to hate me, hate me because I am beautiful and not because of this advice. LOL

#5 lsfinn

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Posted 27 July 2013 - 08:55 PM

Hi Ed,

No hate here! Your advice, as always, is clear and makes a lot of sense.

I am a bit concerned about the semi-permanent nature of your recommendation, though not because I want to use the diagonal elsewhere. I need to pack-up my 'scope between uses in the field. Assuming the necessary clearance in my OTA case can you see any reason why I couldn't leave the diagonal attached in this way when I break-down and stow my gear? Are there other mechanical or strain issues that I should be concerned about?

Again, thanks!

Sam

#6 Eddgie

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Posted 28 July 2013 - 12:22 AM

This is of course a valid concern. Without knowing the specifics of your case, I would have no way of knowing.

If the end of the mirror box does not come into contact with anything except packing foam, I would say you would be 100% OK.

Baader makes very robust stuff and this I think is because they also make heavy eyepieces (Hyperion) and Binoviewers (Mark V) and even their small diagonals seem built to bear substantial loads.

I would think that the 2" would be no different.

And again, as long as it is not bearing the weight of the end of the OTA and will clear the inside of your case, it should be more than OK. Stick a plug into the eyepeice holder and away you go.

I would ask for some other opinions.

Keep this in mind too... I am crazy about stuff like this, and I am first to admit it. The Baader will only make a very small difference in performance and most people would say that I am nit-picking, and maybe they are right. Don't discount this possiblity.

But often these little things add up, andyou as an owner have to make the decision. If you are going to spend the money on a Televue Short Diagonal and short visual back, or on a standard diagonal and a Clicklock, then ask yourself if for the same money, having an f/10 scope with 3250mm focal lenth is better than having an f/10.6 scope with 3400mm of focal length.

It is a very small difference and I am first to admit it.

But considering that one of the biggest gripes about SCTs is the narrow field, anything you do that makes this situarion worse and that is avoidable seems to be well, avoidable.. LOL.

Remember, this diagonal will not stick out much. It is only a little longer than the 1.25" visual back that comes on the telescope.

Also, it is possible that the eyepeice holder may not clear the focuser knob in all orientations, and for this reason, you might want to check to see if somone else has the same configuration.

I like the Baader back, but considering that the Lock Ring would make it easy to rotate the diagonal and you would enjoy the Clicklock eyepiece holder every time you swapped eyepeices, it seems like a very valid option to getting an expensive diagonal and putting it behind an unnecessarily long and somewhat expensive visual back.

Don't forget.. I'm crazy. lol.

#7 Dunkstar

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Posted 28 July 2013 - 06:24 AM

Risking pointing out the obvious but if you're considering the Everbrite or AP or Baader, then you must be concerned by image quality so everything adds up ;)

Mechanically, the Baader diagonal threads on to the rear cell the same as any visual back would. Having read Ed's discussion on this in other threads led me to this solution for my C8. I've only recently implemented this so only have a couple of hours under the stars with it and on an alt-az mount so ymmv but so far so good.

#8 Eddgie

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Posted 28 July 2013 - 08:50 AM

Being Alt Az, you likely don't have any issues with clearance and I doubt that the OP would have any issues because he is using a C9.25, but do you think it would have issues with the eyepeice holder hitting the lock knobs or the focuser on the EdgeHD 8" when used on a GEM?

#9 *skyguy*

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Posted 28 July 2013 - 09:22 AM

Hi Sam,

Take a look at the 2" visual back adapter for the 9.25" SCT from Astro Physics:

Astro Physics 2" Visual Back

Look under "Accessories"/"Focusers, Adapters, Extensions and Covers". It has 3 thumbscrews and a brass locking ring. I have one on my 12" SCT and at $40 dollars it's a steal!

For a quality 2" diagonal ... I'm sending you a PM with information on a soon-to-released, comprehensive test report on over a dozen 2" star diagonals. And the winner is ..... ?????

Jim

#10 Wmacky

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Posted 28 July 2013 - 10:42 AM

Hi Sam,

Take a look at the 2" visual back adapter for the 9.25" SCT from Astro Physics:

Astro Physics 2" Visual Back

Look under "Accessories"/"Focusers, Adapters, Extensions and Covers". It has 3 thumbscrews and a brass locking ring. I have one on my 12" SCT and at $40 dollars it's a steal!

For a quality 2" diagonal ... I'm sending you a PM with information on a soon-to-released, comprehensive test report on over a dozen 2" star diagonals. And the winner is ..... ?????

Jim


Where exactly will this be released?

#11 *skyguy*

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Posted 28 July 2013 - 01:55 PM

A few days ago, I read a draft on a lengthy and exhaustive test report article on a large number of 2" diagonals (not a visual comparison ... but, lab tested) ... written by an eminently qualified individual. Since this is not my work, I can't comment on it in a public forum. However, I can say that it will be a very, very interesting read.

#12 Seanem44

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Posted 29 July 2013 - 11:47 AM

I have the celestron 2" and it works great. No complaints from me regarding it.

#13 Dunkstar

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Posted 30 July 2013 - 01:22 AM

Being Alt Az, you likely don't have any issues with clearance and I doubt that the OP would have any issues because he is using a C9.25, but do you think it would have issues with the eyepeice holder hitting the lock knobs or the focuser on the EdgeHD 8" when used on a GEM?


On the alt az it doesn't need to move so it's all good. On a GEM it could be a problem with the 8 as the eyepiece holder would need an extra half inch of space to clear the lock knobs or focuser if wide angles of rotation were needed. If these are spaced further apart on the C9.25 it might not be a problem.

How much of a problem this would be with the Edge HD 8 would depend on the owner's preference when it comes to viewing position, in which case an alt az mount is likely a better option.

#14 Eddgie

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Posted 30 July 2013 - 08:46 AM

I'm looking forward to your continuing reports.

This though is why I recommended the Televue Short SCT to 2". I know it clears all the knobs on the EdgeHD 8" even with binoviewers and 31mm Nalger.

It has compression ring clamping and is very solid with 31mm Nalger and even with Mark V Binowiewers or the Denk Binotrons used with heavy eyepeices.

Have you tried rotating yours yet just to see? I would love to know if the eyepiece holder clears. My bet is that it does not, but I am sure there would be people that would like to know.

The TV Short works though, and is about as short at you can get with a 2" visual back on the EdgeHD 8".

#15 jrbarnett

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Posted 30 July 2013 - 12:03 PM

Another approach that is not semi-permanent is to obtain the Televue "short" 2" visual back. It will thread directly onto the rear threads of the scope. You'd then need a 2" diagonal with a similarly short nosepiece. Televue makes an Everbrite with a short nosepiece. Another option is the new Baader Maxbright 2" diagonal (the white one with the red band). The latter has a 0.95" long nosepiece which is really short as 2" diagonal nosepieces go. I own the Baader. I think the Televue is better built though. Both are nice enough, and either will allow you to keep the focal length addition to a minimum without losing the efficiency of being able to use your diagonal for other instruments.

If you don't need a premium mirror diagonal, any of the thread on SCT diagonals will also work to minimize added back spacing. They aren't as convenient if you need to rotate the diagonal, but they work well enough otherwise.

But I think having a removable multi-use option for a premium diagonal is the way to go. You'll want to use your better instruments too. You know, the unobstructed ones that show you Sirius B when your SCT hasn't a prayer and deliver better than 0.95 Strehl performance at the eyepiece rather than a maximum of around the bare diffraction limit (which assumes the mirrors themselves are perfect, which they never are) due to design compromises?

On how CO energy displacement mimics SA:

http://www.telescope...obstruction.htm

On how commercial optics typically stack up in mirror quality:

http://geogdata.csun...d/interfer.html

On why you want a minimal CO:

http://www.company7....ks/250f146.html

On the twin "evils" of a large CO; (1) reduces MTF at low and medium frequencies (i.e., reduces low contrast planetary details compared to a lesser or unobstructed system) and (2) makes the telescope materially more sensitive to poor seeing:

http://www.tmboptica...d.asp?cat_id=30

On obstruction and coating effects on system efficiency:

http://geogdata.csun...nd/coating.html

And there are many, many more illustrations of how our SCTs are compromise designs, giving up much to smaller non-obstructed or equal sized lesser obstructed systems on the optical quality and seeing stability fronts in order to achieve supreme compactness and portability (and frankly mass-producability) per unit of aperture.

Love 'em for what they're good at, not for what they're not so good at. But keeping rear spacing additions to a minimum will help you achieve all your instrument is capable of achieving, which is what you want.

:grin:

Regards,

Jim

#16 hfjacinto

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Posted 30 July 2013 - 02:48 PM

I recommend the Baader click lock, it's not that any diaganol will show you more but the click lock is extremely secure. Having a 31 mm T5 fall out once is enough to make me pay the premium for the quick lock.

#17 WesC

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Posted 30 July 2013 - 05:39 PM

Hehe, I bought one of Ed's two click lock visual backs (thanks Ed!) and I really love it. After taking off the VB that came on my Edge 11, I measured them and they are almost exactly the same depth, the Baader being around 1mm deeper. The convenience of being able to quickly, easily and securely rotate my diagonal (which is also a Baader click lock, BTW) is well worth the cost and slight length increase.

YMMV

#18 Mariner@sg

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Posted 30 July 2013 - 09:52 PM

+1 for the Baader Clicklock VB.

#19 Dunkstar

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Posted 31 July 2013 - 01:46 AM

I'm looking forward to your continuing reports.

This though is why I recommended the Televue Short SCT to 2". I know it clears all the knobs on the EdgeHD 8" even with binoviewers and 31mm Nalger.

It has compression ring clamping and is very solid with 31mm Nalger and even with Mark V Binowiewers or the Denk Binotrons used with heavy eyepeices.

Have you tried rotating yours yet just to see? I would love to know if the eyepiece holder clears. My bet is that it does not, but I am sure there would be people that would like to know.

The TV Short works though, and is about as short at you can get with a 2" visual back on the EdgeHD 8".


I expect the TV short is a better solution for the Edge HD 8" with the diagonal clearing all the knobs. The OP has a non-Edge 9.25 which lacks the mirror locks, removing 2 out of the 3 problems :cool:

In its current alt-az configuration, loosening the locking ring to allow rotation of the diagonal gives 35 or so degrees of movement in either direction before hitting one or other of the mirror locks. On a GEM it might be restricted more in counterclockwise movement than clockwise as it doesn't look like the positions of the lock knobs and focuser are exactly at 120 degrees from each other or they're a different distance from centre, but until I get my hands on one it's hard to tell by eye. How practical this is or not would probably be more a case of preference for viewing position, but it might not be possible to present the eyepiece at the 12 o'clock position with respect to the visual back at every position a GEM can move to. I have the click lock 2" visual back for this also but the diagonal doesn't clear the base of the SE mount with it on. On a GEM, the click lock vb allows diagonal rotation easily, but no doubt increases the focal length more than the TV short.






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