30mm 80 degree 2" eyepiece

[fredt]

Hi Folks.
New to this forum and getting back into astronomy.
Just got a 200mm F5 reflector and am looking at eyepieces.
I like to see a lot of sky and saw this eyepiece for what seemed like a reasonable $245 - SIRIUS 80 degree UWA 2" 30MM

Wondering if anyone has experience with these.
Here's the website for retailer:
www.sirius-optics.com.au

Any thoughts very appreciated.

Cheers,
Fred

[MortonH]

Sirius is way overpriced. You can get those new for about $170.

https://myastroshop.com.au/products/details.asp?id=MAS-004H


They are wide but not well corrected. I've used one in the same scope you have and only the central 50% or so of the field was sharp. The edge of the field was rather messy.

If you must try it buy used. Place a Wanted ad here and you'll probably get one for under $100.

[OzEclipse]

Hi Fred,

The link you provided didn't have a photo of the eyepiece. I am guessing it is a rebadged Moonfish Group eyepiece but can't be sure.

http://moonfishgroup.com/catalog/product_info.php?products_id=50&language=en

These same eyepieces are sold on ebay for half the price :

https://www.ebay.com.au/i/192217666367?chn=ps

and were sold for many years by Andrews Communications under an Andrews UWA badge. I bought mine from them more than 10 years ago.

These are very good eyepieces but only for longer focal ratio flattish field instruments.

I've had pretty good results using it with my 8" f9 cassegrain, ED80 f7.5 refractor, and on a friends 12inch f10 Meade SCT.

On all of these instruments, it has a nice sharp field for 80-90% then the focus falls off for the outer 10% of the field. By comparison, a Nagler 31mm will stay sharp pretty much out to the edge, but it distorts the hell out of the image to do it. Look at the Moon through a short focal length refractor and Nagler 31mm and the Moon on the edge of field is sharp but shaped like a football.

I haven't used it on an 8" f5 reflector but my feeling is that it would not perform well.



Joe

[fredt]

Thanks for the quick feedback.
Yes F5 newts tend to be a bit more fussy I guess. I was a little concerned about lack of image at sirius-optics. Sent them an email to see if they'd post pics so maybe get to see them.
I think my main use for this EP is to be able to scan skies a little more easily to find objects and then use other EPs to get close. Just how bad are they at the edge? I guess that's a bit subjective. That one on Ebay looks pretty good Joe. Very similar to the Astroshop product.
Don't want to spend up too big at this early stage. Still getting an understanding of what will suit.
If I posted a "wanted" ad MortonH, do I just do it here or is there some specific ads section on this forum?
Thanks again all.
Fred

[MortonH]

There are various classifieds sections. Here's the Trade/Wanted one:

http://www.iceinspace.com.au/forum/forumdisplay.php?f=32

You should also keep an eye on the Eyepieces for sale:

http://www.iceinspace.com.au/forum/forumdisplay.php?f=28

[Kunama]

I think there is a minimum number of posts before one can use the classifieds to post an ad....

[OzEclipse]

Fred,

I think you need at least 5 posts to post an ad.
People who know each other also loan eyepieces to try out before buying from another source. But you are just too new to the forum for me to risk sending you an eyepiece.


Yes it is subjective. Moreton has used in in a scope like yours and thinks you might only have a decent image in the centre 50% at f5. It is a gradual change to poor images not a hard edge. You'll probably have the stars looking like little seagulls in flight.

If you had an 8" SCT I'd say go for it but I think you might be better off going for a better corrected eyepiece, maybe not a $900 Nagler but something in between.

If you are going to use it like a pseudo-finder, it might not matter or you can use an aperture mask to stop the 8"f5 down to a 5"f8 while you search. You'll lose 1-1.5 stops of light but probably have a wider usable field.

Joe

[MortonH]

I've used an Explore Scientific 28mm 68° in my Newt and it performed quite well. Not perfect at the edges but way better than the 30mm 80°.

A used ES68 28mm would be under $200 if you can get one. They're definitely around as I've had two of them.

[fredt]

I really think this EP is more as a pseudo-finder as Joe said. I used astronomy.tools FOV calculator to look at various magnifications and although a 40mm would be nice the exit pupil was too big. It seemed that for my purposes the 30mm at 80° was about the same albeit with some performance issues towards the edge. Masking didn't occur to me so I guess that's an option. I'll keep checking ads and perhaps something will come up. Otherwise I might go for what's available.
Cheers and great response from this forum.
Much appreciated.
/Fred

[MortonH]

If you use this as a finder eyepiece in your Newt the lack of sharpness near the edges can make stars look like DSO's while you're scanning around. So you think you're finding galaxies and globs all over the place but when you centre them in the field you realise they were just stars. Can be a bit frustrating at times.

[fredt]

I get your point MartonH. I use a pair of binocs to look around - 7x50. Going from them and then trying to find with the inverted view of the newt I find frustrating. I'm still to get used to it and eventually recognising star groups which would make things easier but in the interim I'm guessing the 30mm 80° would be a good compromise.

[OzEclipse]

Using a 40mm lens with an 8mm exit pupil :

An oversized exit pupil wastes some light, some light is transmitted through the optical train that can't enter your eye. If your eye dilates to an entrance pupil of 7mm and the eyepiece has an exit pupil of 8mm, then you lose about 24% of the light. Less than the 60% loss using a 5 inch aperture mask.

At >35mm focal length on an f5, the secondary shadow may become visible as a dark shadow in the middle of the field.

The dark centre shadow becomes visible on any f ratio reflector when you use an eyepiece > 7x f ratio.
f3.5 > 24mm
f 4 > 28mm
f 5 > 35mm
f 6 > 42mm
and so on.

For use as a finder,
A GSO superview 42mm 68deg AFOV will give you a bigger true FOV than the 30mm and better image sharpness toward the edges meaning much more of the 42mm's 2.85 degree field will be usable than that of the 30mm's 2.40 degree field.

When I started out, like you, I used a 40mm Kellner eyepiece as a large aperture finder to find faint objects. It gave me 26x and a 1.7 degree true field.

GSO 42mm----23.8x----68deg AFOV----2.85deg TFOV
UWA 30mm----33.3x----80deg AFOV---- 2.40deg TFOV

AFOV-apparent field of view of eyepiece
TFOV-true field of view through telescope

The GSO might serve you better than the 30mm. With such a wide field, you will probably get coma and need a coma corrector to sharpen the edges.

Joe

[fredt]

Well that makes sense Joe.
Thanks. I'll check them out.
Note mine is an F5 (200/1000) so probably a 35mm/68°.
Saxon had one. Discontinued.
Cheers.