Jump to content


Photo

star test inside of focus

  • Please log in to reply
60 replies to this topic

#26 pstarr

pstarr

    Mercury-Atlas

  • *****
  • Posts: 2937
  • Joined: 17 Sep 2004
  • Loc: NE Ohio

Posted 29 September 2013 - 08:38 PM

The link works on my computer and iPad. You may have to join the Zambuto mirror group to view it.

#27 Tristan

Tristan

    Ranger 4

  • -----
  • Posts: 311
  • Joined: 07 Jun 2011
  • Loc: NC

Posted 29 September 2013 - 09:09 PM

OK, I finally got in to view it. Does make me wonder...I did not see a description of what was going on, but the image entitled tricolor is showing a similar purplish hue to what I've seen on my mirror from an angle at the edges. The hue is not visible straight on, but is visible at an angle. I believe the coatings that were applied are standard and not enhanced. Thanks again for posting the link.

#28 Tristan

Tristan

    Ranger 4

  • -----
  • Posts: 311
  • Joined: 07 Jun 2011
  • Loc: NC

Posted 29 September 2013 - 09:15 PM

You know, thinking about it now, I wonder if the cell is to blame...the cell is the standard GSO 9-point cell. It seems to me I could test this by inserting a rigid plate of some kind between the mirror and the cell and see if the test improves. The idea being if the load is transferred uniformly to the plate instead of to the points of the mirror cell it would be better supported...maybe this is not the case, but as I think about it starts to make a little sense. If the cell isnt providing adequate edge support, I think it could cause the kind of results I am seeing. Does anyone else have any experience with a mirror cell causing issues with a mirror's figure?

#29 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7793
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 29 September 2013 - 11:35 PM

you can set up your own foucault test, not to take measurements, just to see the surface.

also mask off the outer 1/2" and see if your star test changes.

both simple tests.

#30 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10363
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 29 September 2013 - 11:48 PM

I've got enhanced coatings in my 8" and its as perfect as when it was standard coatings. There isn't an issue here at all.

Pete

#31 AlBoning

AlBoning

    Viking 1

  • -----
  • Posts: 592
  • Joined: 06 Mar 2011

Posted 30 September 2013 - 12:11 AM

I have no idea if doing this has any diagnostic value but it is interesting.

I created an aperture mask for my AD8 that converted it from being an obstructed 197 mm f/6.2 newt to an unobstructed 60 mm f/20 newt. The star test required using a much brighter star and it took a couple additional turns of the fine focus knob to get the same amount of defocus. The star test was stunning, near perfect, near identical sets of diffraction rings on both sides of focus. A refractor would have been envious with green CA.

#32 pstarr

pstarr

    Mercury-Atlas

  • *****
  • Posts: 2937
  • Joined: 17 Sep 2004
  • Loc: NE Ohio

Posted 30 September 2013 - 06:38 AM

I've got enhanced coatings in my 8" and its as perfect as when it was standard coatings. There isn't an issue here at all.

Pete


Enhanced coatings are fine. I'm sure yours is. They are just more inclined to produce a clunker every great once in awhile. They have more layers then a standard coating.

#33 Starman81

Starman81

    Surveyor 1

  • *****
  • Moderators
  • Posts: 1992
  • Joined: 06 Mar 2008
  • Loc: Metro Detroit, MI, USA

Posted 30 September 2013 - 10:45 AM

OK, I finally got in to view it. Does make me wonder...I did not see a description of what was going on, but the image entitled tricolor is showing a similar purplish hue to what I've seen on my mirror from an angle at the edges. The hue is not visible straight on, but is visible at an angle. I believe the coatings that were applied are standard and not enhanced. Thanks again for posting the link.


You know, thinking about it now, I wonder if the cell is to blame...the cell is the standard GSO 9-point cell. It seems to me I could test this by inserting a rigid plate of some kind between the mirror and the cell and see if the test improves. The idea being if the load is transferred uniformly to the plate instead of to the points of the mirror cell it would be better supported...maybe this is not the case, but as I think about it starts to make a little sense. If the cell isnt providing adequate edge support, I think it could cause the kind of results I am seeing. Does anyone else have any experience with a mirror cell causing issues with a mirror's figure?


Chiming in before the experts do here... The tricolor is an artifact of coating that I think happens with Majestic Coatings coated mirrors (and maybe some others). I am guessing yours was coated by Majestic. AFAIK, it is only visible at very shallow angles and doesn't have any impact on optical performance.

On the second point, I would think the 9-point cell is far superior than just a 1-point back plate like you describe. I am new to PLOP but I ran it with your scope/mirror combo (assuming 1.5" mirror thickness and 2.1" secondary) and the 9-point came out looking MUCH better than the 3-point cell (no option for a 1-point cell, which would do much worse I think than the 3-point).

#34 Tristan

Tristan

    Ranger 4

  • -----
  • Posts: 311
  • Joined: 07 Jun 2011
  • Loc: NC

Posted 30 September 2013 - 05:04 PM

Thanks again everyone. So, based on the responses I am getting if the figure appears good in Foucault, and the mirror cell isn't the culprit, and the coatings are assumed to be OK, the are most of you thinking this is an issue related to seeing more than anything else?

-Tristan

#35 Mirzam

Mirzam

    Skylab

  • *****
  • Posts: 4447
  • Joined: 01 Apr 2008
  • Loc: Lovettsville, VA

Posted 30 September 2013 - 05:59 PM

I think seeing is the most likely explanation, although to be clear, I cannot assess the correction by eyeballing the foucault and the ronchi images. It looks about right but you really need a quantitative measurement (or an autocollimator) to be sure. Still Gordon Waite knows what he is doing and I think your mirror is probably fine.

Problems with the cell typically would cause astigmatism, not an apparent error in correction or turned down edge.

JimC

#36 Tristan

Tristan

    Ranger 4

  • -----
  • Posts: 311
  • Joined: 07 Jun 2011
  • Loc: NC

Posted 30 September 2013 - 06:47 PM

Thanks Starman. The coatings were from Majestic. As for the mirror, I guess I was thinking if the mirror was uniformly supported and in continuous contact it would be better than having individual points of stress...then again if that were really true we would probably see aluminum platters with heatsinks cast right in to dissipate the heat from the mirror and help it equilibrate faster.

-Tristan

#37 Tristan

Tristan

    Ranger 4

  • -----
  • Posts: 311
  • Joined: 07 Jun 2011
  • Loc: NC

Posted 30 September 2013 - 06:48 PM

As an update- the scope has been out cooling in the shade since about 6:30 with the fan running full blast. Seeing is supposed to be better tonight so I will give it another shot and see what happens. I'll report on my findings a little later- crossing my fingers its just seeing or a boundary layer issue.

#38 Tristan

Tristan

    Ranger 4

  • -----
  • Posts: 311
  • Joined: 07 Jun 2011
  • Loc: NC

Posted 30 September 2013 - 09:18 PM

OK, so I just wrapped up some more observing. Seeing was about good. I did make another change to the scope though for tonight's session- I added some foam weather stripping over the temperature probe for the thermometer on the mirror so the fan was not blowing on it directly. the night started out without a promising outlook, as the inside test continued to look spikey and MUCH more turbulent than outside of focus where rings were well defined and clear with little turbulence. Towards the 9:30 the mirror was showing within 0.2-degrees F of the outside air temperature and occasionally I would get glimpses of some rings. I started thinking if there was a boundary layer trapped in the middle of the mirror what could I do to disrupt it? I found the answer, a can of compressed air. I shot some air down the tube from the front and watched the star test spiral about with the air currents, as it settled the inside focus patter began to clear, and resemble more of the outside focus pattern. I did it a few more times and things continued to improve. the rings were still not as bright or distinct as outside focus, but they were definitely there, and not nearly as spikey as previous or as turbulent. I am really starting to think this is a boundary layer issue coupled with seeing.

I think I am going to create a circular baffle out of black foam core board and located it just above the mirror clips so the air that would be moving up the side of the tubes is instead directed across the front and see if that improves the images farther...If seeing permits I will attempt this coming weekend and report my results.

Thanks again everyone for helping me to troubleshoot this issue, I really appreciate it!

-Tristan

#39 Mike Spooner

Mike Spooner

    Vendor (mirrors)

  • -----
  • Vendors
  • Posts: 147
  • Joined: 06 Aug 2010

Posted 30 September 2013 - 10:01 PM

How tight is the tube around the mirror? I once had a 6" mirror in a 7" tube and it looked like it had a turned edge until I pointed it straight up so the cool air would fall evenly on the mirror (and the heat could escape). I would point it at a star at 45 degrees and in less than a minute or two the star test would look like TDE (soft spiky inside and sharp outside). I put it in an 8" tube and rarely have a problem pushing 500x in good seeing. I suspect it was a boundary layer or stratification along the tube when it was used at an angle and the larger tube was enough to keep it from impinging the kight path.
--Mike Spooner

#40 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10363
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 30 September 2013 - 10:28 PM

I've got enhanced coatings in my 8" and its as perfect as when it was standard coatings. There isn't an issue here at all.

Pete


Enhanced coatings are fine. I'm sure yours is. They are just more inclined to produce a clunker every great once in awhile. They have more layers then a standard coating.


If imagine the larger thin mirrors might be at risk here. I've read dielectric coatings actually squeeze the glass like a vice hence the reason only small pieces of glass tolerate it well like diagonals.

Pete

#41 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10363
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 30 September 2013 - 10:48 PM

OK, so I just wrapped up some more observing. Seeing was about good. I did make another change to the scope though for tonight's session- I added some foam weather stripping over the temperature probe for the thermometer on the mirror so the fan was not blowing on it directly. the night started out without a promising outlook, as the inside test continued to look spikey and MUCH more turbulent than outside of focus where rings were well defined and clear with little turbulence. Towards the 9:30 the mirror was showing within 0.2-degrees F of the outside air temperature and occasionally I would get glimpses of some rings. I started thinking if there was a boundary layer trapped in the middle of the mirror what could I do to disrupt it? I found the answer, a can of compressed air. I shot some air down the tube from the front and watched the star test spiral about with the air currents, as it settled the inside focus patter began to clear, and resemble more of the outside focus pattern. I did it a few more times and things continued to improve. the rings were still not as bright or distinct as outside focus, but they were definitely there, and not nearly as spikey as previous or as turbulent. I am really starting to think this is a boundary layer issue coupled with seeing.

I think I am going to create a circular baffle out of black foam core board and located it just above the mirror clips so the air that would be moving up the side of the tubes is instead directed across the front and see if that improves the images farther...If seeing permits I will attempt this coming weekend and report my results.

Thanks again everyone for helping me to troubleshoot this issue, I really appreciate it!

-Tristan


Ahhh ok I dealt with this demon with my 8" f/9. The majority of the thermal work is handled by rear blowing fan but to complete the thorough equilibrium I also use a side blowing fan. The rear is a 5" fan the side is 3" or 80mm. Both have their own boxy 6 volt battery that seem to run forever.

Here's a hot hot tip...

Make sure your side blowing fan intercepts the side of the mirror. If its an 80mm or 100mm fan have about a half inch to a full inch of the fan aperture below the mirrors reflecting surface. That's right it's blowing on the side of the glass. What's key here is the blade wash is forced onto the boundary layer and it destroys it. It has to directly engage that air blanket.

What I did wrong the first time: I mounted my side fan so the bottom of the blades were just barely even with the mirrors reflecting surface. After all with the airstream that close it'll suck that boundary clean and blow it hither and yon.

It did nothing.

It was literally a zero result. When I dropped it down about .60 of an inch below the mirrors surface level - it was incredible. Yes there's that wasteful air that just blows on the side - but its a non issue the other air is doing the job in style.

Hot hot tip no. 2 - if you install a side blowing fan you MUST have exhaust ports opposite the airflow. It has to exit immediately. Without the exhaust ports (holes) the warm boundary air merely flows up the tube an across the light path. Without the exhaust ports the fan is just redistributing the heat. You ned to get rid of it in the shortest distance possible.

Another hot hot tip: I originally had both fans off one big 6v battery but they ran too slow. Both are rated for twelve bolts but the 6 volt battery each is perfect RPMs. I tried 12 volts - its nutty -way too high.

Final thoughts : a boundary layer is best for the lunar, planetary or doublestar observer whose bent on the last ounce of efficiency. I've found the rear fan about halves the flare and bloating while the side fan running at the same time halves that half. A deep sky observer who happens to like medium to low power could probably pass on the boundary fan but the observer if the formentioned will love it.

And finally : I learned in Connecticut a boundary fan is seasonal. In the colder months (when you can see your breath) it is terrific. The gains are good. In the heat of summer , particularly if its stored without an AC the rear fan is all that's needed and the boundary layer can be virtually invisible like it never existed. Alas the glass isn't 40 degrees or more different than the outside air so that rear fan has easy work. I imagine midway into October will necessitate the boundary fan again.

Good luck.

Pete

#42 Tristan

Tristan

    Ranger 4

  • -----
  • Posts: 311
  • Joined: 07 Jun 2011
  • Loc: NC

Posted 01 October 2013 - 04:43 AM

Hi Mike. I'll be honest, I was a bit surprised to find this was an issue as the tube is about 14" and provides close to an inch of clearance around each side of the mirror. But based on my observations that is very likely what is happening. Someday I'd like to convert this scope to a strut or truss type dob, but I think for now I'm going to try and additional fan or baffle and see if that helps the thermal issues inside the tube.

-Tristan

#43 Tristan

Tristan

    Ranger 4

  • -----
  • Posts: 311
  • Joined: 07 Jun 2011
  • Loc: NC

Posted 01 October 2013 - 04:48 AM

Thanks for all the suggestions Pete. I am going to try the baffle first, and if I have to will put another fan on the side with some exhaust ports. Im just not too eager to drill holes in the side of tube. I've read and heard that for some people it has worked great, others it has made no difference. However, your point about positioning of the side fan is well taken. If the baffle doesn't do the job I will definitely resort to a side mounted fan and exhaust port on the side. Out of curiosity, how do you seal up the exhaust holes when not in use? The reason I ask is because my scope lives in the garage and I like to keep the tube sealed up as much as possible to prevent dust and insects from the making their way to the mirror. Thanks.

-Tristan

#44 AlBoning

AlBoning

    Viking 1

  • -----
  • Posts: 592
  • Joined: 06 Mar 2011

Posted 02 October 2013 - 05:43 AM

On any given night of observing I almost always do at least one star test if only to check the state of collimation. Usually late in the evening to ensure that both the scope and the sky have had a chance to steady down. On those occasions when I'm serious about actually trying to quantify some aberration I make up a cheat sheet with Aberattor and bring it out to the scope so I can reference it while observing the star test.

I've attached my Aberrator Cheat Sheet for spherical aberration (SA) in a newtonian. The first pair of diffraction patterns in the upper left is a perfect mirror and the amount of undercorrection increases by 0.05 wave in each subsequent example. Specifically this is for an 8" f/6 mirror, with a CO of 25%, at 240X, using 10 wavelengths of defocus, and the SA ranges from 0.0 to 0.25 wave in 0.05 steps.

Aberrator can be downloaded from http://aberrator.astronomy.net/

Attached Files



#45 Scanning4Comets

Scanning4Comets

    Markus

  • *****
  • Posts: 13776
  • Joined: 26 Dec 2004
  • Loc: Canada

Posted 03 October 2013 - 01:22 AM

Overcorrection.

My star test always shows:

Outside of focus: Bright outer ring, with inner rings well defined and evenly spaced.

Inside of focus: fuzzy outer ring with hairy look to it and ill defined rings, but evenly spaced. I have made a 1/4" mask for the outer part of the mirror which did nothing.

I get super crisp views of planets and pinpoint stars when the seeing allows. I use a fan on the back of my mirror which has three speeds and leave it on at all times. My scope is well collimated and flocked / modded. I stopped star testing a long time ago, except when I wan to see how my collimation is. Enjoy your mirror and have fun with your scope!

#46 Mirzam

Mirzam

    Skylab

  • *****
  • Posts: 4447
  • Joined: 01 Apr 2008
  • Loc: Lovettsville, VA

Posted 03 October 2013 - 06:50 AM

Last night I was working very close to the resolution limit of my 10" f/6.5 scope while splitting a 0.5" separation double star at ~450x. Seeing was quite good and the rear fan was running. Every few moments the star would lock in and show the barest thin black line between the components. The diffraction pattern during these moments was very clean. Looking at the star test the pattern was similar on either side of focus, however, the in-focus side was a bit fuzzier and seemed to be less stable. The bottom line is that I know the scope is a good one, having made and tested the mirrors myself. The performance certainly bears out the expectations from the testing. And yet, the star test, because of its extreme sensitivity to seeing, does not always indicate a perfect optical system. Remember that when you star test you are looking for that momentary stability, just like when you are observing some critical small detail under variable seeing.

I agree that when a scope is giving excellent views, don't worry about what the star test looks like.

JimC

#47 pstarr

pstarr

    Mercury-Atlas

  • *****
  • Posts: 2937
  • Joined: 17 Sep 2004
  • Loc: NE Ohio

Posted 03 October 2013 - 07:19 AM

Last night I was working very close to the resolution limit of my 10" f/6.5 scope while splitting a 0.5" separation double star at ~450x. Seeing was quite good and the rear fan was running. Every few moments the star would lock in and show the barest thin black line between the components. The diffraction pattern during these moments was very clean. Looking at the star test the pattern was similar on either side of focus, however, the in-focus side was a bit fuzzier and seemed to be less stable. The bottom line is that I know the scope is a good one, having made and tested the mirrors myself. The performance certainly bears out the expectations from the testing. And yet, the star test, because of its extreme sensitivity to seeing, does not always indicate a perfect optical system. Remember that when you star test you are looking for that momentary stability, just like when you are observing some critical small detail under variable seeing.

I agree that when a scope is giving excellent views, don't worry about what the star test looks like.

JimC


Splitting close double stars is not really a good test of optical quality. A lesser optic may actually do better than one of higher quality. web page

#48 Tristan

Tristan

    Ranger 4

  • -----
  • Posts: 311
  • Joined: 07 Jun 2011
  • Loc: NC

Posted 03 October 2013 - 11:51 AM

Thanks for posting this Paul. But I think the author may have miscommunicated something. In once sentence he states that a large central obstruction can decrease the size of the airy disk making it easier to resolve close doubles. My understanding is that if you have a lesser quality optic that more of the light is pushed out of the central portion of the airy disk which would cause it to bloat, right? If that was the case a lesser quality scope with the same size central obstruction, focal, length, and aperture should not be able to better resolve close doubles than a higher quality optic, correct?

#49 Tristan

Tristan

    Ranger 4

  • -----
  • Posts: 311
  • Joined: 07 Jun 2011
  • Loc: NC

Posted 03 October 2013 - 11:54 AM

Thanks for the posts Jim and Marcus. I am starting to think after my experiment the other night that I am obsessing too much about atmospheric and local temperature issue too much. I plan on trying a baffle ring inside the scope to redirect airflow over the primary and will try again on Jupiter if seeing is good Saturday morning. I'll let you know how it goes. Your points about seeing being too finicky to get a reliable star test are well taken.

-Tristan

#50 Tristan

Tristan

    Ranger 4

  • -----
  • Posts: 311
  • Joined: 07 Jun 2011
  • Loc: NC

Posted 05 October 2013 - 06:35 AM

OK- So I got up at 5 AM this morning after installing the internal baffle last night, setup the scopes let the fan run for about 30-45 mins and the mirror showed the same temp as ambient at 60.8 F. When I turned the mirror on Jupiter I was greeted with the best view I have ever had! It was like looking at a 3D picture, it was noticeably spherical with hundreds of tiny tendril like cloud bands. I was able to count 7 larger dark bands on the surface, and saw MANY more small thin ones but could not count them as the planet was travelling through the eyepiece a fast clip (507x). Being very pleased with the view I went ahead and did some star testing. The pattern, while showing less contrast inside of focuse matched VERY closely both sides of focus. My only complaint is that the fan causes noticeable vibration at the eyepiece now with the baffle installed....even at low RPM. I think its because the air coming around the mirror cell is turblent and it smacks into the baffle causing the tube to vibrate...a lot. So, I have to run it for awhile, then I can switch it off and observe.

At the end of the day the views are MUCH improved and what I expected from this mirror. Was the problem solved ENTIRELY by the baffle? I dont think so. I think the baffled made a significant contribution to what I saw, but the seeing was also nticeably better thismorning as well.

Thanks again to everyone who helped me troubleshoot this issue. The scope performed beautifully this morning and I cant wait until my next observing session!






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics