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

[deckardbr]

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.

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

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)

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.

Please be brutally honest.

Thanks

Andrea

[E621Keith]

Don't worry, a 40mm on a 6SE will work. The view don't get any brighter but do not significantly vignette either. I admit I was wrong about the vignetting comments in the past, a 2" diagonal will give sufficient distance to for the light to spread out to illuminate the eyepiece. It's a bit like stopping down a camera lens.

Since you have the 6SE and the 40mm already, why not stick it in the scope and let yourself judge whether it's acceptable?

[VigdisVZ]

I have no idea on if you got ripped or not, but you can always make up for part of the loss by selling it second hand to someone who can use it. So it still hurts, but might be put to good use by someone who would compensate you partly for it.

I myself got semi-ripped by my retailer when I ordered an eyepiece projection adapter separatly (was told I might need one) even though I also ordered a eyepiece set where the same damn adapter was included, so now I have two that I dont plan to use since the scope came with an adapter that fits into the T ring. Oh well

Gonna give one away in my astronomy club or sell it cheaply on the internet.

[michael.h.f.wilkinson]

Have you looked through the combination already? If so: do you notice vignetting?

I have a 2" diagonal with my C8, and I have used a 40mm Paragon to great effect in it (despite people saying it would not work). I cannot see any vignetting by looking through the scope. However, I can detect some vignetting by defocusing, and looking at the shapes of the donuts at the edge of the FOV. My estimate was that I was losing 20-25% of light at the edge. This seems like a lot, but in practice it is hard to notice. Many Newtonians have similar vignetting, due to the trade-off between central obstruction and full illumination of the FOV. You can test whether you see the effect I mentioned, and estimate how much light is lost, by comparing the surface area of the donut in the centre of the FOV to that on the edge. If you are losing less than 30-35 % of light, you might still have a workable solution. Alternatively, if you do not notice vignetting, who cares?

[deckardbr]

Hi Keith,

Thanks for your message. I did try last week, but only on the moon and jupiter. On the former, I couldn't really judge if the increased FOV was only due to the reduced magnification (1500/40=37.5, i.e. FOV= aFOV /37.5 = 1.92 degrees) or by the 2" diagonal.

What I mean is... was there a 1 1/4" solution that would have got me a similar result on a 6SE? Or is there a real benefit that I'm getting out of the 2" diagonal?

Would I be better of re-selling (as suggested) the 2" diagonal and getting a focal reducer instead?

Thanks

Andrea

[deckardbr]

Hi Michael,

Thanks - I need to test that. I honestly didn't know that that's what happens, so I wasn't looking for it.

Next time it isn't raining (!) I'll get something like the moon or jupiter again and the de-focus it and look at the edge of the FOV to see if it gets less bright (significantly).

Cheers,

Andrea

[alan potts]

I would have thought it would vignet with this combo. I think the thing to do in future is to put up a post of what you are thinking and bounce it off a few of the people on here. There really are some very knowledgeable people on the site.

I wouldn't say you were ripped of but you may have been able to get something that would have pleased you at less cost.

Alan.

[E621Keith]

I think the main worry about using a 2" eyepiece on a 6SE is the cropped field and vignetting, which doesn't happen. I use a 42 LVW, which has the widest field stop on the market and vignetting wasn't a problem, so a 40 SWAN should be fine.

A focal reducer isn't suitable for visual use and I'd use a 2" diagonal and visual back on a C6 anyway to increase mechanical strength. Some of my 1.25" eyepieces are heavy and adding a barlow make the problem worse. The 2" connection is more secure than the 1.25"

[alan potts]

If you get a focal reducer you will not be able to use the 40mm 2 inch eyepiece on it. The biggest you would be able to use without vignetting is 27-28mm , this is the case with my 12 inch LX.

I was one of these fools that thought, 2 inch diagonal, massive 40mm eyepiece and then I will be able to see behind me, wrong, very wrong.

With 1 1/4 inch eyepieces you can go higher but you maximum FOV will be with the 32mm, a 40mm will just give you a smaller magnification but no extra field.

Alan.

[alan potts]

A focal reducer isn't suitable for visual use and I'd use a 2" diagonal and visual back on a C6 anyway to increase mechanical strength. Some of my 1.25" eyepieces are heavy and adding a barlow make the problem worse. The 2" connection is more secure than the 1.25".

Keith,

Sorry I can't agree with that, I find a reducer is ideal for, in my case reducing the focal length to 1920mm and I can get a FOV of almost 1 degree with a 28mm 2 inch eyepiece, that is 10% greater than with a 41mm Panoptic without a reducer. I would love to try a 26mm Nagler in the mix but I don't have one.

Alan.

[E621Keith]

If you get a focal reducer you will not be able to use the 40mm 2 inch eyepiece on it. The biggest you would be able to use without vignetting is 27-28mm , this is the case with my 12 inch LX.

I was one of these fools that thought, 2 inch diagonal, massive 40mm eyepiece and then I will be able to see behind me, wrong, very wrong.

With 1 1/4 inch eyepieces you can go higher but you maximum FOV will be with the 32mm, a 40mm will just give you a smaller magnification but no extra field.

Alan.

I think what you meant is a 1.25" 55deg plossl max out at 32mm. A 70deg max out at around 24mm, a 82 deg max out at around 16mm and a 100deg max out at 13mm

[E621Keith]

A focal reducer isn't suitable for visual use and I'd use a 2" diagonal and visual back on a C6 anyway to increase mechanical strength. Some of my 1.25" eyepieces are heavy and adding a barlow make the problem worse. The 2" connection is more secure than the 1.25".

Keith,

Sorry I can't agree with that, I find a reducer is ideal for, in my case reducing the focal length to 1920mm and I can get a FOV of almost 1 degree with a 28mm 2 inch eyepiece, that is 10% greater than with a 41mm Panoptic without a reducer. I would love to try a 26mm Nagler in the mix but I don't have one.

Alan.

But the OP has a 40mm William optics. The only 40mm WO I know is the 2" 70deg SWAN. I don't believe a focal reducer will have any significant benefit for that eyepiece. Remember the C6 have a 1.25" baffle tube

[alan potts]

Keith,

I agree the 40mm 2 inch is of no use with a focal reducer but I use a 28mm 68 degree all the time with the focal reducer and it shows no vignetting and works fine. I did try 32mm ( 60 degree) and 34mm in the 2 inch eyepieces but this vignets.

I think as you said 32mm is your max at 1 1/4. I tried this a long time ago and didn't like it, but the 2 inch option works. I should have got a shorter F/L scope in the first place.

I got all my background for this off Cloudy Nights.

Alan.

[alan potts]

I have just seen what you said about the baffle tube and see you have one of these scope, I will keep my mouth shut. It is just that i have a 6 inch RC GSO and that takes the 40mm without a problem but it is a different type of scope.

Sorry,

Alan.

[deckardbr]

Thanks everyone. To summarize this as if I were a 5 yo :

1. For high magnification/small FOV I'm good using my 1 1/4" highpiece (WO 15mm)

2. For "wider field"... as long as I don't see vignetting the 2" 40mm WO option is viable for a SE6. I'll test this next time I'm out.

In the end... if I don't use the 2" diagonal for NOTHING ELSE other than the 40mm EP... would I be better off selling it and getting a different solution? or would you stick with the 2" cause it will prove useful either with this or future scopes?

The open question I have is... what else do I need the 2" diagonal and visual back for on 6SE?

Thanks

Andrea

[E621Keith]

I have just seen what you said about the baffle tube and see you have one of these scope, I will keep my mouth shut. It is just that i have a 6 inch RC GSO and that takes the 40mm without a problem but it is a different type of scope.

Sorry,

Alan.

I know what you mean, but a 0.6x reducer and 40mm 70deg eyepiece is a bit much. I haven't tried it myself, but I heard a number of people reporting problem when they tried that combination on a C6.

Andrea, personally I'd keep the 2" diagonal. It will be useful when you start using heavy eyepieces in the future. 100deg eyepiece and zoom are heavy.

[alan potts]

Keith,

I am 100% sure a 40mm 2 inch 70 degree eyepiece will not work with a .63 reducer on any telescope. I think Andrea would be better saving for a shorter F/L refractor for the wide field, it dosn't need to cost the earth, there is even a nice one S/H on classified.

Alan.

[John]

You may not actually "see" vignetting even if it is occurring. The way the optical system works I think it might show itself through reduced brightness across the whole field of view rather than at the edges. But you would only notice this if you compare the view through a scope of similar aperture, giving similar magnification that is not vignetted.

When I had a C5 SCT I was advised to use an F/6.3 focal reducer with 1.25" accessories to get a wider field of view. I felt that worked pretty well.

When I moved on to C8 SCT I used a 2" diagonal and a 38mm SWA eyepiece (similar to a Panaview) and that seemed OK too but I was never really satisfied with it as a wide field scope - maybe that was vignetting too ?

My conclusion was that SCT's are not really wide field / low power scopes.

[E621Keith]

Keith,

I am 100% sure a 40mm 2 inch 70 degree eyepiece will not work with a .63 reducer on any telescope. I think Andrea would be better saving for a shorter F/L refractor for the wide field, it dosn't need to cost the earth, there is even a nice one S/H on classified.

Alan.

I agree with getting a frac for wide field and the C6 for planets and DSO.

[alan potts]

John,

When I first tried this with the LX I put the reducer in place and used the 26mm S Plossl that came with the scope. I later tried a 32mm S Plossl 2 inch, from the sam range and that showed classic vignetting around the field stop of the eyepiece. I later found that 28mm SWA and the same reducer gave a good clean view.

The cloudy night forum said 27mm was your max but what is 1mm, nothing I guess. I want to try my 24mm UWA and see what I get but i keep forgetting to do it.

Alan.

[John]

....When I first tried this with the LX I put the reducer in place and used the 26mm S Plossl that came with the scope. I later tried a 32mm S Plossl 2 inch, from the sam range and that showed classic vignetting around the field stop of the eyepiece. I later found that 28mm SWA and the same reducer gave a good clean view......

Thats interesting Alan.

I would have thought the 28mm SWA would have a field stop diameter at least that of the 32mm plossl if not a little bit larger ?

Field stop diameter is more critical than the actual focal length of the eyepiece, I would have thought. Maybe not though ?

[AndyH]

Interesting discussion regarding this here ...............

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/2404097/page/0/view/collapsed/sb/5/o/all/fpart/1/vc/1

Andy.

[deckardbr]

Great link Andy, thanks. I need to read more into this I think. Not straightforward at all.

In general, I get the sense that with my SE6 I'll be able to get "1 and a half moon" at most in terms of wide field. Not "super wide", but enough to get a few DSOs in there... although not the likes of Andromeda.

In the future, I'll explore getting a refractor for proper wide field viewing/imaging.

[michael.h.f.wilkinson]

I can get two and a half moon in my C8. A C6 should get roughly three degrees.

[alan potts]

Andrea,

Sorry I sort of hijacked your post, my intentions were good but I got carried away. It is pouring hard with rain and I think is has had a funny effect on me. It is the first rain I have seen since being in England back in August, my we need it.

Alan.