Buying a Refractor

[eskimo20 (Robert)]

Never had a refractor - thinking I probably should scratch that itch.

Looking at 125mm - 150mm achromats it seems that none has a long enough focal length to reduce chromatic abberation to an acceptable level or to reach the diffraction limit for the aperture (based on the various formulas floating around eg Sidgwick).

Is it feasible to stop down the aperture to meet the constraints the physics for lunar and planetary and open up for deep sky.

Thanks
Robert

[jenchris (Jennifer)]

If you stop down the aperture, why not just buy a smaller, say 100mm, apo?

[Tinderboxsky (Steve)]

What do you mean by an acceptable level of chromatic aberration?
My main visual refractor is a Vixen NA140SS which has superb optics and has well corrected chromatic aberration to the point that I struggle to notice it. It is a superb deep sky instrument.
The only object that shows some noticeable blue wash/fringing that interferes with the sharpness of the image (at high magnification) is the moon. This is easily corrected with a Baadar Fringe killer filter. The faint yellow cast this produces "disappears" after about 10 minutes viewing. I have not tried stopping the aperture down as an alternative but I think if you do the numbers you will be down to about 100 mm aperture to get a long enough focal length. I also have a 103mm APO and I can tell you that I much prefer the views through the 140mm scope to the point that it is my scope of choice for all night sky visual observing. It is gathering twice the light and has noticeably better resolution than the 103. The 103 is now dedicated to solar observing.
Cheers
Steve
PS, your thought of stopping down has me interested to know what the results would be on my scope. I will try making a temporary mask and see what the results are.

[eskimo20 (Robert)]

Originally Posted by jenchris

If you stop down the aperture, why not just buy a smaller, say 100mm, apo?

Well that's an option. But it will probably cost more and I wont have the full 125/150 for the fuzzies.

[eskimo20 (Robert)]

Hi Steve

By "acceptable level of chromatic aberration" I was referring to the theoretical formulas put about by Sidgwick and others.

As i said, I've never owned a refractor and have only looked through a couple over the years.

Quoting from Phil Harrington's book "residual chromatic aberration will not be a factor as long as an achromatic refractor's focal length is equal to or greater that 2.88 times its aperture in inches".

Another version is that the focal ratio should be 3 times the aperture in inches.

I expect that this is only relevant to high magnification viewing of bright objects which is what I want to use my scope for.

Chromatic aberration is not just an annoying false colour, it degrades the image as well.

My original question related to stopping down for the moon, venus and maybe some of the other bright planets where there is already a lot of light.

For deep sky obviously you would use maximum aperture.

I'd be interested to hear how you get on if you do decide to increase the focal ratio of the vixen for lunar/planetary. You might get away without the filter. Even at 100mm the moon is pretty bright.

Cheers
Robert

[MattT]

Originally Posted by eskimo20

Looking at 125mm - 150mm achromats.....

Is it feasible to stop down the aperture to meet the constraints the physics for lunar and planetary and open up for deep sky.

Thanks
Robert

To me that suggests only one scope a 150mm f8 Synta. They are cheap and come up for sale every now and then.

Stopping it down works. I have done it with my 150 f8 stopping it down to be a 100 f12. It does reduce CA by a lot but also dims the view...a lot.

What worked best for me on planets with 150 at full aperture was bino viewers with a Baader semi-apo filter. CA wasn't apparent with that combo.

I then got LFLF ..... built my own 150 f12 that doesn't need a CA filter.

Matt

Edit: LFLF.....long focal length fever

[eskimo20 (Robert)]

Thanks Matt

Going down to 100mm f12 would be the "perfect" solution based on the theory.

Presumably there's compromise point where you don't lose too much light but still get better CA control - say f10.

There are a lot of f10 objectives out there...

Robert

[MattT]

My personal view is CA control starts at f12 for a 4". I have a very nice 90mm f11 that shows a small bit of CA but not enough to buy a SW ED100mm...had 2 of those and sold them. I prefered the 90mm f11 achro view...go figure

A 70mm f13 I bought not so long ago is colour free, and a great lunar scope. Looking foward to using it, the f13, on Jupiter.

Matt

[eskimo20 (Robert)]

Matt
Who made the f11, f13 acromats you use?

[dannat (Daniel)]

you can find carton 4" f12 objectives, canadian telescopes / astrotelescopes among others have a modern 4" f11 which get good reviews, istar make long focus achromatic objectives, D&G don't appear to churn them out like they used to

[anj026]

Robert you have two reflectors which should be good for planets and DSO's. If you are keen for a refractor 5" to 6", f5 to f8 then you can think of it as low power wide field instrument. If you use it at lower magnifications around 6x to 10x per inch of aperture then you will find it gives wonderful images. These refractors are not really best at lunar and planets.

I have a 6" f5 Jaegers and a 6" f15 D&G. The Jaegers will show heaps of violet colour if used for lunar or planets at magnifications over 60x. However it is an amazing wide field scope at magnifications of 25x to 45x and is a lot of fun at a dark sky site. It is made for comet sweeping and cruising the milky way. The D&G starts at 60x (40mm eyepiece) and is good up to 200x for all kinds of objects. It does show some minimal false colour but the detail and contrast are so good that the false colour is easily forgotten.

There is a wonderful book called Starlight Nights by Leslie Peltier. Mr Peltier grew up on a farm in the USA and had use of a 6" f8 refractor. He used it for all kinds of visual astronomy but was most famous for observing and recording variable stars over many years.

I suggest you try a refractor, I think you will enjoy it so long as you have reasonable expectations.

http://www.cloudynights.com/documents/peltier.pdf

[MattT]

Hi Robert,
The 90mm f11 is an old Orion, mid 1990's model, that I bought from Lars on IIS classifieds. I changed the focuser to a GSO 2" single speed crayford.

The 70mm f13 is a Synta scope. Still sold, look for the 70x900mm.

Matt

[eskimo20 (Robert)]

anj026

Thanks for your comments.

The f8 reflector is in the process of being reincarnated (again) and will get used. The f5 truss tube is a bit too heavy to lug around these days and I'd like to have the option of an equatorial mount with tracking. I'll probably sell it.

I suppose I'm trying to tick several boxes with the refractor. I want exploit the qualities of a long focal length scope on planets and the moon and also be able to look at some of the brighter DSOs. Cost is also a consideration otherwise I'd just go out and buy a 6" apo

[eskimo20 (Robert)]

Matt
Thanks for this information

[AstralTraveller (David)]

FWIW, after decades of owning newts my first refractor is a 15cm f/8 Celestron branded Synta. I think I was lucky and got a pretty good lens (I've heard their QA isn't perfect). Initially the CA was objectionable so I got a Baader FringeKiller and placed it on the front of the diagonal so it's always in use. That reduced CA noticeably but introduced a yellow cast. Over time both the CA and yellow cast have 'disappeared' ie. I no longer notice them. Everyone seems to agree it gives quite nice views.

The thing I found more annoying is the shaking when attempting to focus. It's on a GC5 which handles the weight easily but with the focusser a good 50cm from the dec axis you have a lot of leverage when you touch the tube. Initially I upgraded to an add-on motorfocus but eventually the mechanism in the focusser developed so much sideways play that i upgraded to a Moonlite motorfocusser. It's wonderful and makes the scope so so much better to use. Being able to get the focus spot on possibly gains me half a magnitude in limiting mag. That of course translates to better lunar and planetary views as well. Because you don't have to fight the focusser it also keeps you calm and that provides a better viewing experience.

[eskimo20 (Robert)]

Hi David

I always used Newts too and on the few occasions that I have looked through achros I have found the CA to be a bit annoying (coming from the colour-free reflector views).

However, I expect its something you get used to if its not too bad.

I take your point about the advantages of a motorised focusser.

Cheers
Robert

[brian nordstrom (As avatar)]

Hi Robert , I like Matt have refractoritis and have a home built , by me iStar sourced 127mm f8 and it performs so well in all ways I sold a perfect 127mm f7.5 ED triplet APO I had at the time of building the iStar 127mm .

These lens sets are very , very good and the best thing is they have the objective and fully built up scopes in most sizes in stock at great prices , mine took 10 days from Florida USA to Darwin .

My 127's total cost of build including objective ,( iStar) , tube ( IIS classifieds ) , focuser ( Andrews ) , finder and diagonal ( IIS classifieds ) Dovetail and rings ( Bintel ) was only $620 , ,,,, took a short time to get all the gear together but I had the satisfaction of building this awesome refractor myself and my way ..

Here it is a photo fully loaded , so to speak at 2014's partial solar eclips here in Perth , total cost as seen here $1120 , nice scope .

Also here is an un-touched prime focus moon shot taken with my Sony alpha DSLR at 2000mm using a TV 2x powermate , showing the very well controlled CA this iStar produces at f8 ( 1000mm ) .

Hope this gives you a little inspiration , enjoy .

Brian.

[eskimo20 (Robert)]

Brian

Nice work!

Not sure I'm up for another building project. Exchange rate is a bit uglier now too. But I'll check out IStar.

Thanks