CA on a Tak FC100DF

[Littleguy80]

I have a new Tak FC100DF. When looking at Venus there’s very noticeable CA around the limb. I see the same on the moon but it’s much less noticeable. I see this with my Baader Clicklock diagonal and Baader prism diagonal. Tried it with Nagler zoom and Vixen HR 3.4mm. Everyone seems to talk about how these scopes don’t show CA which is making me wonder if there’s a problem with mine. Just wanted to know what other people’s experiences are? I’m either expecting too much or there’s a problem with something I’m using. 

[andrew s]

Prism diagonals can't be CA free. Try without it.  Regards Andrew 

[Victor Boesen]

Another important thing to consider is - are you seeing CA or just atmospheric dispersion? The latter isn't determined by the quality of the optics, and is usually characterized by red/blue colors, whereas CA reveals itself in the form of yellow/purple colors.

Victor

[Littleguy80]

11 minutes ago, andrew s said:

Prism diagonals can't be CA free. Try without it.  Regards Andrew 

I was seeing it with a mirror diagonal too. Planning on trying it straight through tonight to see how that looks

[Littleguy80]

7 minutes ago, Victor Boesen said:

Another important thing to consider is - are you seeing CA or just atmospheric dispersion? The latter isn't determined by the quality of the optics, and is usually characterized by red/blue colors, whereas CA reveals itself in the form of yellow/purple colors.

Victor

Yeah, it was purple so pretty confident it’s CA. 

[Stu]

Just checked with my DC. With a TV Everbrite 2” diagonal and 3 to 6 Nag zoom I would say that it is essentially colour free in focus on Venus. With a Baader T2 Zeiss prism there is some purple CA. It’s still light so I’ll check again later. These are unprocessed smartphone shots from last night, which would show CA much more clearly than with the naked eye. One through the Herschel Wedge, the other through the Everbrite.

[starbob707]

I wonder why anyone would waste their time looking at Venus.  The most boring thing in the sky.  

[Littleguy80]

Well I think it is coming from the diagonals. Top shot is 5mm BGO straight through. Second is nag zoom at 6mm with Clicklock mirror diagonal. 

[Littleguy80]

Looking at the moon with the 5mm BGO it’s barely detectable with the mirror diagonal so looks like the prism is worse for CA as Andrew said. 

[Stu]

51 minutes ago, starbob707 said:

I wonder why anyone would waste their time looking at Venus.  The most boring thing in the sky.  

Perhaps you are not doing it right? Not as much to see as others perhaps but the phases are certainly interesting, and cloud detail is visible with perseverance, albeit some see it more easily than others.

[Piero]

At some point I had both 2" bbhs prism and mirror diagonals.

I sold the prism as it showed CA, whist the mirror did not. The mirror version also cooled down a bit faster. In terms of details, again I prefer the mirror as the views seem more natural.

Lots of members prefer the prism though.. 

[Littleguy80]

4 minutes ago, Piero said:

At some point I had both 2" bbhs prism and mirror diagonals.

I sold the prism as it showed CA, whist the mirror did not. The mirror version also cooled down a bit faster. In terms of details, again I prefer the mirror as the views seem more natural.

Lots of members prefer the prism though.. 

Thanks Piero. I felt sure I’d read somewhere that the Taks do better with prism diagonals. That was one of the reasons I tried the prism. I wonder if getting the BBHS mirror version would remove the CA on Venus. I’m not sure why Venus seemed more prone to CA than even the moon. 

[badhex]

1 hour ago, starbob707 said:

I wonder why anyone would waste their time looking at Venus.  The most boring thing in the sky.  

To each their own, though right?

[Sunshine]

With my TSA-102 I have seen on occasion bluish/red hues around Venus bright stars lower on the horizon and the moon, on other nights nothing at all, I suspect you may be experiencing atmospheric anomalies. If I were to take an average over 10 nights observing I would say that on only two or if lucky three of those nights I see no anomalies. In fact, last night was one of only a handful of times looking at Venus where it was surprisingly clean, absolutely no color but these nights don't come often.

Edited May 25, 2023 by Sunshine

[Mr Spock]

All I see with mine on Venus is the usual atmospheric CA when it gets bright and low. None at all from the scope even at x296 

[Mr Spock]

3 hours ago, starbob707 said:

I wonder why anyone would waste their time looking at Venus.  The most boring thing in the sky.  

With the right equipment Venus shows subtle cloud detail which varies from night to night. Not what I'd call boring at all 

[Sunshine]

3 hours ago, starbob707 said:

I wonder why anyone would waste their time looking at Venus.  The most boring thing in the sky.  

Venus is a challenge, seeing subtle detail requires the right conditions and optics, there are many many objects which can be considered boring until one learns to look at them and be patient.

[Roy Challen]

My FS60-CB shows CA around Venus, as others have said, it's more likely to be atmospheric dispersion, maybe a bit of both. This is with a mirror diagonal. It's actually not much better than my long achro, with the added disadvantage of shorter focal length making high mag viewing more difficult.

Same scope shows virtually colour free image of the moon.

Edited May 26, 2023 by Roy Challen

[Piero]

6 hours ago, Littleguy80 said:

Thanks Piero. I felt sure I’d read somewhere that the Taks do better with prism diagonals. That was one of the reasons I tried the prism. I wonder if getting the BBHS mirror version would remove the CA on Venus. I’m not sure why Venus seemed more prone to CA than even the moon. 

I wouldn't base all my CA tests on Venus. It is a challenging target: it is low (so the atmosphere plays a role in the views), incredibly bright, incredibly white, low contrast and smallish.

The best views of Venus I have had, are still with my TV60 + single polarising filter..

The Tak fc-100 is a doublet refractor. It reduces CA at super low levels, but it cannot fully correct it. I also have some doubts about fixing this using a prism as prisms slightly overcorrect, so whilst "potentially" fixing the blue, the introduced overcorrection would affect the other two colours.

I cannot quantify how much CA you have detected in your tak based on a post here, but I would observe a range of targets to asses this properly.

Said that, my tak FC 100 DF (now FT!) shows a touch of CA on very bright targets, but this is well controlled. I do not have a problem admitting this - perfection does not exist, but it is very often reported as people desire, even ending up believing, that their tool is perfect and can do better than the rest. Similar story about members claiming that their tak 100mm shows views which can be observed with at least 10-12" telescopes.

[Voxish]

I observed Venus two weeks ago at a club event with a 4 inch Tele Vue doublet. There was no observable CA and it was remarked upon by other members for the clean and sharp views it gave compared with the other scopes that night, so.I am wondering about your optics, collamation issues perhaps. I think most of us have unrealistic expectations of what out scopes can offer, and just perhaps a little thin skinned if someone dares point out an issue with a scope owned by a member of a particular brand tribe. I owned Tak, Vixen, SW, TV and Altair refractors over 25 years. The truth is, build quality aside, there wasn't much between them, unless you looked really hard and wanted too. 

Venus is a great target, cloud detail is there on rare occasions. Personally I love watching the phases change over a few months. Still we all have different interests, and that is a good thing. Fuzzy blobs aren't my bag at all, but hey ho...

[Paz]

If you srltraighten out the light cone going through the prism diagonal by putting a barlow in front of it that might reduce some aberrations from the prism.

I always see atmospheric dispersion on Venus. I have an ADC but I don't use it on Venus as for me the added scatter isn't worth the reduced colour dispersion. However I use a lot of dimming amd that means any colour splitting may be dimmed below the threshold of being visible, leaving a cleaner image for viewing.

Edited May 26, 2023 by Paz

[MalcolmM]

28 minutes ago, Paz said:

However I use a lot of dimming

What was recommended to me recently by @ScouseSpaceCadet, and makes a huge difference, was observing when the sky is bright. Similar effect to dimming I suppose. Best view I've had recently was when it was not actually visible with the naked eye! A lot of fun to find though, alternating between binoculars and scope with a wide FOV eyepiece 🙂

I use Tak diagonals and am not aware of excessive (or any) CA. Definitely atmospheric CA at times. Next time I'm observing I'll look out for any CA.

Malcolm

[mikeDnight]

12 hours ago, Littleguy80 said:

I have a new Tak FC100DF. When looking at Venus there’s very noticeable CA around the limb. I see the same on the moon but it’s much less noticeable. I see this with my Baader Clicklock diagonal and Baader prism diagonal. Tried it with Nagler zoom and Vixen HR 3.4mm. Everyone seems to talk about how these scopes don’t show CA which is making me wonder if there’s a problem with mine. Just wanted to know what other people’s experiences are? I’m either expecting too much or there’s a problem with something I’m using. 

 I think your scope is probably fine. A couple of nights ago I observed Venus in the best seeing I'd had for a long time, I was using my FS128, and the limb was sharp and cloud detail as obvious as I've ever seen it. There was also no real CA other than some minor atmospheric dispersion. Last night however, the seeing was abysmal and the whole planet was bathed in colour. I was using a Baader BBHS prism and a Maxbright binoviewer on both occasions, so lots of glass in the light path.  I suspect the issue is due more to local turbulence causing your Image to constantly flit in and out of focus, and possibly the low angle of the planet. Looking through miles of turbulent atmosphere while studying possibly the single most demanding object in the sky is always going to challenge any telescope.

 I used my Vixen 3.4mm HR also, and saw some colour in the view of Venus. The brilliance of the planet is a problem, so you might like to try observing it while its higher in the sky and set against a brighter sky background. Personally I find this more pleasing, especially with regard to atmospheric dispersion and turbulence.  It's also worth noting that some wide angle eyepieces can add their own colour problems.  

 Something else that's worth paying attention to is focus. Venus moves at quite a rate and as it gets lower you see it through a rapidly growing depth of atmosphere. I find I have to refocus two or three times every minute so as to keep the planet as sharp as possible. Coloured filters such as #15 yellow, #21 orange and #80A blue can also help, not only in the revealing of subtle cloud detail, but also in steadying the effects of turbulence.

 So I don't think your scope, or your prism is causing a problem. The DC/DF is a vibrant refractor that can deliver awesome views of the planet's and Moon, but the seeing is so temperamental in the UK that we don't always get the best from our telescopes.

[Franklin]

33 minutes ago, mikeDnight said:

 So I don't think your scope, or your prism is causing a problem. The DC/DF is a vibrant refractor that can deliver awesome views of the planet's and Moon, but the seeing is so temperamental in the UK that we don't always get the best from our telescopes.

Nail on head!

[Franklin]

No one in their right mind would try to observe Venus in a dark sky and most will try at twilight but even then, being so close to the Sun, Venus will not only be very, very bright but also quite low in altitude, not a good combination for high power, high contrast visual astronomy. The best thing to try is to find Venus during the day, when the Sun is still well above the horizon. That way the sky brightness will reduce the glare of bright Venus and the high altitude will help with atmospheric dispersion and unsteady seeing conditions from a low altitude. You don't need fancy setting circles or goto's to do this, as long as you know the general area of the sky where Venus will be it's quite easy to spot. A little trick is, at the end of a nights observing put your lowest power, widest field eyepiece in your scope and focus on the stars, pack up and don't alter the scopes focuser. Next day or next clear opportunity sweep around the general area of the sky where you expect Venus to be and do this when the Sun is still 25deg above the Western horizon at least, be careful NOT to let your scope go near the Sun. An alt-az mount works great because you can start closer to the Sun than you expect Venus to be and just keep panning up and down and AWAY from the Sun till you find Venus. Whilst staring at a bright blue sky and seeing only floaters can at times make your eyes go out of focus and you'll miss Venus when it enters the fov, concentration and practice helps with this and panning low down to a cloud or the horizon for example will help keep your eye focused.

Best views I've ever had of Venus have been in full daylight. (Wratten #21 Orange works well)

Of course, when Venus is a morning object all of this is much easier. Using an equatorial mount, set up on Venus and leave tracking till the Sun is up and then you can observe Venus all day long if you wish.

Edited May 26, 2023 by Franklin