Fast achros are useless for Lunar work

[raymo]

102mm f/5 achro. EP projection-f/l =1500mm. 1/160th sec @ ISO 1600.
Heavily cropped. Image is better if you click on paper clip.
raymo

[Camelopardalis]

Of course they are...look at all the colour fringing ;)

Nice one raymo :thumbsup:

[raymo]

I only just got this old blue tube SW 102mm f/5, and thought I'd check it out on the moon. There was a light haze over the entire sky, so I'm hoping for
some better results at a later date.
raymo

[Nikolas]

have you made the pic greyscale or is it still colour?

[raymo]

Actually Nik, I don't know why I bothered posting it, the moon was shrouded in haze, so I was never going to get the best result. It is greyscaled, because
strangely, it developed a yellow CA tint over the upper part of the image
when I downsized it for posting here. The full size image shows no trace of it.
raymo

[sheeny]

Have you tried a filter? Narrow your bandwidth and fringing disappears...

Al.

[raymo]

Firstly, I've realised why the yellow appeared in the downsized image. It was
already there, but has been concentrated by the offending pixels being
closer together in the downsized image.
I have been looking into various filters in another thread here Al, and will
be experimenting shortly. No filter removes all the fringing, so it's a
matter of trying to get the one that does the best job for the least money.
raymo

[x935418]

Hi raymo
my previous scope was SW 102mm f/5 achro

try use a 62mm stop down, it is better then any filter

[raymo]

Yes, I realise that Rahim, but it rather defeats the purpose of having 102mm of aperture, might as well buy a 60 or 80mm f/10 or 12 in the first place.
raymo

[x935418]

Yes
But i understand in cheap scopes, we can use all of aperture
just for low power (DSO) not for high power (lunar and planets)

In high power, outer part of aperture is useless.
Now i have SW200P and now i use 140mm stop down for high power.
Of course in reflectors, there is no CA but shape of mirror is not
ideal, and it is better to eliminate outer part of aperture.

I think just in high price scopes (near ideal optic) you can
use all of aperture in any situation (high and low power)

[Nikolas]

Still a good photo though considering the conditions

[raymo]

You are talking about two different problems with refractors and reflectors
Rahim. With refractors it is nothing to do with the outer part of the lens
being unusable at high powers. All of the lens produces CA. Stopping it down increases the focal ratio, reducing the CA. If you stop it down far enough, to say, f/15, the CA will essentially disappear. All fast achromatic refractors, regardless of quality will exhibit CA.
Stopping down a reflector has the same effect, namely increasing the focal ratio. All fast Newtonian reflectors, regardless of quality, suffer from coma near the outer edge of the field of view. This doesn't matter to many people for visual use, but it does matter for imaging. Like CA, coma decreases as the focal ratio increases, so an f/8 reflector exhibits far less coma than an f/5 one will. Unless you can't stand a bit of coma, stopping down a reflector for visual use is ridiculous. Stopping down a 200mm to 140mm HUGELY reduces its light grasp and resolution. It's all the more ridiculous when you can use a coma corrector with your reflector, and get rid of almost all of the coma.
raymo

[x935418]

I Agree !





1-light grasp:
reducing light grasp is not important in high power
as i don't use it (stop mask) for DSO
for DSO i use all of aperture and it works great.

2- resolution:
My resolution increase as i stop it down !
I remember somebody in net told: if you stop down a scope
and see increasing in resolution, certainly there is a problem in scope !


you may say, it is because bad collimation , or there is termal issue
but after one year collimation and install various fans, there is
no improvement on image quality for high power.

But stopping down, amazingly solve problem.
Did you think that coma can effect in visual use and is
responsible for image softness ?

i think SW mirror quality is not so high, as i can
see some overcorrection on start test.

Can you get high magnification (200X or higher) on your SW200mmF5 ?

I can not .

[raymo]

With any optical system the quality of the image always deteriorates as
magnification is increased, not too noticeable at lower powers, but very
noticeable as you approach the maximum power that the optics, and/or/
the observing conditions allow.
Firstly, SW mirrors are arguably among the best mass produced mirrors available, although it is always possible that you have a lemon.
Resolution is a tricky one, are you talking about resolution or sharpness?
The larger the aperture of a scope, the more vulnerable it is to
atmospheric disruption of the image, so that at times a smaller scope will give a superior image to a larger one, which is possibly what is happening when you stop yours down. The theoreticall resolution of a scope is purely a function of its aperture. In perfect viewing conditions a larger scope will
always out resolve a smaller one.
You will also find that focusing is easier as you stop it down. It can be
quite fiddly with really fast scopes like f/4 or faster.
Coma has no effect on the image other than the observable distortion of
the stars in the outer portion of the F.O.V.
A bit of over correction should have little negative impact on the image.
Conditions are pretty poor where I live, so I rarely go over about 250x,
but very rarely I have been able to go over 300x.
raymo

[x935418]

preparing an answer for tomorrow ...
please wait !