Difference between an APO scope and an achromat

Post by nF » Sat Apr 10, 2004 11:18 am

Below posts are moved from Samuel's thread on his RX80-L Refractor to this thread due to off-topic. Please do not go offtopic the next time or else post will be deleted. Think before you post in the thread.

Post by nF » Sat Apr 10, 2004 11:19 am

ChaosKnight
If i'm not wrong, high quality achromats, apos and those with diffraction limited optics all have a mechanism to collimate the optics, even though they are refractors. And for a similar telescope, ie a low cost 4" achromat and a 4" apo, the apo will have a tube with thicker walls.

Zong, you seem to be a more experienced telescope user than me. Care to confirm the above observations?

Post by nF » Sat Apr 10, 2004 11:22 am

weixing
I think most high quality refractor will not provide any mechanism for user to collimate the optics - the manufacturer will not want to take the risk for the user to collimate themself. Also, as long as the telescope has less than 1/4 wavefront errors, it is consider to be diffraction limited, so I think most of the decent astronomy telescope out there will meet this requirements.

Have a nice day.

_________________
Yang Weixing

Post by nF » Sat Apr 10, 2004 11:23 am

ChaosKnight
There will always be a way to collimate the lenses, no? If such a mechanism is not available to the user, it must be available to the manufacturer. If you are telling me high-end refractors have their lenses popped in and screwed into their holding cells (like my C102) without collimation of any kind, i find it very unlikely.

Post by nF » Sat Apr 10, 2004 11:23 am

weixing
Hi,
If this is your point... then I think every scope in world can be collimated even your C102...

Have a nice day.

_________________
Yang Weixing

Post by nF » Sat Apr 10, 2004 11:24 am

ChaosKnight
My point is we must match the quality of the tube to the optics if we want to get the most out of it.

My C102 is a low-end scope. As long as the lens cell at the end of the tube is machined to enough precision, pop in the lens and it works fine.

But imagine you are using diffraction limited optics. You want to machine a tube for it. Machining tolerances can easily reach +-0.025mm. You may think this is very good, but if you consider the tolerance of your lens (+- 100nm??), you realize errors when machining will totally overshadow your diffraction-limited capabilities, rendering it useless.

In the end, the tube can only serve to hold your lenses, but not position them accurately. This may be the reason why collimation in a high quality refractor is needed to bring out the best the optics can offer.

Coming back to PVC Vs Aluminium, the issue concerning tubes now is if flexing will also overshadow your diffraction limited capabilities. We can't produce a tube that eliminates the need for collimation, but we can easily reduce flexing.

i did a little research, and the results are quite astonishing. Assuming you have a refractor with the following specs:

fl = 1040mm
aperture = 128mm
tube od = 145mm
tube id = 140mm
lens mass = focuser+eyepiece mass = 500g (conservative estimate)
length of tube at each end outside mounting rings = 0.3m (another conservative estimate)

Assuming also you have ultra sensitive eyes that can see light from 400nm to 700nm.

If you used aluminium, the error at the wavefront reaching the eyepiece is ~0.0547 that of error afforded by your diffraction limited lens. Means if the error of your lens is (1/4)(400nm), the error contributed by tube flex is only 0.0547 of that.

If you used PVC (assuming one of the stiffer grades), the error at the wavefront caused by tube flex is about 18 times that afforded by your lens. Your diffraction limited capabilities are wasted.

Post by nF » Sat Apr 10, 2004 11:26 am

zong
Ohmigod. I'm flattered! Ow, sorry to disappoint you ChaosKnight, but I derived all my "telescope hours" so far from either my school scope or Samuel's scope (thanks Sam!). I think there are many other more experienced people here. I only read more, so I learn more in theory.

I roughly understand what you mean I think. I can trust your initial calculations about the flexing caused by aluminium. But the error caused by the flex in PVC is 18 times that of aluminium? I doubt it, it's way too crazy a number. We need to also take into account the payload of each material per unit length too, see? And in my opinion, even normal PVC has a high payload. So I don't think we need to specially look for aluminium.

Also, I found out that, if by your previous explanation that the wave reaches the receiving eyepiece at a wrong angle, this is not a problem at all. By our normal focusing, we can easily correct this error, because defocused objects are really caused by light reaching the eyepiece at the wrong angle! I wonder why I missed that point out! So silly of me.

About the point on collimation, there is no real need to collimate apo refractors and any scopes like it. The coating itself already corrects the errors. I think in the past such refractors had other different lenses inside it(between the lens and the eyepiece), that's why collimation was needed. But now I don't suppose there is collimation in refractors. Besides, that's exactly why people like refractors (no maintenance needed) right? 8)

Yay, this thread is gettin' more and more exciting, with discussions come alive! I like that But overall, I think no discussions on "theory" can win over a practical session with the scope itself, and the scope has shown that it is "QC passed".

ChaosKnight · Post by **ChaosKnight** » Sat Apr 10, 2004 4:32 pm

I don't think the replies are off-topic. They are a discussion on the choice of materials on making optical tubes and how they affect optics performance.

But then again......the original thread was supposed to be Samuel's moment. Sorry ar, Samuel, for hijacking the thread.

ChaosKnight · Post by **ChaosKnight** » Sat Apr 10, 2004 4:48 pm

Zong, i'm afraid i don't agree with your views. Error in the incident ray on the eyepiece lens is caused by misalignment of the optics. Focusing doesn't correct for misalignment of optics.

i never had the opportunity to use any apos, but i think the coatings and lens elements are designed to bring light of drastically different wavelengths to the same focal point. When installing them in the tube, they still need to be aligned with reference to the focuser.

Post by **weixing** » Sat Apr 10, 2004 5:57 pm

Hi,
Since the topic had been change to Difference between an APO scope and an achromat, I would like to ask the question:
What are the advantages of an apochromatic(APO) refractor over an achromatic refractor??

We all know that the main advantage of an APO refractor over an achromatic refractor is the very small or no chromatic aberration. But does anyone know what other advantages does an APO refractor have over an achromatic refractor ???

Have a nice day.