Supplied Celestron Diagonal and general queries

[Andrew*]

I'm about to receive the C100 EDR which is supplied with a 1.25" "star diagonal".

Since I have a 2" EP, I'll need a 2" diagonal. Which ones are commended?

Can anyone direct me to some info on diagonals, or just answer these queries:

1. Is there a lot of difference between a budget one and a top of the range one?

2. If you've got a rubbish diagonal, does that mean a Nagler in it would be rendered useless?

3. What do "push-fit", "star", "dielectric", and "quartz" mean to a diagonal?

4. 45 degree and 90 degree - differences?

Cheers

Andrew

[GazOC]

Theres an astronomy cliche about the diagonal being a third of the optical chain (ie, the lens/mirror, diagonal and the eyepiece) and the chain only being as strong as the weakest point, IMHO its wrong in practice. Unless the diagonal is very poor or very very good then it has nowhere near the impact on views that the lens/ mirror or eyepieces do.

Just get yourself a half decent 2" diagonal and save yourself a pile of money.

[GazOC]

Oh, and get 90 not a 45.

[Kai]

Andrew i picked up a Enhanced William Optics Diag from the W/O site on Ebay for £46 .

[John]

Andrew,

Like the others I recommend going for a nice 2 inch diagonal. I've used the Antares 2 inchers which are very nice for what they cost (about £35 new) and also the Revelation Dielectric which is pretty much identical to the William Optics version and performs just as well - that costs about £70 new or can be picked up for around £45 2nd hand. I would agree that investing lots of £'s in a fabulous quality diagonal is probably not the best use of the cash.

One thing to watch when using a 2 inch diagonal in your 100 EDR - they chew up a load of inward focusser travel - so it you have any eyepieces that need a little more inward travel than normal you might have problems reaching focus. Thats what I found with my ED100 which I think is more or less the same scope.

John.

[Brass n glass]

1. Is there a lot of difference between a budget one and a top of the range one?

2. If you've got a rubbish diagonal, does that mean a Nagler in it would be rendered useless? They're wrong.

3. What do "push-fit", "star", "dielectric", and "quartz" mean to a diagonal?

4. 45 degree and 90 degree - differences?

1. Yes, but the biggest difference is between prism and mirror ones. The mirror is the one to go for.

2. No, but its performance would suffer a bit. Some people think a really good component can somehow compensate for the performance of a poor one used at the same time. They're wrong.

3. Push-fit means it attaches like an eyepiece, dielectric is an expensive coating for mirrors in diagonals and star is a relic of days long ago when telescopes made in the UK were sold as complete outfits with about 8 eyepieces plus a 90deg prism diagonal like those made today (called in those days a "star diagonal") and a Herschel wedge (called a "Sun diagonal").

4. 45deg (and 60 deg) ones are designed for birdwatchers who want to be looking towards their targets, not vertically into the scope - Hugh.

[ketans]

One thing to watch when using a 2 inch diagonal in your 100 EDR - they chew up a load of inward focusser travel - so it you have any eyepieces that need a little more inward travel than normal you might have problems reaching focus. Thats what I found with my ED100 which I think is more or less the same scope.

John.

This means, the 2" diagonal increases the focal distance due to its size, and it limits the inward focusing ability of the focuser.

John, does this apply to my Omni XLT 102 reftactor (1000mm focalLength)?

More distant objects will need more inward tube travel.

What is the solution for that?

Ketan.

[Andrew*]

very helpful guys. Thanks.

Andrew

[John]

This means, the 2" diagonal increases the focal distance due to its size, and it limits the inward focusing ability of the focuser.

John, does this apply to my Omni XLT 102 reftactor (1000mm focalLength)?

More distant objects will need more inward tube travel.

What is the solution for that?

Ketan.

I think what is happening is that the light path through a 2 inch diagonal is simply longer than it is through a 1.25 inch diagonal because the 2 inch diagonal is pysically bigger all round. In practice this means that an eyepiece would be at focus with the focus tube extended 4 cm with a 1.25 inch diagonal but with a 2 inch diagonal the same eyepiece would be at focus with the focus tube only extended say 1cm - the 2 inch diagonal itself accounts for that 3cm difference (these are not exact measurements - just for example).

With the above example, if you have an eyepiece that is at focus using a 1.25 inch diagonal with the focusser tube extended 2cm, you won't be able to bring the eyepiece to focus with a 2 inch diagonal because your focusser will not be able to rack in that far.

I have experienced this with certain eyepieces where the focus point in the eyepiece is high up in the eyepiece barrel - in my case it was with the Celestron Ultima 35mm and Wide Scan III eyepieces where, to obtain maximum field width, the focus point of the eyepiece is actually designed to be above the top of the chrome barrel. With regular plossls I have not found it a problem although, when using a barlow with my Tele Vue plossls and Naglers there is only about 5-6mm of focusser in-travel left.

The only solution would be to shorten the telescope tube by a few cm :shock: - so I sold the offending eyepieces instead

The scopes I have expereinced this in are Skywatcher ED80 F7.5 and ED100 F9, a Helios 102mm F5 and my current Vixen ED102SS F6.5. I had a William Optics Megrez 90 that had lot more focusser in-travel so it was not a problem with that one.

Sorry for the rather long winded reply - I hope it makes some sense

John

[ketans]

Hallo John,

Long is not the problem., I understood what you said clearly. , thats what is important.

I was trying to get one or two wide field eyepieces(probably GSO Superview, I will have to check whether they will allow focus in-travel with 2" accessories.

see you, Ketan.