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jimegger
Pooh-Bah
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5701687 - 02/26/13 01:31 PM

Not sure yet whether to coat or not. I am leaning towards getting it coated at a later time, certainly not until I am fully satisfied with its performance.

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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: 9.25 inch refractor project new [Re: jimegger]
      #5701798 - 02/26/13 02:36 PM

Food for thought-
The coater should have a really good track record. Not so much the crown, but the finished flint is susceptible to cold drafts. One bad habit when taking out of the chamber, and you could wind up with pieces.
If you leave it uncoated, tweaking later on is handy.
fwiw,
M.


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mikey cee
Postmaster
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Reged: 01/18/07

Loc: bellevue ne.
Re: 9.25 inch refractor project new [Re: jimegger]
      #5701807 - 02/26/13 02:41 PM

And hope to God it doesn't affect it's figure for what seems to be a non consensus on the pros and cons. That's exactly why I lived for 30 plus years with an uncoated R. E. Brandt 8" f/13.3! Probably why outfits like D&G take forever to make a coated achromat. Too many do overs! Anyways Jim I've enjoyed watching this post. Very informative and entertaining too. Mike

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MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: 9.25 inch refractor project new [Re: jimegger]
      #5701833 - 02/26/13 02:57 PM

Jim, could you post the same picture of that mountaintop taken with your 9.25" and with your APO, at the same scale, for comparison? Thanks.

Mladen


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jimegger
Pooh-Bah
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: MKV]
      #5702002 - 02/26/13 04:27 PM Attachment (89 downloads)

For comparison here are 2 shots of the same mountain scene taken with the APO and the ACHRO. Here is the APO shot first.

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jimegger
Pooh-Bah
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: jimegger]
      #5702004 - 02/26/13 04:28 PM Attachment (81 downloads)

Here is the image taken at the same setting through the same dirty eyepiece at close to the same scale only this time through the 9.25 inch achro.

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jimegger
Pooh-Bah
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: jimegger]
      #5702027 - 02/26/13 04:35 PM

I notice the contrast is much better on the APO. This is probably due to the fact that the sun is shining into the maw of the larger 9.25 inch and causing glare on the still not yet blackened lens cell. The sun shade on the 127 Apo is much better protected that way. To be a better test as such it would be best to wait until the air is more stable and the sun not a factor. The sun angle is very low still here in Alaska and is only about 20 degrees away from where this shot is being made.

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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: jimegger]
      #5702847 - 02/27/13 02:43 AM

I managed to get the scope on a really undersized mount I have just for a look at Jupiter tonight. I also set up the 127 APO for comparison. At the same power the APO gave a more crisp image but not much more than the big scope. The lens needs some more work but is very close as the stars were points and I did not notice any color around Jupiter that stood right out. I was pleasantly surprised about the lack of violet. Now all of this is being done before the double pass auto collimation test with the green LED. This will be done soon and these visual tests are a way of gauging improvement after the auto-collimation test and correction.

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MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: 9.25 inch refractor project new [Re: jimegger]
      #5702897 - 02/27/13 04:57 AM

Thanks Jim. The glare might be a factor for the difference. Very telling, indeed. I am sure all of us will be looking for your update on the green laser DPAC test results, and perhaps comparative shots of the mountaintops when you blacken the tube inside. Also, I think a true test of color correction is best made against a bright star. A simple achromat can only do so much.

Thanks for sharing your project with all of us, Jim.

Mladen


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jimegger
Pooh-Bah
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: MKV]
      #5705297 - 02/28/13 12:52 PM

I looked at the auto-collimation green LED test yesterday and got an objective that shows small amounts of spherical aberration such as would be demonstrated by one that has a longer center focus than edge focus giving a look similar to a mirror showing a parabolic shape. Now the trick is to find which surface or surfaces need the attention. I suspect it is the R4 surface as it was not redone during this session. The plan now is to test only the crown and see what it looks like first and do any touch ups needed there. If that needs no work then it is onto R4. I'll try to get a focal gram and post it here.
All in all the lens is not too far off but from my experience with the 6 inch objective I made that little bit makes all the difference in the world when it comes to crisp images !


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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: 9.25 inch refractor project new [Re: jimegger]
      #5705599 - 02/28/13 03:41 PM

Jim, it seems to me that if the center focus is longer than the edge the wavefront is oblate and undercorrected.

Mladen

Edited by MKV (02/28/13 03:41 PM)


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: MKV]
      #5705779 - 02/28/13 05:22 PM

That is what I get from the test results. The shadows look like a paraboloid mirror but on the lens it would amount to an oblate spheroid. So I am back to doing some more polishing now on the crown as it tested out to have that particular shape. I am using some blending strokes in an effort to bring the center up to a sphere.

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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: jimegger]
      #5707675 - 03/01/13 07:29 PM

The old laps for the crown were just not getting the job done so I made 2 more laps from the Gugolz 64 pitch with no additive beeswax this time. From what has been happening it appears that the pitch needs to flow better in order to facilitate making the lens surfaces into the sphere they need to be. Also the fresh pitch should hold the Cerium Oxide better than the waxy surface on the old laps. I am just finishing up on the R2 lap. Polishing will commence tomorrow or possibly later on today and it will be interesting to see the difference if there is any between how these new laps do as compared to the old ones.

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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: 9.25 inch refractor project new [Re: jimegger]
      #5708275 - 03/02/13 07:45 AM

Just me, I find invariably that a good hard 73 will make beautiful spheres.
I fiddled around with the old OSLO design of your achro, and found that shortening R3 by just .015" pushed the green line from the supposed long focus center to about a perfect null @ 586nm. Focus for the green line moved a total of around .006"~. Seems that you could deepen the curve on that flint with less than an hour of polishing, perhaps. How long did the center focus?
M.


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jimegger
Pooh-Bah
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5711062 - 03/03/13 05:32 PM

I have not measured the focus difference for the various wavelengths. I did do a star test on the objective last night and found it to be under-corrected the crown being an oblate spheroid. It seems that the figure is improving with the new pitch laps but it will take some time to get it done with a regular lap so I am planning to make a convexing lap and work in conjunction with the regular laps to get the crown surfaces to a sphere. It seems my extra hard laps with the 1/3 stroke was not able to produce a spherical surface - at least the way I did it. The convexing tool will help speed up the process.
Color looks excellent and the inside focus image as well as the extra focal image showed well blended colors in the proper proportions for a doublet refractor design !


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MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: 9.25 inch refractor project [Re: jimegger]
      #5711846 - 03/04/13 01:25 AM

Quote:

I did do a star test on the objective last night and found it to be under-corrected the crown being an oblate spheroid.



Jim, just curious: why star test the scope when you already did a double-pass autocollimation test, which is more sensitive than a single-pass the star test, and found the wavefront undercorrected?

I don't recall you mentioning it but you should be able to determine if your concave surfaces are truly spherical by the Foucault knife-edge test at their ROCs. Since R2 and R3 are the same, you can then test your R2 against R3 by contact interference method and figure R2 until you get straight fringes. Then you can be sure R2 is also spherical.

Once you know R2 through R4 are spherical then any residual spherical aberration comes from R1.

If this is the case, and you already know R2-R4 are spherical, and you are getting an undercorrected wavefront (i.e. central focus longer than edge focus) then R1 is oblate, (its center is too flat relative to the edge). Either way, 1/3 stroke should bring it to a sphere. Just work on it until you get a clean null using a knife-edge and your autocollimation flat.

Mladen

Edited by MKV (03/04/13 01:54 AM)


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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: 9.25 inch refractor project [Re: MKV]
      #5712074 - 03/04/13 08:33 AM

Flat center on a convex:
= a short stroke which whacks the center lower than the rest of the lens-
Or you are standing in one position while doing the "W"s for too long a time.
*****
In short, the overall wear pattern is simply not random enough. Generally this comes from being overcautious. The shorter radius on a convex's edge has the same analogy as a concave with TDE. It has the same cause.
(this is what I've found out with my meager efforts!)
fwiw,
M.


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kfrederick
Carpal Tunnel


Reged: 02/01/08

Re: 9.25 inch refractor project [Re: Mark Harry]
      #5712125 - 03/04/13 09:08 AM

Jim are the inter surfaces matching RC ? Looks like you are getting close . Did it focus the stars nice? Be a fun ATM project Having the zemax help [Mike ]made your telescope much better . I like the idea of a folded refractor mounted like a dob with the lens at the top /flat in the bottom.Mounted like John Halls 30inch .A 12inch might not be too bad wonder how much the glass is for big lens? I used the 30 inch refractor at the Allegheny Observatory once . Makes me wonder how big they could now AS the 30 was made over 100years back . Congrads Jim on your work

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jimegger
Pooh-Bah
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project [Re: kfrederick]
      #5713756 - 03/05/13 01:42 AM

Ok , here is the skinny on the lens so far. Mladen , I did the star test to double check that the flat I am using is giving me the correct information. Yes Kevin , the flat appears to be doing its job correctly as the star test validates the results from the flat. The star test also gives me an idea how far off the lens actually is as well as a good test for astigmatism. The lens looks excellent for both color and astigmatism but undercorrected enough that I believe the crown will need considerable more polishing. I believe my probably fast short strokes were the cause of the flat center so BINGO Mark! I am now taking longer strokes with my regular lap with a narrow w motion to try and bring the center up. I will give it a couple of hours if need be doing that but if it seems to be taking too long then I'll go with the convexing lap to speed things up. Ive used one before and it worked great. The lens seems to be improving with every polishing session now so it is a matter of time. Now that I know the flat is doing its job correctly the auto collimation tests will continue as will a couple of star tests to see the real results.
I really do not want to use the flint as a test plate for the crown for fear of scratching and I am not sure if they are an EXACT radius match. The sphereometer says they are but that is only accurate down to 1/10,000 ths of an inch. My flints concave is a sphere now by Foucault testing. The crown on the auto collimation test shows the under correction quite clearly. It is polish..... polish .... polish for now....


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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: 9.25 inch refractor project [Re: jimegger]
      #5713864 - 03/05/13 04:58 AM

Jim, you can test R2 on R3 if you use thin shims to avoid scratching. The two do not have to be in physical contact. If R2 is also oblate, you may actually ruin the lens by working only on R1 and assuming that R2 is spherical.

As for precision, that depends on the calibration of your spherometer, to include the precision of the feet separations as well, and not just the dial indicator's stated accuracy. Every spherometer has an inherent error built into it. In other words when you read a sagitta of say 0.0505 it doesn't mean it's 0.0505, but .0505±XXXX, where XXXX is your internal error that has to be calculated.

None of this should prevent you from obtaining interference fringes and determining if the R2 surface is spherical.

You can read more on this test method from this source.


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