Acro vs ED

[starscy]

How do the following refractors compare?

Celestron Omni XLT 120 (CG-4)

Sky-Watcher ED 80 OTA (& Vixen Porta II)

[kniclander]

I have the celestron ed80 which optically i understand is the same as the skywatcher. it is a lovely little scope but if i only had one scope, it wouldn't be it. it's too small both in terms of aperture and focal length.

[Towa]

I've got an ED 80 and Evostar 120mm f/8.3. If doing visual, then the 120mm Achro is a better scope. Mine has given me some wonderful views of Mars over the last two months. The ED gives nice views, but the Achro shows more.

[michael.h.f.wilkinson]

For deep-sky viewing the 120mm achromat with its 2.25x greater light-gathering capacity will beat the 80mm ED any time. Limiting magnitudes differ by 0.88 magnitude, which is quite a bit. Using filters to combat residual chromatic aberrations can greatly increase their performance on planets. Again, the increased aperture will give you a better performance. If I only had space or budget for one scope, I would go for the achromat.

[riogrande100]

From the small time i have been involved in the hobby i have found that if your imaging objects like the moon then the ED would definatley be prefarable, as achromats CA can really distort the color of the image with fringe.

I have taken images using of orion and not sure if CA alos distorts the smoothness of stars.

[michael.h.f.wilkinson]

From the small time i have been involved in the hobby i have found that if your imaging objects like the moon then the ED would definatley be prefarable, as achromats CA can really distort the color of the image with fringe.

I have taken images using of orion and not sure if CA alos distorts the smoothness of stars.

CA affects the point-spread function of the optics, so stellar images will be distorted (extended typically) because the R, G and B images lie at slightly different focal planes (and may have slightly different magnification). Off-axis stars can become elongated "spectra" showing blue at one end, and red at the other. I have not seen this visually in the achromats I have used (most were about F/8 or F/10). The only exception is a very fast (F/5) 70mm achromat I snapped up for very little. I only really use this at 15x magnification, as a cheap alternative for binoculars. At higher magnifications, the CA problem is severe.

Note that when imaging the moon in monochrome cameras, you can reduce this using various filters.

[ollypenrice]

I think it's pretty simple. The ED80 is the world's all time imaging scope bargain and the 120 (and 150) achros are the world's all time visual refractor bargains. I've had both the 120 and 150 and really they give a very decent view of the sky. (I would avoid the short tube versions personally. They have real colour and coma issues.)

Olly

[michael.h.f.wilkinson]

I think it's pretty simple. The ED80 is the world's all time imaging scope bargain and the 120 (and 150) achros are the world's all time visual refractor bargains. I've had both the 120 and 150 and really they give a very decent view of the sky. (I would avoid the short tube versions personally. They have real colour and coma issues.)

Olly

Neat summary. Indeed, avoid short-tube achromats. If the missus lets you, get both scopes:D

[jgs001]

I've used the ST80 for imaging. A SemiAPO filter sorted out all but the worst of the CA on stars, but the quality of the optics led to a number of oddities anyway. The C80ED is made with much better optics.

[F15Rules]

I wonder if your enquiry is to help you decide which to buy?

If so, a suggestion from a confirmed achro fan (value for money) and new apo user ED100, bigger brother of the ED80 (deep down, we all want to be closet Takahashi owners:p)

If you can stretch to it, First Light Optics are selling off their last Celestron ED100s. I bought one a month ago and it is fantastic. It cost me £475 complete with a CG-5 manual mount with 2" stainless steel legs (a great mount), a nice 9x50 finder, 20mm plossl and diagonal. If you don't need the mount and bits, sell them, you should get around £180 for them, so you would have a new apo 100mm OTA for under £300 - compared to the optically identical Skywatcher Equinox OTA only at £630..the latter has an entry level crayford focuser versus the EDs' Rack and Pinion but it is 2" and mine is very smooth, I'm quite happy with it.

The ED100 will show everything on Mars that my 125Achro will, just not quite so bright, but with no false colour at all - so you can image with it no problem if you wish..

The Omni is a fine scope too though, so you really are spoilt for choice:D!

[michael.h.f.wilkinson]

I think the discussion lacks focus a bit because we do not know what the primary use of the scope is going to be, and what other constraints there are (space, weight, etc). If imaging, the APO is better, if visual the larger achros could be better, but then I would consider a 6"F/8 Newtonian to be superior BOTH for planets and deep sky (just to defocus the discussion further. They are quite cheap, have an optical performance close to an APO of the same aperture (without the missus throwing a fit at the price), but bulkier of course.

Of course, some people ONLY want refractors (or even love them), but if I could only have one budget instrument, the 6"F/8 would be it.

[riogrande100]

I think the discussion lacks focus a bit because we do not know what the primary use of the scope is going to be, and what other constraints there are (space, weight, etc). If imaging, the APO is better, if visual the larger achros could be better, but then I would consider a 6"F/8 Newtonian to be superior BOTH for planets and deep sky (just to defocus the discussion further. They are quite cheap, have an optical performance close to an APO of the same aperture (without the missus throwing a fit at the price), but bulkier of course.

Of course, some people ONLY want refractors (or even love them), but if I could only have one budget instrument, the 6"F/8 would be it.

Newtonian better for Astrophotgraphy as well???? So afr all i ahve read points to Refractors rule for photos'

[michael.h.f.wilkinson]

Newtonian better for Astrophotgraphy as well???? So afr all i ahve read points to Refractors rule for photos'

I only mentioned them as an option if the main use is visual, 1200mm is a long focal length for astrophotography, and you would need a coma corrector (though an F/8 is not too bad in most cases).

If the main use if deep-sky astrophotography, by all means go for an APO