Did I get ripped off on my 2" inch diagonal + 40mm EP on NexStar 6SE?

[deckardbr]

Michael: How?

I'll do a more accurate observation hopefully this Thursday... but unless I'm having a bad memory I think the 2"+40mm (my lowest mag setup) did not give me such wide field.

Alan - no hijacking I think. In the end, this is still on topic... and it showed that it is not very obvious and straightforward! There are several discussions on how to achieve wide field on SCT.

[AndyH]

Here's a better thread from someone who has actually measured his tfov on a C6, with a star crossing timing method ...............

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5277629/page/10/view/collapsed/sb/5/o/all/fpart/1

Andy.

[deckardbr]

WO 40mm EP has 72 degrees aFOV. With my 6" inch scope, that means: 1500 /40 = 37.5 magnification. And FOV = aFOV / magnification = 72 / 37.5 = 1.92 degrees

Am I doing the math wrong? or confusing the terminology? I can't see how I would get 3 degrees.

[ronin]

To me you have a long focal length scope that would give therefore be biased to a narrow field ov view immaterial of what is attached at the rear, 1.25" or 2".

Before buying the additional items had you used the scope and if so had you experienced any problems? I would say that to use the 2" eyepiece you would need the diagonal as I suspect that a 1.25" sized mirror would be too small.

The view you should get now, with the 2 items, is around 2 degrees, with a "standard" 1.25" eyepiece you would not realistically get much over 1 degree, I guess 1.25 degrees maximum, so yes you have gained more:- either 50% extra, or, 3/4 of a degree approximately.

To see any gain you need a big object, I suggest the moon is not big enough as with or without it you will get all the moon in view and so it looks the same - a big round moon. Pleiades may be good, M45, M31 better.

All the figures say you will get a wider view, so not I suspect a rip off, whether or not you really need this or will get much from it I am not sure. I say this as there are not that many DSO that are much over 1 to 1.25 degrees. M45 is degree, Pleiades is again 1 degree, only one I know without looking them up is M31 and even 2 degrees is too small for that. Hence my point of did you really need them and how much you will get from them.

A disadvantage to me of the 2" diagonal and eyepieces is the work involved when swapping back to the 1.25" eyepieces for the magnified views.

Not familiar with the ES range of eyepieces but as they have 82 degree 1.25" eyepieces I wonder if one of them may have been any less expensive ultimately, this depends on the focal lengths available.

[alan potts]

I cant see how you would get 3 degrees either who said you could. I think you need to consider a short F/L refractor then 3 degrees is a piece of cake. even more on some. You don't have to spend a fortune, why not try secondhand there are some nice ones on site at the moment..

Alan.

[deckardbr]

From reading the report Andy linked to, it seems that for 6SE the limit is reached at 32mm EP (70 degree aFOV, 2") and it gives you a 1.3 degrees measured FOV.

So it looks like I did overshoot with 40mm EP and likely to have vignetting, although I won't probably notice it right away due to lack of experience.

I'll see how things go. Plenty of practice ahead (clear skies permitting)... and you bet I'll be a lot more careful in the future when researching and buying EP! There is a lot more than the FL... I just had no idea.

Thanks all...

[deckardbr]

And, to partially "redeem" my local retailer... I don't think they tried to rip me off. The EP they advised me to get isn't ideal it seems (40 vs 32), but there is a reason to go with the 2" diagonal. So all in all, not too bad.

I just need to be more careful as I expand my EP collection.

[deckardbr]

Alright, tonight I was so eager to test this out that I assembled my 6SE in front of my flat as quickly as I could whilst the moon was in view.

I was able to observe the moon for a full *5 minutes* before the clouds sealed the sky shut for the evening...

Anyway, I confirm that my 2" diagonal + 40mm WP EP gives me roughly a "2 moons and some" view at most.

How much is that? And how does it compare with theoretical FOV?

I used Stellarium "Ocular" to do a visual comparison. If I set it up with NexStar 6SE and a 40mm WO 72 degrees... it gives me the theoretical 1.92 degrees FOV. But when I look at the screen, it doesn't look like what I was watching just outside a moment ago.

So I used a 28mm 72 degrees EP (simulated), and there it is, with a 1.3 degrees FOV. Now it matches.

Anyway, long story short, with this much less professional experiment I confirm what is linked in the article. With a NexStar 6SE and a 2" diagonal, you reach your limit of FOV at 1.3 degrees. Getting a bigger EP beyond 28mm or 32mm won't do you any good.

In the end, I'm not sure getting the 2" diagonal was worth it, but I'll keep it.

Andrea

[VigdisVZ]

Here's another eyepiece/scope calculator for ya!

http://www.12dstring.me.uk/foveyepiece.php

[deckardbr]

Carl, that is cool! Thanks

[John]

The moon has an apparent diameter of around half a degree. Sounds like you were getting around 1.3 degrees in total.

[deckardbr]

Not too disappointed in the end. I think I've got plenty of stuff to look at in those 1.3 degrees

[Dunkster]

There's the spirit! I think the main thing is whether or not you like what you see.

I also use a 2" (sct) diagonal on my C6 (and C8) and like it for the sturdiness as much as anything (all the more reassuring when using heavier eyepieces), and the compression rings that the bundled Celestron diagonal doesn't offer. The light pollution where I am is not the greatest, so I find my 20mm Nagler (almost 0.5kg!) about as wide as I want to go with the C6 before the sky glow bothers me. I have longer FL EPs for my bigger scope, but one of those weighs almost as much as the C6

[Richard Hather]

Hi I realise this is an old post but I've just purchased a 13mm ethos and I'm thinking about upgrading to a 2 inch diagonal any recommendations?

Richard 

[spaceboy]

On ‎29‎/‎10‎/‎2012 at 15:56, deckardbr said:

Hello folks,

It's never nice to find out you might have been taken for a ~£200 ride... but I need to understand what it is that I'm getting.

I have a NexStar 6SE. Since I was interested in DSO, my local telescope retailer advised me to get the 2" vidual back/diagonal and a 40mm WO EP. This was an expensive solution... but was happy to get the "wider" FOV out of it. Or so I thought.

Your FOV is restricted by the scope design. In this case you have f/10 so with a visual back baffle of 24. "something or other" mm. This means that the widest FOV capable is equal to that of a 32mm eyepiece.

1) What is the actual "wider" FOV that I'm getting with this solution on my 6SE?

Your not getting a wider fov only a lower magnification which I assume the retailer equated to seeing more sky ??? I am though dubious that you would see a significant FOV increase between the 32 and 40mm ep's

2) Would there have been a cheaper option to achieve the same visual result? (e.g. a 2" diagonal maybe is nonsense on a NexStar 6SE... maybe a 32mm 1 1/4" is the maximum I could acheve anyway... or maybe using a focal reducer which would have been good for AP as well, not just visual)

A 150P dobsonian.

From reading other posts on NexStar 6SE EP I'm getting the vibe that someone "took advantange" of my inexperience... but... I let you help me understand if this is the case. If so... live and learn I guess, and will be relying on my own research here vs that of my local retailer, which I thought I could trust.

I don't think your retailer is an astronomer and was only interested in selling you gear that they felt would help. I would personally return the items for a refund and show the retailer this thread if they disagree.

Please be brutally honest.

Thanks

Andrea

 

[iPeace]

2 hours ago, Richard Hather said:

Hi I realise this is an old post but I've just purchased a 13mm ethos and I'm thinking about upgrading to a 2 inch diagonal any recommendations?

Richard 

As the 13mm Ethos is itself a 1.25" eyepiece, I'm not sure a 2" diagonal would provide any additional performance. But I will gladly be corrected on this.

[Stu]

35 minutes ago, iPeace said:

As the 13mm Ethos is itself a 1.25" eyepiece, I'm not sure a 2" diagonal would provide any additional performance. But I will gladly be corrected on this.

There is no visual benefit, only the fact that the 2" diagonals feel (and probably are) more secure holding a heavy eyepiece.

[John]

8 minutes ago, Stu said:

There is no visual benefit, only the fact that the 2" diagonals feel (and probably are) more secure holding a heavy eyepiece.

Yep, I agree with Stu. The 13E does look a little comical in a 1.25" diagonal:

 

[Stu]

2 minutes ago, John said:

Yep, I agree with Stu. The 13E does look a little comical in a 1.25" diagonal:

 

Which side do you look in John, it's hard to tell  

[Richard Hather]

This is were i think I should elaborate on my plans, yes my thinking is more to do with keeping the EP more secure but I'm also thinking about buying 2 more ethos at a later date.

one being the 8E and the other to replace my ES 24mm which will be the 21E... could this work?

Richard

[iPeace]

Just now, Richard Hather said:

This is were i think I should elaborate on my plans, yes my thinking is more to do with keeping the EP more secure but I'm also thinking about buying 2 more ethos at a later date.

one being the 8E and the other to replace my ES 24mm which will be the 21E... could this work?

Richard

You will need a 2" diagonal for the 21 Ethos.

[John]

As above - 2" diagonal required for the 21E (and a sturdy one at that - it's a 2 lbs plus eyepiece)

[Richard Hather]

That's why I want to go down the 2 diagonal route, how will the 21E perform will it be ok in the scope?

Richard

[John]

Just now, Richard Hather said:

That's why I want to go down the 2 diagonal route, how will the 21E perform will it be ok in the scope?

Richard

The 21mm Ethos has a field stop diameter of 36.2mm. There might be a possibility of the baffle tube of the SCT causing vignetting (loss of light) at the field edges ?. I've not tried this particular combination though.

[Stu]

4 minutes ago, John said:

The 21mm Ethos has a field stop diameter of 36.2mm. There might be a possibility of the baffle tube of the SCT causing vignetting (loss of light) at the field edges ?. I've not tried this particular combination though.

Having successfully used a 20mm 100 degree and 30mm 82 degree eyepiece in my OMC140, I suspect it would be fine. It seems that the eye is not that sensitive to mild vignetting so the results are often surprisingly good.