242 replies to this topic

[davidc135]

The lap was a bit too soft so it needed repressing with netting every 5mins. Every few spells the surface was roughed up and refreshed with a brass brush. I checked with a DPAC Ronchi test to see that the surface wasn't losing it's figure too drastically.

 

 

So far so good.

 

David

Edited by davidc135, 26 May 2025 - 02:04 PM.

[davidc135]

A blown up Ronchi image before any polishing showing the roughness effects on the wavefront. My cheap Ronchi grating also contributes to messy images.

 

 

With polishing of R2 more or less done, I'll tackle R1 at the end of the week.

 

David

Edited by davidc135, 27 May 2025 - 01:02 AM.

[davidc135]

R1 turned out to be less straightforward then R2. There was a serious turned edge which I made worse.

 

 

The centre square is a piece of tape to protect against scratches. I'd placed the doublet directly against the flat during testing to save time. There are some sharpie marks.

Edited by davidc135, 02 June 2025 - 05:43 PM.

[davidc135]

With a fine ring lap on the machine, I straightened up the edge leaving a raised zone. 

 

 

Using a craft knife etc as a knife edge is easier and more revealing than a grating at focus. Knife edge from the left.

 

The edge, in shade on the left, is a little turned up whilst the raised zone is roughly a 1/4 wave error, doubled in DPAC.

Edited by davidc135, 02 June 2025 - 11:34 PM.

[davidc135]

The same with a Ronchi test.

 

[davidc135]

At present, the doublet is a fraction under-corrected with some roughness close to part of the edge due to over firm hand polishing, I think. The machine polishing tends to leave record grooves too., mostly cleared away with the last hand polishing.

 

It's close to being finished and I'll wait to see what it looks like when the metal spacers from Jak arrive.

 

 

Virtually all the original cloudiness has been polished away.

 

David

Edited by davidc135, 02 June 2025 - 05:42 PM.

[ETXcetera]

Amazing! Nice work David!

[davidc135]

An outside Ronchi view of #231.

 

 

Polishing both sides of the flint with worked surfaces facing down, to avoid astigmatism, will fractionally make them more concave or less convex. It could be that the colour correction has been altered enough to show up in the above image, even though strokes were very short. I doubt that it will be noticeable in practice, hope not.

 

Any slight wedge in the air space shows as coma.

 

Some of the slight turned edge will be covered by the cell.

 

David

Edited by davidc135, 02 June 2025 - 11:37 PM.

[deSitter]

At present, the doublet is a fraction under-corrected with some roughness close to part of the edge due to over firm hand polishing, I think. The machine polishing tends to leave record grooves too., mostly cleared away with the last hand polishing.

 

It's close to being finished and I'll wait to see what it looks like when the metal spacers from Jak arrive.

 

P6020203.JPG

 

Virtually all the original cloudiness has been polished away.

 

David

 

Wow this is absolutely great work and better presentation.

 

-drl

[davidc135]

Clouds tonight so I set up an artificial star at 27yds. Everything looks reassuring at x100 with no colour error that I can see. Nearby moss lit by a torch was bright and sharp.

 

Inside and out of focus seemed just the same so the important boxes are ticked. After tweaking the collimation to lose some slight astigmatism, the star image showed a trace of trefoil.

 

If tomorrow night is clear, I'll see how it compares on the moon and some stars.

 

All Ronchi and ke test images were taken with an Olympus 4/3 micro, 45mm lens and 1 sec exposure with image stabilisation.

 

David

Edited by davidc135, 04 June 2025 - 04:48 PM.

[kgb]

Hey David. At 27yards, would you be using a 50 micron star or a reflection?

[davidc135]

Hey David. At 27yards, would you be using a 50 micron star or a reflection?

I use a Geoptic 50 micron pinhole.

 

David

Edited by davidc135, 04 June 2025 - 10:56 PM.

[deSitter]

I use a Geoptic 50 micron pinhole.

 

David

 

What about near object induced SA?

 

-drl

[davidc135]

What about near object induced SA?

 

-drl

Yes, I'll need to check against a real star but I think it would be tiny for a 2'' f8 refractor at this distance. Since my mirror diagonal takes up too much optical path, I used a prism diagonal which could be adding a trace of over-correction as against the under-correction due to pinhole distance. Hopefully I'll see tonight.

 

David

[Barry Kawa]

I bought a very early Takahashi FC-100 over the years that eventually developed a similar hazy front objective lens. I took it one time to Starbase Tokyo who sent it back to Takahashi who removed the haze for a cost of about 20,000 yen. Then, the haze gradually returned over a few years so it was cloudy again! I don't think it made a noticeable difference in the sharpness of the image but it probably did and lessened my enjoyment of a fine scope. So, I gave up on it and just sold it off at a tremendous loss and swore off any of these older Takahashi FC scopes. Buyer beware. The owner of the CAT used telescope store here said it's a common problem.

Hopefully, in the States, where there is less humidty and AC available in the hot summer months, that is less a problem. 

Edited by Barry Kawa, 05 June 2025 - 02:01 AM.

[davidc135]

Very little can be seen just looking through the glass.

 

[davidc135]

But back lighting is unforgiving.

 

 

David

[semlin]

zoicks