DavidinFL
member
Reged: 08/28/09
Posts: 52
Loc: Navarre, FL
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Hello,
Never being satisfied with what I have, I decided to build a Foucault tester and use it on the mirror from my XT8. It's marked with a center spot and obviously has dust on it. I built a simple slit-less tester with a LED.
I've attached some pics that I took holding the camera behind the knife edge. They're a little blurry due to my hand shaking but I hope they are usable. I'm just getting into analyzing Foucalt images and am not sure what I'm seeing in the pics. There seems to be a region in the middle that always seems to stand out in the pictures. I'm not sure about what that means. If someone(s) could write a little bit about the pictures I would appreciate it.
Thanks!
David
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Orion XT8 - Orion StarMax 102 EQ
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Quote:
Hello,
Never being satisfied with what I have, I decided to build a Foucault tester and use it on the mirror from my XT8. It's marked with a center spot and obviously has dust on it. I built a simple slit-less tester with a LED.
I've attached some pics that I took holding the camera behind the knife edge. They're a little blurry due to my hand shaking but I hope they are usable. I'm just getting into analyzing Foucalt images and am not sure what I'm seeing in the pics. There seems to be a region in the middle that always seems to stand out in the pictures. I'm not sure about what that means. If someone(s) could write a little bit about the pictures I would appreciate it.
Thanks!
David
David..welcome.
The center *Thing* is the depression of the parabaloidal figure of your mirror.
The center is deeper then the edges and thus permits all the light striking the mirror to focus pretty much in the same spot.
The shadow gram indicates you have a relatively smooth mirror(short of the mega dust).
There are far more experienced mirror makers on these pages then myself...but sufice to say...your shadow grams are pretty good(you did a good job) taking them. One thing I do see is that your figure is a bit abrupt...meaning the transition from the outer zones to the inner zones is harsh. Kind of an edge is present between them.
Hopefully some smart people will pipe up and dismiss/correct my poor observations.
Soooper job. 
Rob
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JohnH
professor emeritus
Reged: 10/04/05
Posts: 734
Loc: vancouver near the wilds of B...
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Very nice photograph.
I would concur with the above opinion that the transition from inner to outer zones is abrupt, almost to the point of a raised zone from 15% to 30% radius, most visible in upper right photograph
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
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I'm not seeing a diffraction ring... could there be a bad edge? Here's a pic of my mirror to show diffraction ring.
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Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
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 DRAACO
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DavidinFL
member
Reged: 08/28/09
Posts: 52
Loc: Navarre, FL
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Thanks for the positive comments! Hmm, is there anything negative about the center area abruptly changing like that? I noticed, as I was trying to move the KE back and forth from the mirror, that the area in the middle tends to always be shown. Sometimes it is very distinct and sometimes only slightly seen, but always present. I also don't seem to be able to view the outer annulus of the mirror without the center area being shown.
Biff, if I understand correctly the diffraction ring is the ring at the outside edge of the mirror that is lit entirely around the circumference? If so, then I see that when I look through the viewer but am not able to get it to correctly come out in the pictures. My mirror has a beveled edge and this is the area that I think is uniformly lit during my viewing. Is this what I should see?
I tried increasing the brightness of the LED to get a faster shutter speed. This did help when taking pictures but it was hard to get the tester in the correct position due to it being so bright. I have attached a picture from this "technique" that is in focus.
Another reason I am looking at my mirror is that I have never been able to get sharp images with my 6mm TMB Planetary (200x mag) eyepiece. For example when viewing Jupiter, it always seems fuzzy even though I have been trying for a couple years. I've collimated the scope several times but it didn't seem to help. My 9mm TMB planetary produces sharp images though. Being in the southeast we don't have the most steady air but I would hope I could push the magnification past 200x occasionally.
Once again, thanks for your help! I welcome any further comments.
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Orion XT8 - Orion StarMax 102 EQ
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Welcome to CN!
You have some artifacts in the center of the mirror, for the most part a ring but also a strange swirl, likely due to fast machine figuring. These mirrors are punched out kind of like doughnuts, judging from the examples I've seen - that swirl is ONE turn of a spindle with a small lap adding roughness as it goes - so this is a beautiful illustration of how these mirrors are actually made.
All the way out to 1/3 radius this is obvious, beyond that to about 1/2 radius it has less effect, but I see an undercorrected (high) area there with overcorrected (low) area taking over inside it and the rough transition. The edge appears fairly decent, but typically the center on mirrors like this is overcorrected and the outer zones are undercorrected. If you add them together you get approximately the right overall correction for about 1/5-1/8th wave PVW. This is a very very typical way to make cheaper mirrors, BTW, as maintaining proper figure out to the edge is a lot of work, and so is doing the the center right.
What is negative about a transition like this? The light energy doesn't end up in quite the right place, compared to a smooth paraboloid. Star testing would show differences inside and outside focus. OTOH defects that are rather glaringly obvious in Foucault can have relatively minor effects on performance, but in this case you're verifying a problem - it runs out of oomph before it gets to 200X... Then it's likely the edge is flat as well. Have you star tested it?
Best,
Mark
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DavidinFL
member
Reged: 08/28/09
Posts: 52
Loc: Navarre, FL
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Mark,
I haven't star tested it yet but have been looking into it since reading your reply. Once it clears up here and I get my scope back together I'll give it a try. Is Polaris an acceptable star to keep it from running out of the FOV too fast?
Thanks,
David
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Orion XT8 - Orion StarMax 102 EQ
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Polaris works for me and a lot of people. 
Best,
Mark
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DavidinFL
member
Reged: 08/28/09
Posts: 52
Loc: Navarre, FL
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The clouds finally moved away and even though it was turbulent I was able to conduct a star test. Here is a drawing of what I saw. The rings were all circular and concentric unlike the rings in the drawings (my inability to draw circles). I noted a bright, dark, bright rings from outside to in on the inside focus. On the outside focus I noted a bright ring, next to a not quite as bring ring then a darker core. On the outside focus image these two bright rings would merge if I went closer to focus.
Hopefully these will help in diagnosing my mirror.
Thanks,
David
PS. Ignore the outside line around the drawings it was just there as a guide and was not something I saw.
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Orion XT8 - Orion StarMax 102 EQ
Edited by DavidinFL (09/24/09 01:38 AM)
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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It kind of looks like spherical aberration.
Below is a simulated star image with about 1/2 wave spherical aberration so if you see something like more light being in the outer ring on one side of focus and more light in the center on the other side; this is classic spherical aberration.
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DavidinFL
member
Reged: 08/28/09
Posts: 52
Loc: Navarre, FL
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I think with spherical abberration there is a bright outside ring on one side of the focus and a bright inside ring on the other side. Thats what I see in your picture at least.
When I was testing I saw two bright rings separated by a dark region with a faint ring in it when I was inside of focus. When I was outside of focus I only saw one bright ring.
I found this chart online from a description of a book called "Star Testing Astronomical Telescopes." http://www.willbell.com/TM/IMAGES/StarTest1.jpg
To me, it looks like my drawing most closely resembles the "Zones" error though I'm not exactly sure what "Zones" is.
Thanks,
David
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Orion XT8 - Orion StarMax 102 EQ
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Since you made a focuault tester, you could simply make a couder mask and measure your mirror to get a quantiative idea of what is going on.
But as Mark alluded to, the foucault test can tell you the mirror "1/4 wave" P-V but still be a "bad" mirror. e.g. light not being where it is supposed to be in the diffraction rings.
What you may want to try is making a Ronchi screen and star testing with that. It will probably give you a better "qualitative" analysis.
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DavidinFL
member
Reged: 08/28/09
Posts: 52
Loc: Navarre, FL
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Ok, I'll give that a try. I didn't realize you could make a Ronchi screen, but I was able to find some links on doing it. I know what I'll be doing today.
Thanks,
David
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Orion XT8 - Orion StarMax 102 EQ
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Printing one a on good laser printer is probably the easiest way
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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The error is zones, when I have some time I'll try to simulate it through FigureXP based on the Foucault pics you showed. It's consistent with the pics.
But for me the star nest is a null - if I don't see any problems I'm happy, but if I do see problems I diagnose them some other way. 
Best,
Mark
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DavidinFL
member
Reged: 08/28/09
Posts: 52
Loc: Navarre, FL
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Well, I fashioned a screen out of fishing line like I saw in one of the links. I ended up with about 100 lp/inch. I attempted a star test with the Ronchi screen and the lines all looked straight to me. I could definately tell a difference inside and outside of focus with the regular star test. I saw no difference with the Ronchi star test. Perhaps my screen is too coarse or somthing else.
Mark, if you can manage to reproduce the images in FigureXP can you (or anyone else) give me an opinion of the mirror. I guess saying how "bad" the mirror is wouldn't be a good way to put it, but maybe the limitations of the mirror? I don't want to worry about a small error in the mirror if that is what it is but I'm not sure how big the error is.
I guess in the end, any opinions or reccodmendations would be appreciated.
Thanks,
David
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Orion XT8 - Orion StarMax 102 EQ
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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It takes a finer Ronchi grating than that - more like 300 lpi, unfortunately.
Mark
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Relativist
Post Laureate
Reged: 10/11/03
Posts: 3081
Loc: OC, CA, USA
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Need something like this?
http://stellafane.org/tm/atm/test/images/atm_ronchi_screen.gif
You could scale it appropriately.
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DavidinFL
member
Reged: 08/28/09
Posts: 52
Loc: Navarre, FL
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Hmm...might have to make use of the laser printer at work. :-D
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Orion XT8 - Orion StarMax 102 EQ
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Here's a rough simulation - I had to go away from a well corrected mirror to about 1/4 wave PVW to get this much deviation - in the simulator I gave it a flat center (undercorrected) and slight undercorrection overall, with the edge zones flatter than ideal and a sharp jump where the Foucault image shows that steep transition. Without measurements I wouldn't be able to get much closer, but to my eye the agreement with the star test and the bench pic (assuming 8" f/6 with a 2" secondary obstruction) is pretty close.
Best,
Mark
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