diffraction rings

[kniclander]

looking through my new celestron ed80 I see what seem like lovely diffraction rings. However, running vertically down the centre of the pattern is a bold dark stripe with two lighter stripes either side of it. As the star move across the FOV, the vertical stripes move across the rings in the same direction ie when the star is on the right of the FOV, the bars are on the right side of the rings.

Is this normal? What causes it?

Thanks

Dan

[John]

Are you using a 90 degree mirror diagonal Dan or a 45 degree prism type ? - I'm asking that because I've read somewhere that the 45 degree prisms can cause issues like the one you describe. I'm also assuming you are using a high power eyepiece for this.

John

[kniclander]

I'm using the 45 degree one that came with the scope - it is a "correct image" diagonal. Thanks for the tip. I will see what else I can find online.

using 75 to about 160 times mag. bars there all the time.

[kniclander]

hmmmm - it seems (from a quick google) that the 45 deg correct image prisms are not the preferred option for astro (particularly star splitting). What do people think?

has anyone compared the ed80 with a 90 deg mirror diagonal against what comes in the box?

Dan

[Merlin66]

If you check out the imaging section threads, you may find the one which talks about the impact of having oversized spacer tabs ( the liitle bits of aluminium foil between the objective lenses)

Have a look into the front of the objective, you may see one of the "tabs" is encrouching into the field and causing the effect.

Just my 2c

[johninderby]

As the CD80 is sold as a "spotting scope" they supply a 45 degree prism that is only fit for terrestrial low power use. For astro use you need a proper star diagonal. One of the 1.25" dielectrics isn't too expensive, although a 2" one would be a good idea if you're thinking of getting any 2" eyepieces later on.

A great buy at the moment is the Celestron 2" XLT 1/10PV diagonal at £73.40 delivered. http://www.telescopesplus.co.uk/moreinfo.php/Accessories/Clearance_Accessories/Celestron_2-inch_XLT_Diagonal_(Refractor)/3278

John

[kniclander]

thanks fellas - Merlin66, i suspect it's not encroaching tabs as I can't see anything and also, the vertical bars on the diffraction riggns are so "regular" that it doesn't seem as if they would be caused by something like that (but I might be completely wrong).

John, thanks for the tip. Any recommendations for a 1,25 inch diagonal? Also, tbh I've been kinda disappointed so far with the scope at high powers (ie 75x plus) - saturn was a sort of mucky yellow with no hint of banding at all and was not at all what you would call "sharp" - I couldn't even make out the gap between the rings and the planet. Also, polaris was splittable at 75x but the little blue component was so faint as to be almost invisible. If it would make a significant difference, I would get a decent diagonal but am I expecting too much from a 3 inch scope?

thanks again guys

Dan

[johninderby]

The CD80 is a great little scope and the optics are the same as the Skywatcher 80ED and it is capable of giving some great views. That 45 degree diag is absolute rubbish for astro as at anything over about 50x to 75x the view becomes so bad it's unusable. A decent diagonal will totally transform your scope, you won't believe the difference.

About the best buy right now is the Stellarvue 1.25" dielectric at £55.00 delivered. http://www.bazaarbuilder.com/cgi-bin/nickaltair/myshop.php?merchant=nickaltair&prodid=598&sinprod=1 as it's absolute top quality and is the one I would buy.

FLO has the Skywatcher Deluxe 1.25 Di-Electric Star Diagonal at £59.00 and it's pretty good. http://www.firstlightoptics.com/products.php?cat=61 The William Optics 1.25" is about £15.00 more, but I've used both and couldn't see any difference.

John

[barkis]

They seem like Fresnel Fringes. You can get them when testing a mirror if a fine slit is used as a light source in the Foucault test. They precede the knife edge.

They can come when light passes through two surfaces, ie prism, or filter.

An article here may explain it than I could.

Then again, yours could be something else altogether.

Ron.

http://spiff.rit.edu/richmond/occult/bessel/Fresnel_Fringes.pdf

[kniclander]

Thanks John - I have ordered the Stellarvue 1.25 inch diagonal (thus avoiding the temptation to buy a 2 inch eyepiece:o)

Ron, think the Fresnel stuff might be a bit beyond me:icon_scratch: but thanks for the link.

Dan

[Izar]

Your observation of Saturn, mirrors that of mine made over the last

few nights with my 80mm APO. Last night was better, with some

faint banding visible, but not great.

BTW, both my 45 and 90 degree erecting diagonals both

produce the effects that you are seeing.

[kniclander]

well, in the end I bought the 2" Celestron diagonal form Telescopes Plus (Hinds online) - delivered overnight with a free magazine!

A decent diagonal makes a HUGE difference - the vertical lines on the diffraction rings are gone and the double double was a lovely easy split at about 167x - certainly cleaner than it ever was in my 10" newt (perhaps it's a case of back to the drawing board for collimation ). I even saw an ariy disc on a focussed star (I think)

saturn was veyr sharp up to about 100x but slightly less sharp at 167 (and no e/p in between) but still no surface detail although that might well be due to the seeing last night.

I don't want to go too overboard in my praise cos this is the only decent refractor I've ever looked through but all in all, one very happy astronomer here .

Dan

[Merlin66]

Sounds exciting!!

Glad things worked out so well!!!

[johninderby]

Congratulations on the new diagonal, bet that's one of the best purchases you've ever made.

I never could understand why Celestron supplied such a rubbish diagonal with what is a very good scope. I wonder how many people who bought the C80 ED were disappointed and blamed the scope not the diagonal.

John

[steppenwolf]

That sounds like a real result then - excellent news. As I am not a visual astronomer I had not heard of this problem before but it has been an educational thread for me!

[kniclander]

. As I am not a visual astronomer I had not heard of this problem before but it has been an educational thread for me!

are you never tempted to take just a little peek?

[kniclander]

Congratulations on the new diagonal, bet that's one of the best purchases you've ever made.

I never could understand why Celestron supplied such a rubbish diagonal with what is a very good scope. I wonder how many people who bought the C80 ED were disappointed and blamed the scope not the diagonal.

John

I suppose cost - also, if you look at early discussions about the scope, (eg on Cloudy Nights) it took a while to take off as an astro scope cos it was marketed (oddly IMHO) as a "spotting scope" for which the supplied prism was probably ok.