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25585

How much CA in an Evostar 150mm?

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Being a 1200mm f8 achromat,  how much CA would there be in the SW Evostar 150mm? 

Edited by 25585

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How would you like it quantified ?

I've owned 3 of them. The CA seemed about the same in all 3. Quite noticable around the moon, brighter planets and stars brighter than around mag 2/3. I tried to measure it around the moons limb (it varies with the magnification used, increasing as more mag is applied). I seem to remember that the depth of the CA halo around the limb was similar to the apparent diameter of the crater Copernicus so something around 60 arc seconds at perhaps 150x :icon_scratch:

I also owned a Meade AR6 which is a 152mm F/7.9 and that showed a similar amount. It's a difficult thing to quantify through because some people seem to notice it more than others.

All the above scopes had a degree of SA (spherical aberration) as well which had some impact on their performance at higher magnifications.

There is probably some example to example variation on both CA and SA though so others may have had better experiences ?

Using a Chromacor to correct the bulk of the CA and SA was very educational for me with these scopes and why I've ended up with ED doublets now.

 

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I ought to add that I'm not over-critical on these chinese 6" F/8's. For what I paid for them (all were under £200 on the used market) they provide a big refractor / unobstructed aperture experience and I generally enjoyed using them (I did own 4 of the things, but not all at the same time !). A 6" ED doublet is going to cost around 9x as much on the used market I'd have thought. Skywatcher are due to launch a 150mm ED doublet later this year so it will be interesting to see what that costs.

 

 

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I've only owned one 6" f/8 achromat the Evostar 150 with collimateable cell.

I really enjoyed owning one although mounting it was often challenging due to the size and weight. 

I am one of those who can observe through an achromat refractor with little distraction from adverse effects of CA but admit that the use of a semi apo filter is almost a given to tidy up the views on brighter objects (roughly a 20-25% reduction). CA will still be dominant when using a filter and often a yellowish hue to the overall views will be evident as a result.

 

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I owned a 6" f/8 Evostar.

I liked it in many respects and it often performed well on a number of objects. We had a very memorable view of the Veil Nebula unfiltered with it. I had primarily bought it for planetary and lunar viewing but found the CA to be a distraction . Also It is a big piece of glass so I would suggest considering a dew band or at least extending your dew shield.

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I really liked my Helios 6" F8 refractor which I bought in 1999. It was a great scope for sweeping up fuzzies and comets, and excelled when it came to galactic clusters, globulars and galaxies. It even gave pleasing views of the Moon and planets, although the CA was definitely evident on bright objects. I was content with the 6" F8 until the 3rd January 2003 when someone donated a 4" Vixen fluorite F9 to my local astronomy club and I saw Saturn's rings in all their intricate glory. The 6" F8 was massively out gunned by the smaller aperture, but higher quality Vixen apo when it came to the Moon and planets. The rest is history!

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