Diagonal 2”

[Bob tries hard]

so think of getting a 2” star diagonal.

but my thoughts are around 1.25” eyepieces will do anything for them I only have two lenses the one that came with my Nexstar 8se and a 10mm Luminos.  
want to make sure that I head in the right direction.

Planetary is my main objective at the mo I’ll take the odd image but more looking for my self. 

[Mr Spock]

The only reason to get a 2" diagonal is if you are going to use 2" wide field eyepieces. It won't make any difference to 1.25" eyepieces and will be considerably heavier than a 1.25" diagonal.

[Louis D]

It will also increase the focal length of your SCT by 3mm for each 1mm of increased optical path length.  This might amount to 150mm or so moving from a 1.25" to 2" diagonal and visual back.  It will also slightly increase spherical aberration by moving the focal point off the design focal length.

[Alfian]

On 19/02/2021 at 17:49, Mr Spock said:

The only reason to get a 2" diagonal is if you are going to use 2" wide field eyepieces. It won't make any difference to 1.25" eyepieces and will be considerably heavier than a 1.25" diagonal.

Yes, I agree. The added bulk and weight is very noticeable and this in turn affects the balance of the 'scope. If planetary is your main aim then I'd look to spend money on another EP.

[Bob tries hard]

Thanks for info, when enquired at a shop they didn’t mention an draw back. 

[Cosmic Geoff]

I don't have a 2" diagonal. I could never see the point.  You also need a 2" visual back, and it adds up to a significant expense just so you can get a somewhat wider FOV with a low-power eyepiece.   You can go to about 32mm focal length in the 1.25" format.

 

[Louis D]

8 hours ago, Cosmic Geoff said:

I don't have a 2" diagonal. I could never see the point.  You also need a 2" visual back, and it adds up to a significant expense just so you can get a somewhat wider FOV with a low-power eyepiece.   You can go to about 32mm focal length in the 1.25" format.

 

If you already have a bunch of 2" eyepieces for Dobs and fracs, and 2" diagonals for fracs, it just seems natural to invest in a 2" visual back for your Cats.  They're not very expensive at all.

[Pixies]

On 19/02/2021 at 21:10, Louis D said:

It will also increase the focal length of your SCT by 3mm for each 1mm of increased optical path length.  This might amount to 150mm or so moving from a 1.25" to 2" diagonal and visual back.  It will also slightly increase spherical aberration by moving the focal point off the design focal length.

Hi. Sorry - could you explain this, as I thought the focal length was a function of the mirror (and associated corrector plate)? I don't understand how adding a diagonal could change that?

Only have a Mak, so no SCT for experience.

Many thanks.

[johninderby]

With any scope that focuses by moving the primary mirror the focal length isn’t fixed but varies a bit as the primary is moved closer or further away from the secondary mirror so adding anything that changes the focus point will change the focal length. 

Edited February 22, 2021 by johninderby

[Pixies]

2 minutes ago, johninderby said:

With any scope that has focuses by moving the primary mirror the focal length isn’t fixed but varies a bit as the primary is moved closer or further away from the secondary mirror so adding anything that changes the focus point will change the focal length. 

Ah right. I guess that's because the secondary isn't flat?

[johninderby]

No it’s just the fact that the primary moves that causes the change.

A Classical Cassegrain is different as it  has a fixed primary and a seperate focuser so the focal length doesn’t vary .

Found this quote.

This is due to the moving-mirror method of focusing. In effect the moving mirror varies the telescope's focal length to project an image at various distances behind the rear port. This is one of the strengths of an SCT - it can accommodate at large range of back focus needs, compared to refractors or Newtonian reflectors.

Edited February 22, 2021 by johninderby

[Louis D]

4 hours ago, Pixies said:

Ah right. I guess that's because the secondary isn't flat?

Actually, you guessed more or less correctly.  An SCT primary is about an f/2 and the f/5 secondary effectively slows it down to f/10 due to its curve.  Varying the distance between them varies the resulting magnification.  If the secondary were a simple flat, the focal length wouldn't change any more than it would on a Newtonian by moving the primary forward in its cell to get more back focus.

[Bob tries hard]

This subject seems to have open a can of worms.

So to confirm a 2” star diagonal has no advantages. 
 

I was considering a Celestron luminos barlow which is 2”

 

Are we saying stare clear I thought it would give more choices which I like the idea of as I’d still have the 1.25 diagonal?

 

thanks for info other than my lack of knowledge it’s great.

[johninderby]

A 2” diagonal may have advantages over the 1.25” as you would be using the centre of the mirror with a 1.25” eyepiece and the mirror tends to have any distortions or other problems at the edges. With a 1.25” diagonal you are using most of the mirror. A minor thing really but some think it can have an effect. 

With small maks and SCTs the added size and weight of a 2” diagonal can be a downside though.

A decent 1.25” diagonal will be fine though.