Jump to content


- - - - -

Synta 120mm f/8.3 Achromat combined with Aries Chromacor

This is a review of three products.

  1. The Synta made Skywatcher 120 F8.3 achromatic refractor
  2. The Aries Chromacor
  3. RR Achromat 125/F1000 lens

The Skywatcher 120 refractor needs little introduction. Made by Synta Technologies in China, this scope is a modestly priced medium fast achromat. The Aries Chromacor is a product that came in to being largely as a result of the mass appeal of such inexpensive large aperture Synta refractors. The RR Achromat is a replacement objective lens supplied by Sky Objectives

Skywatcher 120

I purchased the Skywatcher from Scopes Direct in the UK. I opted to purchase the optical tube assembly only, as I wanted to use the scope with my existing Vixen GP DX mounting.

Included with the optical tube assembly are mounting rings and a dovetail bracket, suitable for placing the scope onto the Chinese EQ5 or Vixen GP and GP DX mounts.

In addition the scope came complete with two eyepieces, of 20mm focal length and 10mm focal length (I believe these eyepieces are typical Plossl designs.) and a 6X30 finder scope, 2" focuser with 1.25" reducer. The scope looks smart with its glossy blue paint and adjustable lens cell. In use the Skywatcher held few surprises.

Optically it is surprisingly good, although I would never claim to be an expert at star testing, I am experienced enough to determine when a scope star tests poorly. The Skywatcher displayed none of the trademark optical faults I have seen in some inexpensive telescopes. Certainly I could detect no signs of astigmatism or zones. The star test reveals a modest amount of spherical over correction.

Under the night sky, views were pretty sharp. The scope gives the impression of being very sharp until the view is compared with an optical system that is very sharp. This occurred when I combined the 120 with the Aries Chromacor. I attribute this to the moderate amount of spherical over correction.
Not surprisingly for a medium F ratio achromat of this aperture, there is considerable false colour visible on bright stars, the moon, Jupiter and Venus. Even Saturn with its lesser surface brightness displays a blue halo of unfocussed light at higher powers.

Never the less, the views of the gas giants through this scope were impressive, notwithstanding the false colour. For example the Skywatcher has no problems resolving Saturn's Cassini division all the way round the rings, it also reveals Jupiters GRS residing within its hollow together with other low contrast Jovian cloud detail.

Placed side by side with a Pentax 75 SDHF APO and Stellarvue 80mm Achromat, the Skywatcher easily surpassed both the planetary and deep sky performance of both the smaller but more expensive scopes . It was useable at higher powers and revealed Jovian cloud detail that was just not visible in the smaller aperture refractors.

Wide field, deep sky views are surprisingly good, although no 5" aperture scope is ever going to rival the views through much larger instruments, the views through the Skywatcher are contrasty and its dark sky background works well in the moderately light polluted conditions of my home.

All in all, the performance of this scope is good in real terms and even better when you take into consideration the low purchase cost of £198.

Aries Chromacor

There has been much talk of this device on the various news groups, there are three existing reviews of this accessory on Cloudy Nights, but both these reviews are in respect of the 150mm Synta/Chromacor combination.

I will restrict my comments here to my experience of using the Chromacor with the Skywatcher 120.

There are two principal advantages to be gained by using a Chromacor with an achromatic refractor

  1. Much improved colour correction
  2. Potentially much improved spherical correction.

In my opinion, both of the above advantages are of equal importance.

Spherical aberration is an aberration which occurs when different zones of the objective lens or mirror have a different focus. When the objective has an undercorrection of spherical aberration - the outer zones have a shorter focus than the inner zones. When an objective has overcorrected spherical aberration, the outer zones have a longer focus than the inner zones. The actual difference of focus length is related to the amount of spherical aberration

Of course, the best performance happens when all zones of the objective and all colours have a common focus.

In an average achromat with some amount of spherical aberration, neither the objective's zones and different wavelenghts of light have a common enough focus - so the image suffers from both. The Chromacor will fix both these optical aberrations simultaneously.

The Chromacor, by using various unspecified and presumably exotic glass elements, brings various wavelengths of light to a much tighter focus than the standard achromatic lens can, thereby significantly reducing the amount of false colour seen in the image and thus improving contrast and definition as well as giving a more aesthetically appealing image.

According to Aries, the colour correction when used with the Synta 120 F8.3 scope is 1.5 times better than when used with the 150mm F8.

It is worth taking into consideration the necessity to use a 2" Star Diagonal. The Chromacor screws into the 48mm thread of most 2" diagonals and it is important to ensure that the distance between the Chromacor thread/flange and the focal plane is as near to 161mm as possible. Used with an Intes 2" diagonal and the Synta 2" to 1.25" reducer this distance is almost exactly 161mm.

It is also worth noting that according to Aries, when used with the 120mm F8.3 Synta, the colour correction can be improved even further by extending this length to approx 181mm. At this point the theoretical colour correction is 2X better than achieved with the 150mm F8 Synta/Chromacor.

I decided to put this theory into practice. I had an extended diagonal barrel machined, this increased the distance between the Chromacor and the focal plane to approx 180mm. Indeed the colour correction of the system was improved further still, although not by a large margin. It was also a little more sensitive to collimation in this guise.

In addition to its colour correcting abilities, the Chromacor can be supplied with a pre set amount of spherical under or over correction. The purpose of this is to nullify the spherical aberration of the telescopes Objective lens, thus providing sharper images and better performance at higher power.

That's the theory! How does it work under the night sky?

I initially used a Chromacor O-1, which in view of the slight over correction on the Synta lens was the wrong choice as the O-1's built in over correction will simply add to the scope's spherical aberration rather than reduce it.

The Chromacor O-1 had been purchased for use with my under corrected Celestron CR150 refractor and I initially used it with the Skywatcher 120 in the full knowledge that it was the wrong choice in view of the spherical aberration issues, but I was keen to see how much improved the colour correction would be.

Dealing with the colour correcting properties first, I found that provided I took care to ensure the 2" focuser was correctly squared on to the objective, the colour correcting performance of the Chromacor was very effective.

It is important to ensure correct collimation of the focuser, to assist this I used a Cheshire eyepiece.

Using my favourite BW Optik Bino viewers at magnifications between 150X and 240X I can detect no false colour halo around the limb of Jupiter or Saturn.
It is worth remembering, that when observing Jupiter in particular, without the Chromacor the scope throws up a large halo of unfocussed light that extends several Jovian diameters around the planet. Bright stars likewise show a large improvement.

Rigel for example normally displays a large halo of unfocused light which appears ever more obvious the higher the power used. With the Chromacor, this halo is totally invisible at low to medium powers and only a very faint hint of violet unfocused light can be seen at higher powers.

Using the O-1 and notwithstanding the doubled up spherical aberration ,the views were still an improvement over the standard scope.

I decided to obtain a Chromacor U-1 from APM Telescopes (APM-Telescopes.De), this model has a built in under correction. In theory this should counter the over correction and result in a virtually null corrected optic.

The difference was immediately obvious.

The star test using a green filter showed the improved spherical correction. Before, the outer fresnel ring inside of focus was thicker and had a slightly "woolly" appearance. With the U-1 installed I could not determine any difference in the outer rings on either side of focus.

Although I shy away from providing actual estimates of spherical correction, to give an illustration of the Chromacor's spherical correcting properties, if one assumes the scope has over correction of say 1/5 wave, dividing this by the Chromacor's 1/6 wave under correction (1/5/1/6) the net result is a scope with 1/30 wave over correction.

This is simply outstanding and explains why this combination delivers such sharp views. The Chromacor U-1 therefore has a two pronged approach to image improvement. It corrects for much of the light left unfocused by the simple achromatic object lens, thus improving image sharpness, definition and the overall aesthetic appeal of images delivered. It also corrects for the spherical aberration of the objective lens, delivering sharper images and allowing the effective use of higher powers.

But the most important test for me came when I turned the scope onto Jupiter. Again the difference was immediately obvious. The scope snapped straight into focus in a way that demonstrated perfectly the improved optical performance. There is a definite improvement in image sharpness when comparing the views with the U-1 to the O-1 or the telescope without either Chromacor. Using the slightly coarse Synta focuser, there was never any doubt when the scope was in perfect focus.

Jupiter was beautiful because of its sharpness, the limb was hard edged against the dark sky background, as the GRS crept into view, the darker outline around one side of the hollow was visible with a clarity the scope had not been able to achieve before. One night whilst I was conducting this test, the Chromacor U-1 and Synta 120 combination delivered the best views I have had of two faint festoons extending from Jupiters NEB into the equatorial region The scope retains its sharpness at high powers, the seeing seems to be the limiting factor at my observing location.

RR Achromat 125mm/F1000 Objective Lens

I obtained this lens from Hans Winkelaar of Sky Objectives in The Netherlands.

The lens was received before the U-1 Chromacor and I was interested to assess its optical quality and to determine whether it would be a better match for my O-1 Chromacor.

The lens was easy to assemble, it is an air spaced achromat that fits perfectly into the Synta adjustable lens cell and gives a clear aperture of 120mm. There are clear alignment marks on the two lens components and I experienced no collimation problems.

Star testing the lens showed it was indeed superior to the stock Synta lens, it took me a long time testing before I eventually concluded this optic has a very small amount of spherical under correction. I could see no evidence of any other optical defects.

I tested the lens without the Chromacor. It threw up an identical amount of false colour to the Synta lens and was very sharp and operated well at higher powers.

Hans Winkelaar was very helpful and supportive answering all my questions.

Ian King




Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics