Tonight's crescent

[Cyclops]

Ridiculously windy out there but I had to have a go! First at 44x, good clear view with a little CA.

Then I tried adding the Barlow and first I put it between the scope and the diagonal. Couldn't focus, no infinity focus. I guess the scopes tube is too short. So I put it in the other way, scope, diagonal, barlow, 9mm kellner. Certainly bigger but lots of CA and the image is softer. I think I need a better ep...

Anyway I went back to 44x and got a photo with my phone hovering over the ep-not very easy at the best of times!

 

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[vlaiv]

Apart from focus issues - barlow magnification varies with distance, so putting it in front of diagonal is going to give you much larger magnification (often not usable).

If I remember correctly you have fast achromatic refractor? CA is just a fact of life with such scopes, even slow ones need to be F/10 - F/15 (depending on diameter of the lens, even up to F/25) to be almost free of CA. It probably has nothing to do with eyepiece.

[Cyclops]

8 minutes ago, vlaiv said:

 

If I remember correctly you have fast achromatic refractor? CA is just a fact of life with such scopes, even slow ones need to be F/10 - F/15 (depending on diameter of the lens, even up to F/25) to be almost free of CA. It probably has nothing to do with eyepiece.

Its specs are 400/70

[vlaiv]

There is something you can do to lessen CA to some extent. Here is a good guide of what one can expect in terms of CA:

There are two "standards" in CA levels that can be calculated from objective diameter in inches and focal ratio of telescope.

As is your telescope has this number at 2.07, so below both of these standards, but still in "filterable" region. This means that you would need to invest in minus violet filter, or fringe killer or even simple #8 yellow wratten filter.

But there is other way around this with no investment needed (just a bit of free time and black cardboard will do to try it out). You need to fashion so called aperture stop / aperture mask to improve this ratio and reduce CA.

Given that you have 9mm EP and x2 barlow to give you about x90 magnification, suitable aperture mask would be one of 50mm (or even 45mm - you don't want to stop your scope below half of magnification you are after).

If you make 2" aperture mask (and maybe you don't have to make it at all - some scopes come with already made hole of 2" diameter in their lens cap - check your lens cap you might already have suitable hole) this would raise you CA index to:

(400 / 50 ) / 2" = F8 / 2" = 4

This would place your CA index right in between two standards (3 and 5) - which means that there will still be some CA but it will not be bothersome at all.

There are some drawbacks to this - image will be dimmer, and you won't be able to use higher magnifications with aperture mask (higher than x2 diameter of aperture mask in mm), but since you have fast achromat, higher powers simply won't work because of CA in the first place - fast achromat is not suitable scope for high power.

Anyway, you should give this a try. If you want to make one, it's just a bit of black cardboard and some glue, let me see if I can find DIY instructions for it. Have a look here:

https://10minuteastronomy.wordpress.com/2017/02/11/why-and-how-to-make-a-sub-aperture-mask-for-a-refractor/

Or maybe this one:

Just make sure your aperture hole is centered and round.

[Cyclops]

Thanks vlaiv, my lens cap is in two parts, I wondered why it has a central hole with cap! I have used it in daylight with the middle hole removed with no loss of light. It would be fine for lunar observing I guess! 

Now, I'm not sure where you got the number 2.07 from. That confused me. If I read that CA chart correctly I see the number 1.09 under mine (70mm/3), in the red!

[Cyclops]

My  2 part lenscap

 

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[vlaiv]

Oh excellent - just the right size - Synta scopes seem to have such smaller hole on most scopes (Skywatcher, Orion, Celestron, ...)

To calculate CA index you need two numbers and you need to divide them.

First one is F/ratio of scope - this in it self is ratio, so you again need to divide two values, but it's often written on scope as well - like F/5 or similar. In your case you need to divide focal length with aperture

so 400 / 70 = ~F/5.7 (5.71428 .... to be precise)

Other one is diameter of lens in inches - that one is easy 70 / 25.4 = 2.756"

If you divide F/ratio with diameter in inches you will get CA index so, 5.7143 / 2.756 = 2.0734..., or ~2.07

When you use aperture mask - same thing again, except this time you will not use 400 and 70 - specs of your scope, but rather 400 and 50 (or 2" which is 50.4, don't know exact diameter of hole in lens cap but it should be about that).

So F/ratio will be ~F/8 (400/50) and aperture is about 2", hence when you divide those two F/8 / 2" = ~4 CA index - much better, as you should be able to tell when observing moon - just pop your lens cap on (with central plug removed) - and you should instantly see two thins - image getting a bit dimmer (not such issue on moon) and CA going away

 

[Cyclops]

Thanks again vlaiv. I'll try next time we have a clear night (peeing down again now. I'm glad I went out when I did!)