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CA on a Tak FC100DF


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Thank you everyone for the comments. It wasn't something that really bothered me other than the question in my mind of is this normal/expected? The views on the Moon after all my playing around with the Vixen HR 3.4mm were fantastic!

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14 minutes ago, Franklin said:

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.

Great post here from @Franklin 👍

I don't observe Venus against a dark sky if I can help it. Jupiter also looks at it's best with plenty of light still in the sky. 

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On 26/05/2023 at 10:19, John said:

Great post here from @Franklin 👍

I don't observe Venus against a dark sky if I can help it. Jupiter also looks at it's best with plenty of light still in the sky. 

Yep, I always observe Jupiter early doors if I can.

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  • 4 weeks later...
On 25/05/2023 at 19:58, 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. 

Congrats on getting a Tak Neil!

I don't recall seeing any noticeable CA when I owned the 76.

Chris

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  • 7 months later...

Reviving this thread to try and put to bed the little niggle on CA once and for all and before warranty runs out on the Tak. I was wondering if there’s a good way to test the Tak to see if alignment/collimation is as it should be? Is it as simple as a star test or are there other things I could try? I’ve been refining my Tak setup over the past few months and I think it’s almost there now. I’d love to be able to get confirmation that all is well as I think I’ll probably stop noticing it after that 😂

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It's well worthwhile doing a star test and checking that all is well at focus. Out of focus there may well be colour effects.

A 50 micron artificial star 20 m plus away will be more independant of the weather and seeing than a real star. There may be a touch of spherical under-correction.

And it's fun.

David

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I use the cheshire collimator check inside and, if clear, a star test to follow up.

That should pick up any mis-alignment of the focuser or objective.

 

 

 

Edited by John
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28 minutes ago, John said:

I use the cheshire collimator check inside and, if clear, a star test to follow up.

That should pick up any mis-alignment of the focuser or objective.

 

 

 

Same. Doublets are easier to diagnose than triplets with a Cheshire as there are less reflections to see. 

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Thanks all. I have a Cheshire but it’s tricky to see past the crosshairs. Looking past them I see dark green circle and a lighter green circle. Not sure if I need to try and remove the cross hairs to test it properly? IMG_3797.thumb.jpeg.2ebd53cedc5e58ab8fd472c815a2f284.jpeg

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21 minutes ago, Littleguy80 said:

Thanks all. I have a Cheshire but it’s tricky to see past the crosshairs. Looking past them I see dark green circle and a lighter green circle. Not sure if I need to try and remove the cross hairs to test it properly? IMG_3797.thumb.jpeg.2ebd53cedc5e58ab8fd472c815a2f284.jpeg

I use two, one with and one without. The crosshairs allow you to determine if the reflections are central and without gives a clearer view that the reflections are concentric.

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7 minutes ago, Littleguy80 said:

Removed cross hairs. I think it looks ok. Hard to capture in a picture. Not sure if it works better using daylight to light up the Cheshire? 

I use artificial light generally but daylight works the same. Yours looks OK to me. If you star test your diffraction rings at high power, either side of focus, should be concentric around the central airy disk. I reckon yours will be looking at that.

Edited by John
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41 minutes ago, John said:

I use artificial light generally but daylight works the same. Yours looks OK to me. If you star test your diffraction rings at high power, either side of focus, should be concentric around the central airy disk. I reckon yours will be looking at that.

Cool, thanks John. I’ll try that next time it’s clear. Reassuring that it looks ok to you too :) 

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Do you know of anyone reasonably local, perhaps someone in your astro society with an FC100D Neil? May be even another 4" ED brand that you could use alongside your Tak on the same star, under the same conditions; then you can assess performance more easily.  If you aim your scope at Polaris and check the diffraction pattern is concentric, ensuring your scope is cool, you can be confident its collimated. Also, the diffraction rings should be near identical either side of focus. It's unlikely there's an issue with the lens itself as it was produced to a very high spec by Canon Optron. 

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4 minutes ago, mikeDnight said:

Do you know of anyone reasonably local, perhaps someone in your astro society with an FC100D Neil? May be even another 4" ED brand that you could use alongside your Tak on the same star, under the same conditions; then you can assess performance more easily.  If you aim your scope at Polaris and check the diffraction pattern is concentric, ensuring your scope is cool, you can be confident its collimated. Also, the diffraction rings should be near identical either side of focus. It's unlikely there's an issue with the lens itself as it was produced to a very high spec by Canon Optron. 

Thanks Mike. I’ll take it along to the next meet up of my local astro society. There’s normally a good collection of scopes there for comparison. 

Ok, I’ll check the diffraction rings on Polaris. Cooling might be part of the issue. I tend to look at a planet or Moon as first target when coming out so perhaps the scope has cooled sufficiently when I’m noticing the CA. I’ll try to note if it looks better after I’ve been outside for awhile. 

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13 minutes ago, Littleguy80 said:

Thanks Mike. I’ll take it along to the next meet up of my local astro society. There’s normally a good collection of scopes there for comparison. 

Ok, I’ll check the diffraction rings on Polaris. Cooling might be part of the issue. I tend to look at a planet or Moon as first target when coming out so perhaps the scope has cooled sufficiently when I’m noticing the CA. I’ll try to note if it looks better after I’ve been outside for awhile. 

It should only take 15 or 20 mins for the Tak to cool enough for testing.

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Looks good to me as well. I'll bet that the star test is good as well.

I don't see collimation as the possible cause of any CA having seen that. 

Maybe go back to your theory that the diagonal is the cause ?

 

Edited by John
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4 minutes ago, John said:

Looks good to me as well. I'll bet that the star test is good as well.

I don't see collimation as the possible cause of any CA having seen that. 

 

Perfect. Thanks John. I don’t think that leaves many options for an issue with the scope after that. 

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I always use an artificial star for my testing. Pretty much all doublets at F/7.5 and faster will show CA out of focus - even those using fluorite. My FC-100DC had a clear purple ring out of focus. In focus, it was mostly CA-free, but on the brightest objects, a bit of false colour could be seen. This is perfectly normal. My TV85 is the same. I think sometimes “CA-free in focus” gives the impression that no false colour can be provoked even on the most difficult targets, when this is normal for medium fast doublets. Far more important is control of spherical aberration, in my view. If you’re only seeing tiny bits of CA at high powers, I’d ignore it and just enjoy the scope. The only star test I’ve done with no false colour in or out of focus is with a triplet, though maybe slower fluorite doublets might also fit into this category?

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3 minutes ago, Highburymark said:

I always use an artificial star for my testing. Pretty much all doublets at F/7.5 and faster will show CA out of focus - even those using fluorite. My FC-100DC had a clear purple ring out of focus. In focus, it was mostly CA-free, but on the brightest objects, a bit of false colour could be seen. This is perfectly normal. My TV85 is the same. I think sometimes “CA-free in focus” gives the impression that no false colour can be provoked even on the most difficult targets, when this is normal for medium fast doublets. Far more important is control of spherical aberration, in my view. If you’re only seeing tiny bits of CA at high powers, I’d ignore it and just enjoy the scope. The only star test I’ve done with no false colour in or out of focus is with a triplet, though maybe slower fluorite doublets might also fit into this category?

Thanks Mark. The point on focus is actually quite pertinent as I’ve struggled a bit with focus due to a wobbly tripod. I’ve only recently replaced it with a Berlebach Uni 18. Plus I’ve just installed a MEF3 in the last few weeks. Certainly makes me wonder if what I’ve been seeing is just down to focus being slightly off.

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7 hours ago, Littleguy80 said:

 Certainly makes me wonder if what I’ve been seeing is just down to focus being slightly off.

It really is something to be aware of. I know I've to regularly tweak the focus to keep the image sharp, because the object on view is moving across the sky and constantly changing focus due to different thicknesses of atmosphere. I also used an mef3 when I had my DC. Some even go the whole hog and fit a feather touch, but the scope really is worth it!

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  • 2 weeks later...

Last night, I was looking at the Moon with a new to me eyepiece. It was incredibly sharp, I couldn’t spot an CA along the terminator where I was focusing. There was some along the limb (yellow) but that I expect to see unless I’m focusing on the limb. Same eyepiece on the Sun in white light this morning delivering really sharp views. My feeling is that issue I had was the precision of my focusing, or lack of. The stabile tripod and fine focusing with the MEF3 seem to have been the answer. The Tak is seeing a lot more use than the dob these days. Thank you to everyone for helping me work through this. I learned a lot through the process :) 

Edited by Littleguy80
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