Highburymark

There’s one other thing about the ‘branches test’ that’s informative. When focusing on one particular branch, the majority of other branches in the fov are inevitably slightly out of focus. So if you see purple quilting all around, but your focused branch is colour free, then that seems to be a perfect demonstration that your telescope’s control of CA is pretty good. The background sky does need to be fairly bright to provide a stern enough test, and deally, conditions will be still, so your chosen target isn’t blowing around.

Appreciate all the contributions to an interesting thread. 300 years after the first crown and flint doublet, it’s amazing how something as simple as two discs of polished glass still challenges our curiosity.

This does make sense, yes. Though presumably the lenses would have to be thicker on the larger scope to retain the same refracting power?

The question here is why should the average 60mm F/6 scope have better colour correction than a 100mm F/6 scope, assuming they are doublets and use the same glass? I have now read conflicting reports on this. One says the light bends less with the smaller scope - hence is less liable to false colour. Another claims that scopes with the same focal ratio should be equal in CA, no matter the aperture - as Andrew states in his first post above. If there is a difference in CA between the two scope sizes, I don’t think it’s because light is bending more. But perhaps it’s because overall, more light is bending? The larger glass is refracting a greater amount/surface area of light away from the central axis of the objective lens. Absolutely. More tests to do.

I remember this diagram but didn’t remember the relative difference in CA control between big and small apertures. It certainly suggests my experience with the 60ED is not particularly unusual. But why is a good question. Is it easier to manufacture smaller aperture lenses accurately to iron out false colour, or is the light being bent less? A quick Google search suggests it’s the latter.

Thanks chaps. Of course colour correction is only one part of the equation. The artificial star test exposes other interesting features. The TSA and TV85 are almost textbook - identical in and out of focus (apart from the TV85 showing the expected colour difference - purple one side, yellow/green the other), but otherwise perfect, clearly contrasted rings. The Tecnosky, in comparison, shows clear, black rings one side, and much fuzzier rings on the other. Slightly less foggy with a green filter. Possibly due to spherical aberration? To determine that, I’ll need to do the 33% obstruction test. Must admit, as someone who never did sciences beyond O level, a lot of this stuff is over my head. I’m fascinated, but only as a layman. All these scopes work very well indeed.

I have owned five apo/semi-apo refractors over the past decade. From the start, I’ve subjected them to the same rudimentary visual tests, one of which is to determine how well they control false colour. I thought my impressions might be worth sharing after I recently took delivery of a little 60ED which performs rather well. Not much science involved, I must confess. I simply test them with an artificial star, and then observing tree branches against a bright background sky. I have mostly used orthoscopic eyepieces for the tests, together with Plossls and high power Takahashi TOE eyepieces. I also test them with Baader Zeiss T2 prism and Baader T2 BBHS mirror diagonals. And I repeat the tests over time, whenever possible side by side, to ensure comparisons are as meaningful as possible. The telescopes are as follows: SW Equinox 80ED, FPL53, F/6.25 doublet. Takahashi FC-100DC, F/7.4, fluorite doublet. TeleVue 85,F/7, ED doublet. Takahashi TSA-120, F/7.5, FPL53 triplet. Tecnosky 60ED, F/6, FPL53 doublet. Here’s how they perform in ascending order, with CA control rated out of ten: 5th: 6/10 - SW Equinox 80ED CA was most obvious with the Skywatcher. Violet edges to branches and on the lunar limb, even in focus. Though it always gave sharp and pleasing views, even the planets at 180x, its relatively fast speed was clear on bright objects. 4th: 7/10 - TV85 The TV85 is a wonderful scope. So flexible and beautifully built. But it can exhibit small amounts of CA under demanding conditions. When doing the tree test, marginally out of focus branches show purple fringing. And at very high magnifications (200x and over), colour can be perceived in bright sunlight. Although it’s only fourth in this comparison, the TV85 is probably the scope I’d keep if I could only have one refractor - its value as a solar/travel/spotting/night vision/grab and go instrument means it gets the most use of all my current scopes. 3rd: 8/10 - Tak FC-100DC Third is the Tak FC-100DC. Such a lovely scope, pretty much colour free when focused. But at high magnifications, just slight amounts of CA were discernible…… I’m being very picky with both the TV85 and FC-100 here. Both would impress the vast majority of visual observers. 2: 9/10 - Tecnosky 60ED. Well here’s a surprise. The fastest telescope tested. Chinese. A little over £300 new. Absolutely colour free in daytime tests. Even defocused branches don’t display any false colour. To see if the diagonal made any difference, I swapped the mirror for the Zeiss prism - reputed to not work so well with fast scopes - but the same result. Completely colourless branches, in or out of focus. Doing a bright star test, then the familiar purple and yellow rings are obvious when defocused, but a very impressive performer. 1: 10/10 - Takahashi TSA-120. Zero colour on the brightest stars/branches in or out of focus. Superb colour correction on all targets - but at a price! Although I no longer have the Equinox or FC-100, all these scopes were/are great to my eyes. Unless false colour is noticeably diminishing views, then it’s of marginal importance. Hence why I would be happy to keep the TV85 as my one ‘do everything’ scope. But visual tests like these are still of technical interest, and show that Chinese FPL53 doublets can reach or even surpass some of the more expensive brands in colour correction.

I use my 135 in alt-az mode with a Gitzo 5 series carbon fibre tripod advertised as supporting up to a 40kg payload. Mostly carrying a 5” Newtonian or 85mm refractor. It’s very stable. I was slightly less confident using a Gitzo 4-series that has a 25kg payload. Always use counterweights if any concerns about imbalance, but with the 5-series and my modest telescopes they are not needed.

It’s going to be the best year for solar activity for some time. The first two years of Cycle 25 have exceeded expectations and we have lots to look forward to as activity continues to increase. Even a small refractor and some Baader solar film will produce great detail around sunspots. Hydrogen alpha scopes will open up much more - filaments, active regions and prominences in the solar chromosphere. If conditions for night astronomy are difficult, solar provides regular opportunities to get out and observe or image.

Excellent - thanks.

Very nice set up Nigella. What refractor are you using for CaK?

That is a tempting price. But I would definitely give it a few months (or longer). The 135 benefitted from very close working between Rainbow and owners to iron out early quirks and problems - most of it documented on long CN threads. With a small number of users and quick responses from the manufacturer, things were worked out pretty quickly. ZWO will sell far more units, and communication will be more difficult. All the coverage so far has focused on imaging, but these are excellent visual mounts too. I use my 135 mostly for visual, but plan to start solar imaging with it in the spring. It was designed to be used with carbon fibre photo tripods, but the ZWO - 50% heavier - is a slightly different proposition. I hope it delivers as these are game changing products, but wise to wait and see how reliably it performs before hitting the buy button.

Nice report. No question the Baader should be sharper than the ES Victor. Minimum glass is usually the way to do on the Sun, even if barlowed.

Very nice Nigella. It’s an epic - will keep you going until next Christmas. And even better news: the latest prediction from solar boffins is for a much stronger Cycle 25 than anyone thought.

What a Christmas present!

Sure - astrograph.net

If you’re in the UK, Astrograph sells the APM range. Despite supposedly giving up large eyepieces a few years ago, I’ve just bought the 20mm for well under a third the cost of the 21mm Ethos.

You have some great eyepieces! The zoom has a fine reputation, and I’m sure your new Pentax will be stunning. For the first 15 years in the hobby, I only had some Meade Plossls and loved them. Not until relatively late in life did I start buying more expensive EPs (funnily enough coinciding with the time I joined this forum)…..

Difficult question Dave. For planets: Tak TOE 4mm. For solar/lunar binoviewing: Leica HC Plan S 25mm. Night vision: TV plossl 55/67mm. Probably my most used eyepiece is Panoptic 24. I know it has pincushion distortion but not once has that been a problem during observing. It’s just an all-round excellent eyepiece. A modern classic. It’s a useful finder eyepiece. A great DSO performer. The first EP in the case when travelling. And brilliant for binoviewing, including on the Sun.

These are beautiful! You’re lucky to have a very good Quark.

Excellent report Victor. Thanks for posting.

Very nice! Thanks for the report. I bought a little Tecnosky 60ED and have been impressed with the build quality. Yet to get it under the stars, but they seem to offer excellent value

Definitely Stu - they don’t seem to come up often on the classifieds any more. And even new it’s difficult to find stocks of Fujiyamas at the moment

Thanks Dave! Very similar to the Antares orthos too. Adding to the general green and blackness of my eyepiece collection.

Beautifully packaged in individual canisters and completely mint - thanks to Sylvain from France - a set of Kasai orthos.