DIY 7 inch Cassegrain telescope - 4, Figuring finished.

[Chanlunlun]

Dear ATMs,

There is very little mentioning about how to figure the hyperboloid secondary mirror of a Classical Cassegrain telescope. If you try googling on the web, you only end up with very superficial description about the topic, the true how-to knowledge is just not appearing.

May be that the only source of related material that is in depth enough for one to successfully making a Cassegrain secondary is from Jean Texereau’s book – but the method employed needs a master concave surface of exactly the same radius of curvature together with an interference test. The equipment and technique involved is way outside the hands of an average ATM, and especially outside the capablity of me.

WARNING - Below discussion is rather technical and for non-ATMs it may seem rather boring and even confusing. But for those who want to build a Cassegrain ( I doubt if there are any ) the following paragraphs may be found useful.

Calculation shows that the hyperboloid surface required in this particular 7 inch Cassegrain differs from the BEST FIT SPHERE of about 0.4 wave. And this greatest departure occurs at the 0.7 radius zone of the useful mirror surface.

The figuring process thus involved removing this tiny surplus amount of glass away exactly at the required zone. To achieve this we must first calculate the radius of this zone ( easily done on a computer by OSLO light ) and draw on the back side of the mirror. A very tiny pitch lap was then made, the diameter of the lap must be exactly at 30% of the useful mirror diameter.

Using this tiny lap on the required zone QUICKLY ( in molecular scale ) removed glass away. After a few rounds of figuring the whole telescope was then taken to star test ( the target star is actually a chandelier about 350 m away ) and the process was repeated until star test showed good enough result. Because of the extreme big f/ratio the star test was very sensitive but as usual I only aimed at 0.85 Strehl ratio ( 0.25 wave wavefront error, just pass the Rayleigh criteria ) and stopped there. This left an overcorrection in star test but the final image was very sharp, sharp enough to charm an average visual observer.

The actual glass touching lasted no longer than 10 minutes but the whole figuring process took several days to finish, due to the fact that a rather lengthy effort was spent on the star test.

Now the figuring has been finished and this is the beginning of the end. The next will be the machining and adjusting the baffle tube – the baffle tube looks simple but it require very troublesome machining, adjusting and, most of all, the internal wall treatment.

Best regards
Chan Yuk Lun
30 April 2012

[BossKwan]

It is an art of finger exercise, I can't imagine, for me, how I could hold this tiny pitch lap with my stupid fingers to finish a good figured secondary. These are all dedicated works and need much much patience....

Dear Mirror God,
Is it really a binoculars objective lens? (Seems truth shown on your photo)
As I know, the back surface of an usual bino-lens is flat surface, is your a special one with curve surface?

[Chanlunlun]

It is an art of finger exercise, I can't imagine, for me, how I could hold this tiny pitch lap with my stupid fingers to finish a good figured secondary. These are all dedicated works and need much much patience....

Yes it requires a feather touch with a slow motion plus steady applying presure. With practice this is possible

Is it really a binoculars objective lens? (Seems truth shown on your photo)
As I know, the back surface of an usual bino-lens is flat surface, is your a special one with curve surface?

May be you have overlooked the back side of all common binocular objective are convex. The curvature is, however, quite small so that one may think that the surface is flat at the first glance. But they are convex.

Most of the binocular objective have too strong a curvature to be useful. However I find this objective lens from a Meade 8 X 50 finder, just at the right curvature to be a Cassegrain secondary

[Eddy So]

[蔥寶恭喜樣] [蔥寶恭喜樣] [蔥寶恭喜樣]

[BossKwan]

May be you have overlooked the back side of all common binocular objective are convex. The curvature is, however, quite small so that one may think that the surface is flat at the first glance. But they are convex.

Most of the binocular objective have too strong a curvature to be useful. However I find this objective lens from a Meade 8 X 50 finder, just at the right curvature to be a Cassegrain secondary

Seemingly I overlook this.. I should also have some spare bino-objective lenses in my tool box and let me examine them someday....your good idea aroused my interest in this. Indeed and at least, using an available lens saves lots of working hours for grinding a secondary from a flat glass. However, the following procedure "figuring secondary" is yet a challenge for me...

[Chanlunlun]

May be you have overlooked the back side of all common binocular objective are convex. The curvature is, however, quite small so that one may think that the surface is flat at the first glance. But they are convex.

Most of the binocular objective have too strong a curvature to be useful. However I find this objective lens from a Meade 8 X 50 finder, just at the right curvature to be a Cassegrain secondary

Seemingly I overlook this.. I should also have some spare bino-objective lenses in my tool box and let me examine them someday....your good idea aroused my interest in this. Indeed and at least, using an available lens saves lots of working hours for grinding a secondary from a flat glass. However, the following procedure "figuring secondary" is yet a challenge for me...

The figuring is not difficult. Yet the making of a tiny lap and the star test is energy consuming.

[anyone]

鏡神，新鏡預計幾時開光？

[Chanlunlun]

鏡神，新鏡預計幾時開光？

Actually this small Casesegrain is finished. All we need to wait is the silvering of the secondary mirror and clear sky.

It's a pity that this telescope is finished just after the opposition of Mars.

[anyone]

鏡神，你將會自己動手做 silivering 嗎？

[Chanlunlun]

鏡神，你將會自己動手做 silivering 嗎？

Yes. The process will be reported here. Actually silvering is a rather easy job.

We did silver the 30 inches mirror by ourselves before.

[PTS]

鏡神，新鏡預計幾時開光？

Actually this small Casesegrain is finished. All we need to wait is the silvering of the secondary mirror and clear sky.

It's a pity that this telescope is finished just after the opposition of Mars.

Why not make some observation of the Sun before silvering.


pts

[Chanlunlun]

鏡神，新鏡預計幾時開光？

Actually this small Casesegrain is finished. All we need to wait is the silvering of the secondary mirror and clear sky.

It's a pity that this telescope is finished just after the opposition of Mars.

Why not make some observation of the Sun before silvering.


pts

Setting up an equatorial mount under the sun is a little bit too demanding for me.

[BossKwan]

鏡神，新鏡預計幾時開光？

Actually this small Casesegrain is finished. All we need to wait is the silvering of the secondary mirror and clear sky.

It's a pity that this telescope is finished just after the opposition of Mars.

There is never any pity, we still have other beautiful planets to observe, like Saturn, with its greater and greater opening rings, that's good enough to enjoy testing your new DIY cassegrain.

By the way, I am quite interested in watching the process of silvering a 30 inch mirror. I was just thinking, how an amazing moment when you were holding up such a huge size mirror, shining your eyes with flash silver surface. Did you feel so excited that moment?
I also wonder what size of a tank you used for the silvering process, a glass or plastic tank, or made a temporary small sized swinning pool? Did you record a short video clip?
It is worth putting on YouTube for sharing...

[Chanlunlun]

鏡神，新鏡預計幾時開光？

Actually this small Casesegrain is finished. All we need to wait is the silvering of the secondary mirror and clear sky.

It's a pity that this telescope is finished just after the opposition of Mars.

I also wonder what size of a tank you used for the silvering process, a glass or plastic tank, or made a temporary small sized swinning pool? .....

Dear Boss Kwan,

There was no tank large enough to hold the 30 inch mirror. Instead we built a dam around the mirror and the mirror itself is the "tank" for reactions to take place.

Of course for the very secondary mirror we are discussing we only need a CDROM container to be a tank.

Best regards
Chan Yuk Lun