Skywatcher 100p f/4 vs Takahashi FS60 f/6 Shootout

[Stu]

Sounds good. I still think the moon has got to be worth another shot as it will show CA differences, sharpness etc

[Chris]

1 hour ago, Stu said:

Sounds good. I still think the moon has got to be worth another shot as it will show CA differences, sharpness etc

You're right, it is the best target for showing aberrations and image quality, plus it's easier to find on the small camera chip. I'm just reluctant to cover the same ground twice. 

Maybe if I bought a slightly more powerful Barlow lens to really push the scopes on the Moon?

[FLO]

In theory a Newtonian telescope has zero Chromatic Aberration because it uses mirrors, not lenses. 

Steve

[Moonshane]

I wonder if uncorrected coma could create CA in the other optics when using an f4 newt? For me, although the newt does amazingly well, the Tak images are in a different class in terms of contrast and sharpness (in the finished images at least).

[Chris]

1 hour ago, FLO said:

In theory a Newtonian telescope has zero Chromatic Aberration because it uses mirrors, not lenses. 

Steve

Indeed you would think so, whatever the blue is I don't think it can be CA?

[Chris]

16 minutes ago, Moonshane said:

I wonder if uncorrected coma could create CA in the other optics when using an f4 newt? For me, although the newt does amazingly well, the Tak images are in a different class in terms of contrast and sharpness (in the finished images at least).

You could be onto something here, any fast f4 mirror will fall apart off axis, in other words there could well be a bit of colour separation off axis which appears similar to CA.  The only thing I would say against this is that I'm using a very small sensor which should be 'cropping' out the worst parts of the mirror....I think? 

I posted the 100p Moon images on Facebook, I was that pleased with them, Then the next night I kind of wished I'd held off and posted the Tak pictures instead ?

[Stu]

They both actually show similar CA in terms of red/blue separation around crater edges etc. Is this not just atmospheric? The 100P shows much more, when it should show less.

[Chris]

Stu, thanks for posting, at this scale it is clearer to see.

It could be a combination of fast mirror off axis distortion and the atmosphere. The nights didn't seem that different so I would lean towards f/4 vs f/6 optics being the culprit for showing more aberration on the 100p. Neither night was the best seeing, but the idea around stacking 1000 shots is that you tell Registax to keep the best 70% to get around the bad seeing to some extent and give a sharper final image. 

I do hear what your saying about the atmosphere, we've all looked at a planet low down with a reflector and wondered why there was CA at the top and bottom of the planet! The Moon was fairly high both nights, certainly close to equal in elevation. 

[Chris]

p.s yes, the only way to settle this is a same night test which I will do......round 2!

[Stu]

Yep, seconds out, round 2!

Perhaps try to rule out as many variables as possible.  Centering a distinct crater in both images so you can judge on axis performance as well as possible may simplify things. There are many other factors which could cause differences, cooling, tube currents, larger aperture picking up seeing more, blah blah blah.... but at least by taking at the same time, same altitude you may be able to rule a few things out.

Regardless, it's a great comparison, unknown I have been impressed by my 130P and just how much it will show. I find it harder work and less pleasurable to use that the Tak for instance, but for £50 off Astroboot it does an amazing job.

[Chris]

Just had a thought regarding the colour fringing on the 100p, I was using a Revelation achromatic 2.5x Barlow lens! Maybe I should buy an ED Barlow? Any suggestion for one around 3x? before I re do this test.

[Stu]

1 hour ago, Lockie said:

Just had a thought regarding the colour fringing on the 100p, I was using a Revelation achromatic 2.5x Barlow lens! Maybe I should buy an ED Barlow? Any suggestion for one around 3x? before I re do this test.

That could well contribute to the CA Chris, good thinking. Perhaps you need a x2.5 PowerMate

[Chris]

19 minutes ago, Stu said:

That could well contribute to the CA Chris, good thinking. Perhaps you need a x2.5 PowerMate

lol yeah this could get expensive if I'm not careful! 

Actually I would buy the explore scientific equivalent for less than half the price of the Power mate if it wasn't for that horrible taper on the barrel. It would never work with the twist lock type fitting on the Tak unfortunately.....maybe a power mate will come up second hand