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For NexStar 8SE, what (if any) is the advantage of upgrading to a 2" diagonal and eyepieces?

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#1 Brawlingsaz

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Posted 18 September 2020 - 03:51 AM

This may open a large can of worms, but I see some suggestions that upgrading my 8SE to a 2" diagonal might provide better viewing.  And others which say that actually their 1.25" diagonal is just fine, thanks.  And some say "get a better diagonal than the Celestron stock model, while some say "the stock Celestron is fine".  This conflicting information is as natural as a preference for chocolate or butterscotch topping - personal taste is what it is, and we all have our own.  But there has to be SOME objective reality when something is clearly better, or clearly more clear. So...

 

1> Can someone please explain the reason why 2" is better than 1.25"?  Is it wider field of view?  More light gathering/transmission?  Better manufacturers or precision at the top end?  Is it substantially more money, for very little difference?  I really have no idea why one is superior over the other, if indeed there is an empirical, objective difference, rather than just personal preference.  ??

 

2> I see some EPs that are both 2" and 1.25", and adapters to allow each size in the other size of diagonal.  Is an EP that fits both just a 1.25" EP that has a built-in 2" adapter?  Or is it different than that simplistic supposition?

 

3> I would assume that once you go one way or the other, that you've picked your foundation and now everything you buy or use matches up to that diagonal decision.  So, if you buy a 2" star diagonal, you just trade in all your 1.25" EPs for 2" EPs, and that's that, now you're a 2-incher.  Or vice-versa.  But it sounds like some - perhaps many - people in CN actually have both sizes, and use both.  ??  Wouldn't you just pick one size and go with that size for everything, from then on?  Or does the answer take its cue from the answers to question #1, about there being times when 2" is preferred, and times when 1.25" is preferred, so that eventually you end up with some of each?

 

Thanks in advance for your answers and advice.Also, if this belongs in the "General Viewing" area instead of the newbie area, please let me know and I'll see about getting it moved to where it should best go


Edited by Brawlingsaz, 18 September 2020 - 03:55 AM.


#2 Redbetter

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Posted 18 September 2020 - 05:54 AM

As to whether or not you need to replace the original 1.25" diagonal (as in with another 1.25"), my answer is only if you identify a problem with the current one.  If the diagonal is not well-collimated that can introduce some problems and you should replace it.  Diagonal collimation is often an issue with the way the nosepiece and eyepiece holder are threaded into the body of the diagonal, if they aren't square with one another it causes havoc with image illumination, etc.  Similarly if there is astigmatism in the diagonal you will see a tiny cross pattern to the airy disk at focus on a star at high power.  If you remove the diagonal and the cross goes away, then you know it is the diagonal at fault.  

 

1.  The advantage of 2" diagonals and eyepieces is a wider true field of view.  (A focal reducer can also achieve this in 1.25", * more on that in a minute.)   A 1.25" mirror diagonal and eyepiece are good for ~0.76 degrees of field because the max field stop of the 1.25" barrel is about 27mm.  A 2" mirror diagonal can be used with eyepieces with field stops up to 46mm, which will provide a true field of view of about 1.22 degrees with an SCT threaded diagonal (it is a bit smaller for a refractor diagonal with 2" visual back adapter because the focal length of SCT's changes as the focus is moved to account for the new focal plane position behind the scope.)  [NOTE:  Despite what you will read, this works with C8's for visual even though the rear baffle is only ~37mm.  There is vignetting but this is not so easy to see visually at night--imaging is another matter.]  

Some advantages:

  • While the vast majority of objects visible in an SCT fit within the field offered by a 1.25" eyepiece, a few of the biggest and brightest showpiece objects need more field of view, especially if they are so large you still need to pan around even with a 1.2 degree field. 
  • 2" eyepieces can provide a brighter field of view, useful for diffuse nebula with filters.  It also helps with reflection nebulae such as in the Pleiades.  The Horsehead can be presented at higher brightness with a filter than with a 1.25" (sans reducer/corrector.)  
  • 2" diagonals provide more secure (rather than rotation slipping) attachment for "turkey leg" heavy and long 1.25" eyepieces.  This is something I realized early on in the old days with an original 9mm Nagler.  
  • 2" often provides more comfortable/more immersive wide field viewing.
  • Panning and star hopping is easier/more productive with a 1.2 vs. 0.76 field of view.  
  • So far, I have yet to try a 2" mirror diagonal that was not well aligned or that had major optical flaws like astigmatism.  1.25" diagonals on average had a lot of problems, at least in the samples I have tried--I assume the 1.25" are bad except for the Takahashis (I have two) and other individual samples that I have actually tested and passed.

Negatives:

  • A very important consideration is that 2" diagonals might not always fit well between the back of the OTA and the base of the mount.  Check with other Nexstar 8SE owners to make sure the close coupled SCT threaded diagonal will work.  This wasn't an issue with my old clock driven, dual fork C8, even with a longer refractor diagonal, but mounts have changed.
  • 2" diagonals and the widest field eyepiece are heavy and stick off the back of the OTA more.  This requires more consideration in balancing.  That may or may not be possible with your scope.  (I set my C8 up with a dovetail weight system from the star, so the transition to 2" was not an issue.)
  • 2" diagonals are affordable, but wide field 2" eyepieces are pricey, and so are 2" filters.  While you can thread 2" filters on eyepieces or refractor diagonals, you can't thread them onto an SCT threaded diagonal.  Cost is a major "fork in the road."
  • As mentioned earlier, the operating focal length of the system changes slightly because of the 2" diagonal spacing.

*Alternative approach (which I have not tried.)  You can get the same effective max field as 2" using a combination of 1.25" eyepieces and a 0.63 focal reducer/corrector.  You likely won't be able to get much more field because you will be nearing the "cut off" of the light path.  

Advantages:

  • No balance issues.
  • Should not have clearance issues.
  • Much less expensive.  You stay in 1.25" mode and use 1.25" filters.
  • The corrector aspect flattens the field, so field curvature effects commonly seen with an SCT should be removed.  (This is the reason I should try one.)

Negatives

  • 1.25" diagonals tend to be less stable with respect to rotation.
  • Some of the longer focal length/long eye relief 1.25" eyepieces might be less aesthetically pleasing/comfortable to use than 2".
  • When you want to use planetary or double star magnification levels you might find the R/C problematic:  either you need shorter focal length eyepieces, a Barlow, or you will need to remove the R/C.
  • Some have also said that they believed planetary images were better without the R/C (I can't confirm this.)

2.  Generally dual type eyepieces can be used with either size diagonal, they just will fit each differently depending on the type.  

 

3.  1.25" vs. 2" eyepieces is not "either/or" except when you only have a 2" diagonal.  With a 2" diagonal or focuser (in Newt/Dob) we usually employ both types of eyepieces.  If one only has a 1.25" diagonal or focuser then 1.25" eyepieces will be used and true 2" are not an option.


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#3 BPoletti

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Posted 18 September 2020 - 08:14 AM

I use a 2" diagonal in a Nexstar 8SE.  A few observations about using the 2" for observations....   

 

  • No particular weight issues
  • It does NOT clear the base   
  • As with every change, the balance point of the tube must be checked.  BTW, this also includes finding the right balance point with a dew shield and certain large 2" eyepieces.  I put a piece of masking tape along the side of the tube and mark the balance points for certain observing configurations.  
  • Having the capability of 2" eyepieces opens up a great list of wide field and low power eyepieces.  Might as well use a 2" for low power eyepieces such as an Agena 38mm 70 degree eyepiece.  Great views.
  • Though I have no experience with an f/6.3 Reducer, it makes sense that the field of view will be much better with a 2" diagonal and wider field lower power eyepieces (even if they are lower power 1 1/4").

The only real operational drawback with a 2" diagonal, from my perspective, is the inconvenience of not being able to view objects directly overhead.  The diagonal will not clear the base.  But I find that viewing objects directly overhead is kind of a pain (literally, at my age) when setting up the scope for comfortable viewing of other objects.  

 

The cost of a used Celestron 2" XLT diagonal is quite reasonable and the resale, if you don't like it, will be about the same.  It's worth experimenting with it.  And with a 2" eyepiece or two.  I won't give up mine.  

 

 

Just remember to keep the balance close.  On my scope, the gears really grind if the system is not balanced.


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#4 spaceoddity

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Posted 18 September 2020 - 06:01 PM

https://agenaastro.c...r-diagonal.html

 

I have this diagonal on my 8se and it threads right on with no need for any adapters. Works fine but the only advantage is the use of 2" wide angle eyepieces which will provide a little bit more field of view than a 32 or 40 plossl. 



#5 BrandonLive

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Posted 19 September 2020 - 10:12 AM

I recently purchased my first serious telescope and went for the 8SE. I did a lot of research and went back and further on whether to get a 2” diagonal, and ultimately decided not to for now. I’m not an expert *at all* and because of the smoke situation I have not been able to use my setup at all yet. But I wanted to do things “right” and it’s bonus season so I spent a good chunk of change on things. I decided on:

Celestron f/6.3 Reducer/Corrector
Explore Scientific 82 14mm
Televue Panoptic 24mm
Televue 2.5x Powermate (instead of traditional Barlow)

The last of those arrived yesterday. I’m eager to get to try all this out. Hoping I won’t regret these choices, but I can always add stuff later if some clear reason to add some 2” EPs emerges.

Edited by BrandonLive, 19 September 2020 - 11:40 AM.

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#6 gnowellsct

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Posted 20 September 2020 - 01:45 AM

I can't believe one would want to do it any other way.  (That is, any other way except 2").  I thought it vastly changed my experience and enjoyment of the c8.

 

I sometimes get a feel that people offer advice on SCTs, hmm, like this.  You're going to a place in NY called WORLD FAMOUS CHEESECAKE and everyone around you is ordering cheese cake and you are trying to decide what to get and a bunch of helpful people tell you to get vanilla ice cream.

 

That's what I think the situation is with SCTs and two inch diagonals.  You have the potential for WORLD FAMOUS CHEESECAKE views of the universe and people talk you into a half melted bowl of vanilla ice cream.  And then the say sprinkle your vanilla ice cream with a helping of budget eyepieces ("it doesn't matter on a long focal length SCT") and sagely advise: "use a reducer corrector" just to completely smush out the experience to make it as un-spectacular as possible.

 

I treat my C8 just like my apo and I get great views.  This is not to say you "have" to go out and spend money but I will say that the c8 is worth every dime you put into it and that includes excellent eyepieces and a good two inch visual back and two inch diagonal.

 

Greg N

 

c8 visual back in the field.jpg

 

c8 and stowaway visual backs.jpg


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#7 Brawlingsaz

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Posted 20 September 2020 - 10:11 AM

Greg, I appreciate the insights, and am intrigued by "...a good two inch visual back and two inch diagonal."

 

Having just got my C8, and just started out with using the scope and getting familiar with it, I have no frame of reference for digital imaging devices or the 'visual back' approach.  Can you give me a quick rundown on this aspect of how you view the heavens?

 

1> Since you have no EP to directly view, you must have some sort of screen, tablet, laptop, monitor...  ??  What do you use to display the image in the field?  Presumably something with a battery and brightness control.  If you've used multiple display devices, what have you found to work best for you?

 

2> What is the connection out of the digital imager?  Some form of standard display connection like HDMI, USB, DVI, VGA?  I can imagine that imaging devices have wireless hotspot capabilities and you just use a tablet or phone connected to their hotspot/website to view the output... my Pentax camera actually uses such an approach for remote viewing and camera control, for example.

 

3> What do you use for your imager? How high of a pixel count, brand(s) you use, and what price range for decent digital kit?  I'm sure that - as always - it depends, and you can no doubt go quickly broke on the high end of the spectrum, but if I do get into this side of scope viewing (many posts advocate this approach), I want to have decent quality without totally breaking the bank.

 

4> Being decently schooled in photography principles, I can imagine the advantage of a 2" aperture for light gathering, but in general larger format optics and imagers cost quite a bit more, in the camera world.  But to your point, dumming down and going cheap on the "final link", after spending serious coin on the scope and associated kit doesn't make a lot of sense, since your imaging and views will only ever be as good as the lowest quality item in the optical path.  Any comments on relative costs for 2" vs. 1.25"?

 

Any comments on this from anyone are welcome and appreciated.  Good  info, always interested in alternatives and suggestions in support of this excellent money-hole of a hobby...  grin.gif


Edited by Brawlingsaz, 20 September 2020 - 10:13 AM.


#8 Echolight

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Posted 20 September 2020 - 10:34 AM

Obviously it's the wider field of view.

 

And for me, because I have another scope that can make use of 2 inch eyepieces and all future planned scopes(big dob/little apo) could also use 2 inch eyepieces, it makes sense for me to invest in 2 inch eyepieces.

 

Others will say get a reducer. Which may give you the wider field of vew, but then limits your ability for high power without removing the reducer.

 

The 2 inch image train is just more simple and versatile. You can use either 2 inch eyepieces, or 1.25 inch eyepieces with a simple adapter.

But this also makes eyepieces like the Morpheus that come with both 1.25 and 2 inch barrels more attractive and convenient.

 

Only thing is, with an 8SE, you need to be aware of clearance with the type of 2 inch diagonal, and maybe also with a reducer.



#9 gnowellsct

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Posted 20 September 2020 - 07:07 PM

Greg, I appreciate the insights, and am intrigued by "...a good two inch visual back and two inch diagonal."

 

Having just got my C8, and just started out with using the scope and getting familiar with it, I have no frame of reference for digital imaging devices or the 'visual back' approach.  Can you give me a quick rundown on this aspect of how you view the heavens?

 

1> Since you have no EP to directly view, you must have some sort of screen, tablet, laptop, monitor...  ??  What do you use to display the image in the field?  Presumably something with a battery and brightness control.  If you've used multiple display devices, what have you found to work best for you?

 

 

There is an eyepiece in that bottom picture, it is just rotated so you can't see it.  The first picture also shows the c8 with the eyepiece plainly in sight.

 

If you want to do EAA (electronically assisted astronomy) that's a whole different ball game.  We have enthusiasts for that here.  I don't do it yet.  I just wait for the feeling to go away.  I don't see what the kick is of watching an electronic image when I can just google stuff at home.  But you certainly can detect much deeper (fainter) objects and can use the tool in light polluted areas.  There is a separate CN section for EAA.  

 

The wires hanging off the rear in the second (bottom) picture are to the control paddle of the mount.  I hang it there so I know where it is when I want it.  --GN

 

Greg N


Edited by gnowellsct, 20 September 2020 - 07:09 PM.


#10 mjbetancourt

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Posted 05 October 2020 - 06:21 PM

I recently purchased my first serious telescope and went for the 8SE. I did a lot of research and went back and further on whether to get a 2” diagonal, and ultimately decided not to for now. I’m not an expert *at all* and because of the smoke situation I have not been able to use my setup at all yet. But I wanted to do things “right” and it’s bonus season so I spent a good chunk of change on things. I decided on:

Celestron f/6.3 Reducer/Corrector
Explore Scientific 82 14mm
Televue Panoptic 24mm
Televue 2.5x Powermate (instead of traditional Barlow)

The last of those arrived yesterday. I’m eager to get to try all this out. Hoping I won’t regret these choices, but I can always add stuff later if some clear reason to add some 2” EPs emerges.

Man I did 3 of the 4 things (did not get the 24....yet) on your list also!  I must be on the right path...... ;-)  Just waiting for my 2.5 which is on backorder right now.....


Edited by mjbetancourt, 05 October 2020 - 06:21 PM.


#11 seasparky89

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Posted 07 October 2020 - 09:44 AM

Another advantage of the 2” diagonal is that it gives a much more solid mounting for video or, in my case, DSLR imaging.



#12 PNW

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Posted 07 October 2020 - 10:01 AM

In my humble opinion, I think it all depends if you plan to use the SE mount. If you do, you'll have balance issues and clearance issues. Not to mention the payload stresses on the mount with the heavier 2" eyepieces. If you plan to ditch the SE mount, the only limits to the weight of accessories will be determined by the load capacity of your new mount.

 

Edit: I just checked the payload capacity for the 8 SE mount. It is 12 pounds.


Edited by PNW, 07 October 2020 - 10:35 AM.



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