Need a education in refractors. Help please!

[mental4astro]

Hi all,

I'm looking to dabble with what is really my first experience with refractors. What I'm looking at getting/making my ultimate rich field scope, so something between an 80mm f/5 & a 4"f/4 achromat.

This won't be for photography or high power, nor lunar or planetary, so chromatic abberation isn't a concern for me. But I imagine that a fast refractor will introduce its own set of quibbles. "Field curvature" is one, but how can one best deal with this? Better quality EPs handle this better?

Anything else I need to be on a look out for? Focuser? Design if it comes to a DIY using existing components? Other advantages of Apo over Achro other than chromatic abberation. Anything.

Cheers,

Mental.

[goober]

Hmm, I remember a guy named Phil from Melbourne who built his own refractor. You may see some posts about it here... this was a few years back... 2007?

[MattT]

Mental, use the link in my signature to get to Istar scope club. It's all about refractors.... nothing else. As a dedicated refractorholic it's the best place as is cloudy nights refractor forum.
I have a 4" f5 with a 28mm 68deg ES eyepiece that gives 3.7 degree FOV I think which at a dark site would be amazing a bit curved similar to a fast reflector but nothing that really bothers me too much. Check out Istars lenses on the Istar site...cheapish round $500 for a 6" achro.
Matt

[brian nordstrom]

:) Hi Alex , just what do you want to use it for , Visual or AP ?
The best all round refractor has to be the ED80 , awsome for AP and short enough to give well corrected wide field views using 25 - 45mm 2inch eyepieces , Great quality for little outlay , I recon . :thumbsup:.this is what you need either way , mate.
Brian.

[mental4astro]

Purely for visual!

Oh, & sketching!!!!:D

[brian nordstrom]

:thumbsup: ED 80 for sure , the FPF53 glass in these would be perfect for you are wanting to do Alex , nice by the way , the colour's would be way more true to life than a short focal length achro , sad but true . :shrug:. And in another league quality wise , for a song .
I think there is one in IIS classifieds now .
Brian.

[AG Hybrid]

Alex 4" F/4 refractor is going to be like looking through a fish bowl. You'll need drop a decent amount of cash to compensate the field curvature.

Next time we get a night out I'll give you some time with my 4" F6.5. Pretty damn good as a rich field scope. With my 30mm it give a true field of 3.5 degrees. To put that into perspective that's all 3 stars in the Orion's belt in the same field of view with a bit of room to spare.

An ED80 is a good choice and decent value. But I suspect you'll be wanting for aperture. TBH, and this is my opinion only. The views from smaller then 100mm just don't really throw up a particularly interesting view when compared to a 100mm telescope or above.

[Poita]

I'd second that, the differences between my 105/650 and my ED80 are considerable.

If possible, I'd go for a 4" or larger refractor. You can borrow my Lomo 105/650 if you want to see how it looks.

[mental4astro]

I, for once, am not chasing aperture! I'm chasing WIDE FIELD VIEW!!!:D

Maybe if you read the following experience of mine from a week ago will give you some insight on my thinking:

http://www.iceinspace.com.au/forum/showthread.php?t=92811

What I now see that I've omitted in my original post is that a with an 80mm f/5 scope, & a 2" 35mm 68deg FOV EP will give me a huge 6 deg FOV! My 11x70 binos have 4.5deg FOV. I'm now GREEDY for more!!!:lol: :D:help:

To go large aperture, & longer focal length is a step backwards.

I've found a 100mm f/4 achro lens on the net. Would stopping it down to f/5 help?

Adrian, I found the view through my 11x70 binos absolutely staggering. Maybe it was the RFT nature of them, but I can't see myself going for an f/6 and slower scope. That's why I'm asking about the pain that's involved with fast refractors. RFT's have really gripped me, & now I also want WIDE :)

[cometcatcher]

Generally speaking, scopes faster than F6 won't give pinpoint star images very far out from the center sweet spot unless you use some very expensive glass.

I'm not sure why a visual observer would have F ratio fever? Consider that the field of view with an F8 scope and 30mm eyepiece will be the same as an F4 scope with a 15mm eyepiece. And the view through the F8 scopes will have less noticeable "seagulls" near the edge of the field.

Another thing to consider is exit pupil size. If you're a young fella you might get 7mm. As we age it's more like 5mm. If the ultra fast scope gives an exit pupil larger than our eye is capable of, the extra light is wasted.

To calculate exit pupil size, divide the aperture of the scope's objective lens by the magnification of the eyepiece. For instance if you have an 80mm objective say at F4 with a 40mm eyepiece, first work out focal length 80 x 4 = 320mm. Then work out magnification 320 / 40 = 8. Now that you have a magnification of 8, 80mm / 8 = 10mm exit pupil.

If you're middle age and have a 5mm max eye pupil size, that fast scope is just going to waste with any eyepiece over 20mm. You can still get wider views, but it won't be any brighter.

F ratio fever is for photographers. ;)

[brian nordstrom]

Well , if moneys no object then grab the TAK FS60c thats just been posted in the classifieds , Alex .
I have one and there is none better , the views thru mine using my 82 degree Celestron axiom LX are the best , these lil scopes do punch way above their weight .
Perfect colour correction and flat field , nice.
Just a thought ?
Brian.

[mental4astro]

How about binoviewers? Any good? I know nothing about them :shrug:

[gregbradley]

ED80 is the best allround bang for your buck widefield scope. I think its F7 or F8 but you can get a reducer.

Its also good for astrophotography.

I am not aware of any F5 or F4 refractors except FSQ106ED F5.

The lower the f ratio the higher quality the glass and optics need to be otherwise aberrations of various types will kill it. Even high end APO refractors are rarely faster than F7. You can get reducers to bring them down to mid F5 but those reducers are usually worth more than you are probably considering for a scope.

If you want F5 best go with a Newt. They are cheap, light, collect a lot of light and have cheap accessories like reducers and flatteners and are also good for widefield astrophotography.

F4/5 in a cheap refractor is probably an unreal prospect.

Greg.

[marki]

See if you can get hold of one of the meade 5000 series 80mm apo's. Not F5 but F6, lovely little scope with a triplet lens and wide FOV.

Mark

[cometcatcher]

I believe Alex already has an 8" F4 Newtonian.

The Kson Ekinox F5.5 ED80 seems fine to me visually with a good eyepiece. To go wider than that, I'm thinking binoculars.

Or if a person isn't that fussy... maybe even a Skywatcher achromatic F5 refractor. But I haven't seen through the F5 versions.

[mental4astro]

Hi all again,

Ta for the input.

As Kevin mentioned, I already have an 8" f/4 Newt.. The coma it produces in my 30mm 68deg. EP is not a problem for me. For me, I think these aberrations are overstated and overrated. When the object is close to the edge of the FOV you will move the scope to accommodate it, regardless of the quality of the image at the edge with visual use! I don't mind the coma and even some astigmatism in the eyepiece for this reason. A coma corrector is wasted on me as they also kill photons.

Remember, I am wanting to get the WIDEST rich field I can (to within reason and exit pupil). From the replyies, field curvature at low powers is the biggest aberration with fast refractors. From my reading and dissection of binoculars, this is delt with by introducing a smaller field stop to the eyepiece of low power binos. The AFOV tends to be bigger in higher power binos than low power ones of the same aperture to remove the field curvature aspect. Most binos use the Erfle EP design, which is capable of delivering upto 70deg, even 80deg AFOV, but this would be lost in binoculars. Efles produce a fantastic wide field image in slower instruments than fast.

Take my 11X70 binos. The FOV is 4.5deg, giving an effective AFOV to the EPs of 49.5deg. That's smaller than a standard Plossl EP of 52deg.

Now, as I understand and am happy to deal with the aberrations of optical systems, like my 8" f/4 Newt., so a fast refractor, for visual purposes, I'm seeing as quite a feasable beastie.

I completely understand the consequences of such aberrations to astrophotography. My visual philosophy sees me happy with them though, :).

Kevin's new 80mm f/5.5 semi APO looks very interesting... :) :question: :D

[Larryp]

Hi Alex
Long Perng make a quite nice 90mm f5.5 refractor-it is actually the same lens as the Megrez 88mm, as Long Perng made the Megrez for William Optics. Image quality is very nice if you don't mind a bit of chromatic abberation on bright stars and planets.
Andrews Communications sell it for $799.00

[cometcatcher]

I was talking to Luke at Andrews because I was after one. They are out of stock of both the 80mm and 90mm LP and won't have any of those in stock again for months.

[Larryp]

Hi Kevin,
My LP90 mm will be for sale, as I have ordered a triplet apo from o/s.
Pm me if you are interested.

[Poita]

That's why I mentioned the Lomo, it is an inch wider than my ED80, but the same focal length, so I enjoy the wide views.

[omegacrux]

Sounds like your after what I got 150x750 acro with 25 or 30mm ep goes pretty wide !
25 mag on Eta or Orion fits all omega cent great
Pick em up $500 good value

David

[robz]

Hi Alex, I picked up a 6 inch F5 Achromat from Andrews a while back and can say that it is absolutely awesome on wide field low power views.
I think they still have them....................worth a try.

Highly recommended!;)

Rob.

[Wavytone]

Alex,


If you want an RFT you can use f/7 - there's no real need to go down to f/5 these days unless compact size is what you really want. The benefit of f/7 is that the ED f/7 doublets can also give perfect high powers too, far better than any f/5 can.

Basically, it comes down to being able to

a) reach the upper limits of magnification feasible from a good 4" refractor, when you want to (bright doubles or the planets) with eyepieces that are still comfortable to use, which makes it useful on weekends away.

Faster 4" refractors will be quite inferior at high power (an f/5 is not much good, actually)

b) it can also operate as an RFT at f/7, if you have a 40mm 2" eyepiece that has no field stop (I have one).

By way of example:

High power: at x1 per mm of aperture you should jut be able to discern the maximum detail in images that the scope can provide. If you use more power (say 1.4X per mm) you should clearly see diffraction rings around bright stars, but not really any more detail.

For an f/7 scope, a 7mm eyepiece gives x1 per mm, and a 5mm gives x1.4 per mm. On my scope, 102X and 148X respectively.

Low power: if you assume your exit pupil could be 6mm, for a 4" scope that means the lowest useful magnification is 17X. This corresponds to a 42mm eyepiece, and I happen to have a 40mm (close enough) which means it will work fine as an RFT.


Happy hunting !

[Wavytone]

Alex,


If you want an RFT you can use f/7 - theres no real need to go down to f/5 these days unless compact size is what you really want. The benefit of f/7 is that the ED f/7 doublets can also give perfect high powers too, far better than any f/5 achromat can.

By way of example:

Low power: if you assume your eye pupil could be 6mm, for a 4" scope that means the lowest useful magnification is 17X. This corresponds to a 42mm eyepiece, and I happen to have a 40mm (close enough) which means it will work fine as an RFT.

Note that this lowest power is determined by m = D/d
where D the ratio of the objective of the scope, and
d = the diameter of your eye pupil.

It is not governed by the f/ratio of the scope. If you choose an f/5, it will only need a 30mm eyepiece to give the same exit pupil and the same field I have in mine.

High power: at x1 per mm of aperture you should just be able to discern the maximum detail in images that a scope can provide. If you use more power (say 1.4X per mm) you should clearly see diffraction rings around bright stars, but not really any more detail.

For an f/7 scope, a 7mm eyepiece gives x1 per mm, and a 5mm gives x1.4 per mm. On my scope, 102X and 148X respectively. On an f/5, those eyepieces will be 5mm and 3mm. And I don't like using eyepiece that short.


Happy hunting !

[overlord]

EXACTLY!

If I want refractor it is gonna be one of those old style f/15.

This also cuts down on the chromatic!

yay!