Just wondering if anyone has used one and if so, how much CA did you experience?
I've seen one fairly cheap brand new (about half price) and was thinking about it, but I don't know if the CA would annoy me, or if I can get a filter to mask it a bit.
Any thoughts on the matter would be great!
I've never really used a refractor before as I've been a reflector person all the way. The last time I looked through one was in 1995!
f5 or f8?
There are some reviews here: http://www.cloudynights.com/page/art...mm-refractors/
The general consensus seems to be that the f5 has a fair bit of CA, and so a number of people use a minus-violet filter for planetary viewing with these.
I think you will find that Skywatcher, Synta, Celestron, etc. are all re-badged from the same factory.
The CA gets worse with aperture for a fixed f-number. I have not looked through one of these scopes but on paper it should have lots of CA. A naive paperless analysis tells me that you need to increase f-number in proportion to aperture. So eg a 75mm f/4 would have the same CA as a 150mm f/8, all else being equal.
You can tame CA with filters and in even more clever ways (but filters are the easiest and cheapest off-the-shelf solution). If you can easily afford the scope and you're curious to try it out then get it. You'll have fun with it and you'll learn stuff.
Tolerance to CA is a personal thing. Depending on what you are observing there will be times when you don't see any at all such as lower powers and deep sky. It will be worst on very bright objects like Venus, Jupiter, Sirius and the moon.
I used to have a Jaegers 6" f5 and it was wonderful for deep sky observing and comets.
There was a famous amateur astronomer by the name of Leslie Peltier. He wrote a wonderful book called Starlight Nights. He was a dedicated observer of variable stars and discovered many comets. His primary instrument was a 6" f8 refractor.
The CA gets worse with aperture for a fixed f-number. I have not looked through one of these scopes but on paper it should have lots of CA. A naive paperless analysis tells me that you need to increase f-number in proportion to aperture. So eg a 75mm f/4 would have the same CA as a 150mm f/8, all else being equal.
You can tame CA with filters and in even more clever ways (but filters are the easiest and cheapest off-the-shelf solution). If you can easily afford the scope and you're curious to try it out then get it. You'll have fun with it and you'll learn stuff.
One of the other simple ways to tame CA on bright objects is to use an aperture stop. If the scope is stopped down to 120mm aperture then it becomes an f10 scope and if stopped down to 100mm it becomes an f12 scope. CA will be significantly reduced and the scope will still gather sufficient light from bright objects to give a well illuminated image. Combine this with a filter as mentioned and you will be quite surprised. Some cardboard, glue and some packaging tape is all you need to make some effective aperture stops to test the idea. have made a set myself and they work a treat.
Whether you use an aperture stop or use the yellow/ green filter to filter out the defocussed halo of blu/violet light at the green focus - you are simply throwing light away.
I've owned one of the achros for a number of years . I find the highest magnification I can use for deep sky where the image looks acceptable is about 18mm . Higher magnification than that and you cant escape the stars all looking like slightly fuzzy yellow balls . At the same time the view is pretty nice with 32 to 24mm eyepiece and large and bright clusters and nebulae can look good.
I think its just a case of not having too high expectations and being occasionally pleasantly surprised . Saturn being fairly yellow and monochromatic can look pretty good at high power but medium to high power on the Moon is really marred by secondary spectrum.
As previous poster said everyones sensitivity to chromatic aberration varies , but I guess as a Newtonian user for years I don't adjust easily to it . I think its well worth getting hold of a second hand 100 to 120 mmm ED , than a 150 mm achro.
For $595 new I would think it's a bargain, though I could be wrong.
I think it is the same scope: made by Synta.
Looking around they seem to be selling (new) for well over $1000, and with f8 you won't have as much trouble with CA, so I would think that price is pretty good. (For that price I personally would be very tempted, but I think my wife might notice: it is a big telescope!) I suspect that you will love the views at low to medium power compared to a reflector, because the star images are so much nicer, and you should get really nice contrast as well. It should be great for nebulae and clusters. (I have seen some comments that it more than holds its own against 8" reflectors for DSO's, but is not so hot on the moon or planets at high power because of the CA.)
So it is unlikely you will really need a filter or resort to reducing the aperture unless you are going really high power on bright objects like Jupiter and get annoyed with the colour fringes.
By the way, for what it is worth I have corresponded frequently with a guy in Germany who is a bit of a guru on optics, especially binoculars. His "go-to" scope is a 150mm f5 Synta, and he is very happy with it for low and medium power fields (although he does use Swarovski and Zeiss eyepieces that tend to negate a lot of the CA).
Good luck, and let us know if you get it and your "first light" impressions.
(I have seen some comments that it more than holds its own against 8" reflectors for DSO's, but is not so hot on the moon or planets at high power because of the CA.)
I think it really boils down to your experience level and how casual an observer you are . I own an 8" F4 Newt which gets a lot more use than the 6" F8 Achro - I can use any focal length eyepiece and it is sharp and colour free ( with a low cost coma corrector fitted ).
I have had a chance once to look through the 6" F5 varaint and found the CA to be completely intolerable - the view were bordering on psychedelic.
Those achros I 'm afraid are one trick ponies - limited to low power view if you want the impression of chromatism and clarity.
FYI I owned a Celestron branded of the 6" Synta which sold here for $400 which was the going rate after a lot of people bought them when Andrews first stocked them and dumped them after a few years . $600 is probably a fair price for a tube as I think they are around $1200 new now.
This is all of course just my opinion as an owner - I know that peoples imporessions of scope performance and tolerance of CA blur can vary wildly.
Good price Tim for a lot of scope .
I have the Saxon version and it is a great performer on all objects , shows the planets very well up to around 300x on a good night and the moon is spectacular !!! .
It also performs very well on deep sky showing almost as mush as a friends 8inch Meade SCT when set up side by side , I like these scopes ,,,,,, BUT , you need a good mount as they are LONG and HEAVY so I recon an HEQ5 as minium . I do have a friend with the light blue SW version on a Vixen Super Polaris mount and tall stainless tripod that is OK for visual .
Also any Minus Violet filter works well at taming CA on brighter objects like Jupiter and Venus , I find the moon pretty CA free without one fitted , but you eyes might be different as has been mentioned by anj025 and Mark .
Here is mine on its HEQ5 and HD wooden tripod , a nice solid set up that performs brilliantly .
Brian.
ps. the white rings on the OTA are white nylon ( 4mmx25mm ) wrapped and riveted to the tube so I can leave the rings loose and rotate the OTA without it slipping downwards and losing balance , this works surprisingly well .
I'm not as sense to CA, I fond the f8 version acceptable, a light yellow filter #8 helps too. Even the f5 I'm not bothered by, but like mark said looking at something like venus is psychadelic
I use mine with a Baader FringeKiller and find the CA just acceptable, though it took me a while to adjust to it after using a newt. all my life. A few people who have looked through it were pleasantly surprised, probably because they didn't expect much.
In the longer term, the biggest issue I found is the focusser. At first it worked OK but it is not possible to get a perfect focus because you inevitably shake the tube while focussing (this is on an EQ5). So I installed an 'after market' motor focusser. That was OK for a few years but eventually the slop in tube guides was so great that the image shifted significantly every time the focusser changed direction. Now I have a motorised Moonlite focusser and it's a different scope. I reckon just being able to get the focus spot on gains perhaps about 0.5 magnitudes (though I'm a poor judge).
Yes I hear you on the focuser ( pretty carppy ) but the money is in the optics and that's where these shine when collumated and cooled , when I got a Bintel 10-1 Crayford for my ED80 the first thing I did was replace the R&P focuser on the 6 inch Saxon with the single speed ED80 Crayford , fitted straight in and made a huge difference to focusing .
Brian.
Quote:
Originally Posted by AstralTraveller
I use mine with a Baader FringeKiller and find the CA just acceptable, though it took me a while to adjust to it after using a newt. all my life. A few people who have looked through it were pleasantly surprised, probably because they didn't expect much.
In the longer term, the biggest issue I found is the focusser. At first it worked OK but it is not possible to get a perfect focus because you inevitably shake the tube while focussing (this is on an EQ5). So I installed an 'after market' motor focusser. That was OK for a few years but eventually the slop in tube guides was so great that the image shifted significantly every time the focusser changed direction. Now I have a motorised Moonlite focusser and it's a different scope. I reckon just being able to get the focus spot on gains perhaps about 0.5 magnitudes (though I'm a poor judge).
I had the f5 version , I really enjoyed it yeah the ca was bad on bright things , but on most objects , Orion and dimmer things there fine
Would I have another yes
For $595 new I would think it's a bargain, though I could be wrong.
I've use a Celestron 150mm F8. It was a good scope and I regret selling it now. I'm contemplating getting the Skywatcher version. The C.A. is not really an issue and can be reduced further with a fringe killer. They make a great visual RFT. As mentioned else where, mount is, and always is, King.
I loved mine, I regret selling it. Here is a single shot image I took with it.
I spent a lot of time adjusting the air gap on the primary to reduce spherical aberration. http://cdn.astrobin.com/images/thumb..._watermark.jpg
I loved mine, I regret selling it. Here is a single shot image I took with it.
I spent a lot of time adjusting the air gap on the primary to reduce spherical aberration. http://cdn.astrobin.com/images/thumb..._watermark.jpg
I've had a Skywatcher 150mm F8 for a couple of years and really like it. I've also used a Newtonian, a Mak Newt, a Ritchey Cretian and a small apo and of these the Skywatcher is my current favorite scope for visual. I recommend upgrading the the focus mechanism as it's is pretty ordinary, but the original didn't stop me having some great nights of planet viewing.
The CA is controlled quite well with a fringe killer, although if you're used to a reflector then you'll still notice it on Jupiter and the brighter stars. I personally like to view stars without a filter. The fringe killer takes away a lot of the colour variation between them and objects like the Pleidaes can look quite pretty with the 'enhanced' blue from the CA.
Others below reckon that they're only for low power viewing but I think it depends more on the seeing and the quality of your eyepieces. Three of my best visual nights have been with this scope. I regularly use a 4mm William Optics UWAN or an 8mm Radian with a Tele Vue 2x barlow. These two eyepieces (at 300x) are good about half the time (given that I only get the scope out when the seeing is good). I've had one night recently, viewing Mars and Saturn near opposition, where 4mm was clearly not the maximum capable power. I tried the 2x barlow with the 4mm and although it was too much power, it wasn't far off. I got the most detail that night from a Long Pern 3mm, which I don't rate as a great eyepiece.
I've seen craters within craters on the moon, polar caps and other details on Mars, four distinct bands and the Great Red Spot on Jupiter, and I can regularly see cloud bands and the Cassini Division on Saturn. There are heaps of better scopes, of course. But for the money and ease of use, I definitely enjoy mine.
Good price Tim for a lot of scope .
Yes I hear you on the focuser ( pretty carppy ) but the money is in the optics and that's where these shine when collumated and cooled , when I got a Bintel 10-1 Crayford for my ED80 the first thing I did was replace the R&P focuser on the 6 inch Saxon with the single speed ED80 Crayford , fitted straight in and made a huge difference to focusing .
