Diagonal / erecting prism / visual back - confused a tad.

[Starslayer]

So I have googled and found some videos  but  I remain confused. 

I understand that a view straight through my sct will invert the image left to right and up to down.

I understand that a 90 degree prism ( like my 2" xlt celestron diagonal ) will invert just left to right and yes it does just that. 

I understand ( from you tube ) that a 45 degree erecting prism will show a correct image in both planes

What I do not understand is why therefore the stock visual back that came with my scope also shows the image correct in both planes notwithstanding to me it looks like my 90 degree 2" albeit smaller at 1.25"

Is there something else about the way a visual back is constructed and works that I am not only totally missing but also cannot find an answer to in the world of google?   Is a visual back not a 90 degree prism? I do try and find these things out before running to you guys. 😇. Thank you. 

 

[Starslayer]

.... and then I find this.      Still does not explain things.  Do I need to get a visual back that accepts 2" accessories to take full advantage of my 2" diagonal yet keep things all in the right plane?  

[NGC 1502]

38 minutes ago, Starslayer said:

 

What I do not understand is why therefore the stock visual back that came with my scope also shows the image correct in both planes notwithstanding to me it looks like my 90 degree 2" albeit smaller at 1.25"

Is there something else about the way a visual back is constructed and works that I am not only totally missing but also cannot find an answer to in the world of google?   Is a visual back not a 90 degree prism? I do try and find these things out before running to you guys. 😇. Thank you. 

 

The stock diagonal used with your SCT will flip the view left and right but not invert it. This is the same whether it’s a prism or a mirror in the diagonal.

It’s possible to purchase a “correct” image diagonal but the problem with that is it degrades the image.

[Alan64]

Firstly, your 2" "XLT" diagonal is a star-mirror, not a star-prism.

Yes, you need a 2" visual-back to insert your 2" star-mirror diagonal.  Else, you cannot use it.  If you have been able to insert your 2" diagonal all along, then the telescope already has a 2" visual-back.

The visual-back is the hole into which you insert an eyepiece, a barlow, a diagonal.  This is my Maksutov-Cassegrain, and similar to your Schmidt-Cassegrain...

It has a 1.25" visual-back, therefore I cannot insert a 2" anything.

What is the exact nature of your Schmidt-Cassegrain?  Is it a Celestron C6, C8, C9.25, or is it a Meade?

[Starslayer]

1 hour ago, NGC 1502 said:

The stock diagonal used with your SCT will flip the view left and right but not invert it. This is the same whether it’s a prism or a mirror in the diagonal.

It’s possible to purchase a “correct” image diagonal but the problem with that is it degrades the image.

Erm - no.  The stock set up does not flip anything.   Edit - maybe the stock one IS a correct image diagonal as supplied. 

Edited July 30, 2021 by Starslayer

[Starslayer]

53 minutes ago, Alan64 said:

Firstly, your 2" "XLT" diagonal is a star-mirror, not a star-prism.

Yes, you need a 2" visual-back to insert your 2" star-mirror diagonal.  Else, you cannot use it.  If you have been able to insert your 2" diagonal all along, then the telescope already has a 2" visual-back.

The visual-back is the hole into which you insert an eyepiece, a barlow, a diagonal.  This is my Maksutov-Cassegrain, and similar to your Schmidt-Cassegrain...

It has a 1.25" visual-back, therefore I cannot insert a 2" anything.

What is the exact nature of your Schmidt-Cassegrain?  Is it a Celestron C6, C8, C9.25, or is it a Meade?

Again, still confused. See pics attached above.  My 2' diagonal will screw direct onto the back of the scope ( although I also question the benefit of that as the 'hole' at the back is only 1 1/4 anyway - another issue ). I therefore do not need another visual back to attach it to the scope to use it.  

[Cosmic Geoff]

Did you buy your C8 used? I think the source of your confusion is that the previous owner fitted a 2" diagonal that screws straight onto the SCT thread at the back of the scope, eliminating the need for a 'visual back' (which is basically a circular metal adapter). Some people like a 2" diagonal (etc) because it offers a slightly wider FOV, and a more solid base on which to mount accessories.

If you are confused about whether the setup is inverting or reversing, try aiming it at some distant signage.

Edited July 30, 2021 by Cosmic Geoff

[Starslayer]

7 minutes ago, Cosmic Geoff said:

Did you buy your C8 used? I think the source of your confusion is that the previous owner fitted a 2" diagonal that screws straight onto the SCT thread at the back of the scope, eliminating the need for a 'visual back' (which is basically a circular metal adapter). Some people like a 2" diagonal (etc) because it offers a slightly wider FOV, and a more solid base on which to mount accessories.

If you are confused about whether the setup is inverting or reversing, try aiming it at some distant signage.

Hi Geoff.  Not sure where you got this information from. I have not got a C8, it is a Celestron Starsense Explorer 6" DX purchased new from FLO May '21. So the stock is 'stock' as supplied.  Likewise I am not confused at all by what I am actually seeing. Established this by locking on to views of local hospital in daytime some 2 miles away from 3rd floor window.  Nurses looked lovely!! 😇  I do also take your point about why some people like the 2" diagonal as the 'hole in the back end or the tube' remains the same and is smaller. The visual back I see has no optics, it is just a means of attaching things. I can only deduce form this then that the diagonal supplied as stock is a 'correct'; image one. ie - left is left and up is up. 

[Starslayer]

OK is this the answer -  checked the spec and this scope comes with an erect image diagonal.  viz:

Star Diagonal:

Erect image 90° (1.25")

I have sort of answered my own question maybe although I have an sct and it says 90 not 45 degrees. Apologies if I am coming across a bit stupid here but maybe this will help others too. 

Edited July 30, 2021 by Starslayer

[johninderby]

Not a lot of point in using a 2” diagonal on the C6 OTA that you have due to the small diameter of the baffle tube. Upgrading to a 1.25” dielectric mirror diagonal is worthwhile though. The stock diagonal is a lightweight plastic bodied affair. The better diagonal will also use a compression ring fitting for the eyepiece which will be kinder to your eyepieces than the screw type holder. 

A 90 degree erect image prism is useful though for terestrial observing.

BTW although your scope is called a DX the OTA itself is commonly known as the C6.

[Starslayer]

1 minute ago, johninderby said:

Not a lot of point in using a 2” diagonal on the C6 OTA that you have due to the small diameter of the baffle tube. Upgrading to a 1.25” dielectric mirror diagonal is worthwhile though. The stock diagonal is a lightweight plastic bodied affair. The better diagonal will also use a compression ring fitting for the eyepiece which will be kinder to your eyepieces than the screw type holder. 

A 90 degree erect image prism is useful though for terestrial observing.

BTW although your scope is called a DX the OTA itself is commonly known as the C6.

Thank you.. Makes sense.  The 2" was actually a well intentioned present.  The stock set up I find works well and I add in the Baader Hyperion zoom.  It actually seems pretty robust too but I do take your point regarding the advantage of compression rings on the more expensive option.  I often just drop the Baader in unless getting towards zenith and do not tighten. Dodgy maybe but I find it also catches on the tightening screw unless I edge it back a tad.  The stock one marks the stock eps a little but I can put up with that for now..  

[NGC 1502]

1 hour ago, Starslayer said:

Erm - no.  The stock set up does not flip anything.   Edit - maybe the stock one IS a correct image diagonal as supplied. 

 

Ok my apologies, perhaps I should have said that the stock diagonal traditionally supplied with SCT’s flips the image left to right but doesn’t invert. Sorry if my original post confused.

Ed.

[Pixies]

Oh Matron! Does 'terrestrial' viewing now mean nurses?

(note - I make no differentiation between male and female nurses)

Edited July 30, 2021 by Pixies

[Starslayer]

1 minute ago, Pixies said:

Oh Matron! Does 'terrestrial' viewing now mean nurses?

(note - I make no differentiation between male and female nurses)

Correction - not terrestrial VIEWING.  Purely terrestrial alignment and testing procedures that happened to focus on a certain area of view!  - Believe me, I AM a doctor!! 👨‍⚕️

[Starslayer]

Will a dielectic diagonal still invert image left to right ?  If so I may as well save the money and use the 2" despite the extra weight on the back end. 

Edited July 30, 2021 by Starslayer

[Pixies]

6 minutes ago, Starslayer said:

Will a dielectic diagonal still invert image left to right ?  If so I may as well save the money and use the 2" despite the extra weight on the back end. 

Yes, it will.

If you stick an eyepiece straight into the visual back, the view will be reversed left<->right and also up<->down. Basically, rotated 180 degrees -just like a Newtonian.  A mirror diagonal flips up<->down again, so only left<->right is reversed.

[johninderby]

Surprisingly one of the cheap Chinese 1.25” dielectric mirror diagonals is not bad at all and has the eyepiece compression fitting and metal body.I bought one to see if it could be any good and was pleasantly surprised. 

Have seen them as cheap as £10.00 on eBay if you  don’t mind waiting a bit.

https://www.amazon.co.uk/Archuu-Dielectric-1-25inch-Connected-Telescope-default/dp/B08D386NK6/ref=sr_1_9?dchild=1&keywords=1.25”+mirror+diagonal&qid=1627656870&s=electronics&sr=1-9

Edited July 30, 2021 by johninderby

[Starslayer]

2 minutes ago, johninderby said:

Surprisingly one of the cheap Chinese 1.25” dielectric mirror diagonals is not bad at all and has the eyepiece compression fitting and metal body.I bought one to see if it could be any good and was pleasantly surprised. 

Have seen them as cheap as £10.00 on eBay if ypu don’t mind waiting a bit.

https://www.amazon.co.uk/Archuu-Dielectric-1-25inch-Connected-Telescope-default/dp/B08D386NK6/ref=sr_1_9?dchild=1&keywords=1.25”+mirror+diagonal&qid=1627656870&s=electronics&sr=1-9

Hahaha.. New £17.  Don;'t mind waiting a bit?  Free delivery tomorrow.  Not sure I can wait THAT long. 😇  Aside from that the original point is the view.. I think I need to persevere with the 2" if I will get flipped left to right any way. I am sure I will get used to it in the dark when fine 'knob twiddling' ( he says not wishing to set @Pixies off again.!!  )

[Starslayer]

So with thanks to SGL members once again I shall try and sum this one up and put my confusion to bed. ( seek confirmation that I have sorted it in my head ). 

A visual back has nothing to do with what I see inversion wise. It is a screw on bit that affords access to attach other things. 

What I have as stock is an erect prism diagonal and that is why what I see is correct in both planes. 

I could swap out the stock diagonal for a dielectic one which although giving me the possible benefit of increased sturdiness and also the desirable compression ring lock down to protect eps it shall also invert image left to right. Not an issue once I get used to this when making adjustments at night on my manual mount. 

My 2" xlt diagonal is fine as it is big and sturdy and has compression rings but the downsides are that it is heavy and it does not enhance the views due to the limitations of the SCT 6" baffle ( 27mm ish ).  

My conclusion - this has been a terminology issue compounded by me being relatively new (ignorant) to all this.  I have had some great results thus far with my stock " erecting prism diagonal " especially with BHZ ep so I think immediate future expense will go on a focal reducer.  Later maybe swap out for the dielectic but I see no hurry there as I have the 2" kit anyway which will do the same, offer the same protection for my eps and I then still live with left to right inversion. 

Back to nurses ( for those that have read the whole thread).  Regards to all and thank you again. 

Edited July 30, 2021 by Starslayer

[Alan64]

7 hours ago, Starslayer said:

Again, still confused. See pics attached above.  My 2' diagonal will screw direct onto the back of the scope ( although I also question the benefit of that as the 'hole' at the back is only 1 1/4 anyway - another issue ). I therefore do not need another visual back to attach it to the scope to use it.  

The purpose in getting a 2" diagonal in the first place, is to use low-power 2" eyepieces, with their wider, yea, widest, fields-of-view.  There is a 2" 56mm Plossl on the market, or was as it was branded "Meade", and it is sometimes chosen by owners of the larger Schmidt-Cassegrains.

But the primary-mirror of your C6 has a too-small hole in the centre of it to make use of 2" accessories...

Using the 2" diagonal would essentially be the same as using a 1.25" diagonal, yet at a greater, unnecessary expenditure.

This is the hole in the centre of a C8...

As you can see, the prospect of using 2" accessories is improved.  This increases further as you go up in aperture.

In the end, I would suggest, only to suggest, selling the 2" diagonal.  Then, to suggest the Celestron star-prism instead; for example...

https://www.rothervalleyoptics.co.uk/celestron-90-degree-star-diagonal-125.html

In the past, that's the diagonal with which these Celestron SCTs came equipped, and for good reason.  The C8 may still come equipped with that one.  You might also consider the Baader...

https://www.firstlightoptics.com/diagonals/baader-t-2-prism-star-diagonal-32mm.html

It is modular, and allows you choose how to configure it.  Then, to combine either one with a 1.25" 32mm Plossl, and for the lowest power and widest view possible.  Some also choose a 40mm, but I digress.

There is the Celestron focal-reducer, however it might not live up to your expectations...

https://www.firstlightoptics.com/reducersflatteners/celestron-f63-focal-reducer.html

You're not the first in desiring all-round performance with a Schmidt-Cassegrain, but the telescope is simply not designed and configured for low-power, wide-field views.  Your C6, similarly as my Maksutov, is for medium-to-high powered observations.  They're like microscopes, but for the night sky.

If 2" eyepieces and accessories to improve finding one's way round the sky, I suggest looking into a more capable finder.

It is curious as to why your telescope came equipped with an Amici, erect-image diagonal.  I have to wonder if it's intended to work with the "StarSense" system.