50 replies to this topic

[StarryEyedSwede]

I had another go at estimating the resolution capabilities from my photos above. I looked up the apparent field of view, here of the Celestron 7.5 mm Ultima Series eyepiece, and with that information and the magnification I calculated the true field of view as TFOV = AFOV/mag. Or 51 degrees / 333x = 0.15 degrees. After that I simply measured in the photo. As far as I can tell the thinnest branches that can be resolved seem to be about 1 arc second wide. So is it correct to say that I reached roughly 1 arc second resolution in this photo? Or am I fooling myself by looking at a dark object with a lighter background?

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[StarryEyedSwede]

The whole day yesterday the rain was pouring down but when I woke up this morning the sun was shining and there was a light sprinkle of snow on the ground! The forecast even says that the sky will be clear tonight. So I took the old lady out for a look at a distant cell tower this morning. I have located this tower in Google Earth and measured the distance to 8.8 km. At this distance it was obvious that the air was in motion and "boiling". Not so strange after all the rain and snow being melted by the early morning sunlight. Anyway, looking at the cell tower with my 7.5 mm eyepiece and again trying to measure the smallest details I could resolve. Seems like the narrowest parts in the truss structure are about 1-2 arc seconds in width and as can be seen in the photo these are easy to resolve without even zooming in. And this is through 8.8 km sea level atmosphere which is surely more air than between me and the stars at higher elevation angles. I am kind of impressed since I haven't spent any efforts on collimation or cleaning yet. Will do that next (collimation tonight, cleaning the outside of the corrector plate tomorrow).

 

[StarryEyedSwede]

The seeing was better in the afternoon. Now I could definitely resolve down to 1 arc second. Maybe even more, not sure. There is a little cable loop in the photo. For comparison, I have drawn two gray circles to the left and right of it in the zoomed inset. The left one has a thickness (not diameter) of 1 arc second and the right has 0.6 arc seconds. I think the actual cable loop is somewhere in between but my wife thinks the left one resembles it more. What do you think?

 

[StarryEyedSwede]

Update: Spent a frustrating evening trying to understand collimation. Out of focus was easy: the shadow of the secondary looked perfectly centered. This was both with low and high magnification and different degrees of "out of focus" from a tiny donut to a big one filling 1/3rd of the field of view. But I never got any wiser from the in-focus view with the high mag eyepiece (333x, same as in daytime photos above). There was too much vibration from the winds and the less than perfectly stable wooden deck that I had put the scope on so the star kept jumping around. Also, difficult to get precise focus.

 

I did finish off by looking at the moon and Jupiter. The level of detail was ok but not to my expectations so I probably need to figure out the collimation better and maybe get better seeing conditions.

 

Btw, the out of focus views showed the need for cleaning in no uncertain terms. So I just spent the morning cleaning the finder scope, the eyepieces, the diagonal mirror, and finally the outside of the corrector plate using some isopropyl alcohol + destilled water mix with a small drop of washing fluid. They all look good now, especially the corrector plate had a gray film that is now gone. But now that the outside is very clear I can see that the inside has a similar film and may be even more in need of cleaning. I am sure it can be done, I am just not sure that I am brave enough (or stupid enough?) to try to disassemble it. Some encouragement or helpful advice would really be appreciated!

[Rick-T137]

The corrector plate is really easy to take off - just six Allen bolts hold it in place. The key is ensuring that you put the corrector back in exactly the same orientation as it came out. You can use a piece of tape to index it. Once you take the bolts off, the plastic retaining ring will come off (tilt the scope upwards to ensure everything doesn't tumble out when you remove the bolts). You may notice that the corrector will already have a white spot on the edge of it that aligns with a white spot on the OTA. All of my Meade SCTs had this done already at the factory.

 

Once you know you can get the corrector back in the right way, you can lift it out of the OTA by grabbing the secondary housing. Also note there will likely be some small cork shims that hold the corrector in place and centered - those likely will just stay stuck to the OTA, but some may come loose. Just make sure you note where they came from and go back in the same spot.

 

Now you can clean the inside of the corrector. The last thing I'd note is be careful not to get anything on the secondary mirror itself. I have cleaned my corrector many times, but I've never touched my secondary or primary mirrors - they both have a bit of dust on them, but not enough to warrant me pooching them by trying to clean them.

 

Clear skies!

 

Rick

[Bomber Bob]

You may notice that the corrector will already have a white spot on the edge of it that aligns with a white spot on the OTA. All of my Meade SCTs had this done already at the factory.

 

I like that.  I may need to do an 8" SCT Swap - 2080 for my orange '86 C8...

[StarryEyedSwede]

Thanks Rick! I did gather courage by reading a bunch of threads and watching a couple of youtube videos on the subject. As you write, it was surprisingly easy in the end. And boy am i glad that I did this! Not so much for ending up with a very clear corrector, although that was nice and easily done. But much more importantly, when I removed the ring that held the corrector in place I could immediately see two things: first, that it had been removed before (not by me so presumably by the original owner) and whoever it was had not payed attention to the orientation when putting it back. There was a really nice little pair of white dots that I presume are original from the factory. Though these were on completely opposite sides!

Second problem was that one of the three cork pieces that should hold the corrector radially in place had basically turned to dust and disappeared. This I fixed with a little roll of adhesive tape, carefully judged to be the same thickness as the other cork pieces.

 

I did blow a bit of dust of the primary and secondary mirrors with a bulb but didn't physically touch any of them as they looked perfectly fine otherwise.

 

So now I have a squeaky clean scope but presumably the collimation is now totally out of order. Will need to wait patiently for the next opportunity to try. Judging from the weather forecast this won't happen until next month...

[Rick-T137]

That is AWESOME! Yes, sounds like someone had removed it and didn't understand the white markings are important.

 

OH! When you put the Allen bolts back on, make sure you didn't over tighten them. They just need to be snug.

 

Clear skies!

 

Rick

[StarryEyedSwede]

Yep, was very careful with the tightening as I read some horror story about a cracked corrector plate.

 

Now I am hoping that the aligned corrector will solve everything and that I will have razor sharp views in the future. I suppose we can all afford to dream a bit, right?

[DAVIDG]

 Let my explain for the 100000 times from someone who actually makes optics, the rotation of the corrector makes NO DIFFERENCE. What counts is that the corrector and the secondary is centered.  There is good 1/8"  to 1/4" of side play in corrector in the cell and about 1/8" of side to side play in secondary cell in the hole in the corrector. Optics are made as figures of rotation so they are symmetrical as you rotate them. If not you will not correct the astigmatism in one by rotating them. The errors would have to be opposite in sign and also magnitude.

   What is critical is you get the corrector centered in it's cell and the secondary centered.   So you can put that white dot exactly were the factory put it but if the corrector is not centered you will not be able to correctly align the optics no matter how hard you try.

 You need to make sure that the secondary is centered on the mounting plate since it can be off. Next you center the corrector in the cell  as best you can since many times the corrector is not perfectly round. Then you measure from the inner edge of the corrector cell to the center of the secondary cell and make sure it is centered by measuring at a number of different places. Now you'll be able to correctly align the optics. 

     

               - Dave 

[StarryEyedSwede]

 Let my explain for the 100000 times from someone who actually makes optics, the rotation of the corrector makes NO DIFFERENCE. What counts is that the corrector and the secondary is centered.  There is good 1/8"  to 1/4" of side play in corrector in the cell and about 1/8" of side to side play in secondary cell in the hole in the corrector. Optics are made as figures of rotation so they are symmetrical as you rotate them. If not you will not correct the astigmatism in one by rotating them. The errors would have to be opposite in sign and also magnitude.

   What is critical is you get the corrector centered in it's cell and the secondary centered.   So you can put that white dot exactly were the factory put it but if the corrector is not centered you will not be able to correctly align the optics no matter how hard you try.

 You need to make sure that the secondary is centered on the mounting plate since it can be off. Next you center the corrector in the cell  as best you can since many times the corrector is not perfectly round. Then you measure from the inner edge of the corrector cell to the center of the secondary cell and make sure it is centered by measuring at a number of different places. Now you'll be able to correctly align the optics. 

     

               - Dave 

Hi DAVIDG,

 

What you are writing about centering all optical elements makes perfect sense, but on the other hand the manual specifically talks about the importance of rotational orientation in this warning:

 

WARNING: Do not, in any case, remove the correcting plate from its machined housing for cleaning or other purposes. You will almost certainly not be able to replace the corrector in its proper rotational orientation and serious degradation of optical performance may result.

 

I also didn't see any obvious way of tampering the with position of the secondary mirror on the corrector. But at least I got rid of the potential for the corrector to shift around due to the missing cork piece. That in itself seemed critical since otherwise it could shift (even bang around?) due to gravity when rotating the OTA.

 

(speculating) Could it be that Meade purposefully designed and manufactured the correcting plate with a slight off-center asymmetry? If so, one could move the true optical center by rotating the corrector and by that optimize the centering. So perhaps Meade tested the optics with a few different orientations and marked and shipped the scope with the best (most centered) one. (/end speculating)

 

Edit: I guess it doesn't have to be purposefully. Achievable tolerances can be the reason.

Edited by StarryEyedSwede, 19 February 2024 - 04:56 AM.

[DAVIDG]

  The hole in the corrector plate is larger then the secondary cell and if you can unscrew it and  move side to side. Also over time, it can come loose and move.

   Why Meade says not to rotate the corrector is because the secondary many times is not perfectly centered on it's mounting plate and cell is not perfectly centered in the corrector   So in the factory they put the secondary assembly in the corrector plate, put  that assembly in the corrector cell and spin it around trying to find a position were they get things to be being close to optically centered.. Note I said close to optically centered not perfectly optically centered.  That is good enough for them and they ship the scope. When you  actually get these scopes up on the test bench  you will see that they are not figured to "1/10" wave. In many cases they are barely 1/4 wave. So getting the optics close to aligned in good enough for them.

   Here is typically what I see when I test one of  these.  The  Ronchi lines should be very close to dead straight yet they shows zones and other problems.  There are many other examples on this site as well that show similar results. 

    

 

 

   Optics are suppose to be made has figures of revolution, When you  have errors in the figure they are also rotationally symmetrical. So you are not going to cancel a rotationally symmetrical error in one surface by rotating it against another rotationally symmetrical error in another surface. The exception is astigmatism.  To correct for that you need to have another element have the same amount of astigmatism but of the opposite  amount. That is how eyeglasses work since the lens is made to have the opposite amount of error and is rotated to the same position as the error in your eye. So if you look at the prescription for correcting astigmatism for eyeglasses it states the power of the lens and the rotational angle it needs to be mounted in the frame.  

   Here is the problem in a SCT,. all the optics are made with processes to make them rotationally symmetrical.  Next the corrector has almost zero optical power,  spherical primary and the secondary have greatly different optical power. So if you have astigmatism some how you need to have an element that was suppose to  made to be symmetrical, not be AND have the same error has one of the other elements and also have the opposite amount of error.  The odds of that are very low and it also means you have a defective manufacturing process. 

   So all this talk about getting the corrector in same position as the factory will make no difference unless you get the optics centered. That is what counts.  I can put that white dot in any position I like as long as I get the optics centered. If it makes you feel better put it in the factory position BUT get the optics centered since that is what counts.

 

                 - Dave 

Edited by DAVIDG, 19 February 2024 - 11:22 AM.

[StarryEyedSwede]

Thanks, very educating! And fascinating!

 

In your experience, how much off-center in mm (or inches if you prefer) causes poor alignment for a 10" f/10 SCT? Just so I get a feeling for what kind of accuracy is needed. Also, is there any way to test this for an amateur like me without access to a dedicated optical test bench? Adjusting the radial position of the corrector (possibly even the secondary) sounds like something anyone can do, but it is of no use without knowing in which direction to adjust.

[DAVIDG]

Thanks, very educating! And fascinating!

 

In your experience, how much off-center in mm (or inches if you prefer) causes poor alignment for a 10" f/10 SCT? Just so I get a feeling for what kind of accuracy is needed. Also, is there any way to test this for an amateur like me without access to a dedicated optical test bench? Adjusting the radial position of the corrector (possibly even the secondary) sounds like something anyone can do, but it is of no use without knowing in which direction to adjust.

 In a SCT you have a very fast primary and 5x  aspheric secondary so the optical centering is critical. I would say no more then 1/2 mm or less but you should be able to get it perfect.  Having the secondary centered is more critical then the corrector. The secondary should  be  mounted centered  on it's mounting plate. If so then you can use center bolt in the secondary cell as a reference and measure from it to the inner edge of the corrector plate cell in a number of places to get the secondary centered.  If the secondary is centered on the it's mounting plate you should have no problem getting it perfectly centered and most likely better then they did in the factory. 

  The correctors many times are not perfectly round so you try to get  the  average space between the edge of the corrector and in inner edge of the corrector cell to be even as possible..

    You can collimate the optics in the daytime using a glint of sunlight off the cellphone tower as a  "star"  since you  can better see the adjustment screws and thing are not moving. 

   If your optics are centered, when you get the shadow of the secondary to be dead centered on one side of  focus it will stay centered on the other side. If it drifts off center from one side of focus to the other the secondary is off center and  you'll never get a good an image as possible,

 

                     - Dave 

Edited by DAVIDG, 19 February 2024 - 11:51 AM.

[Bomber Bob]

Your Meade's metal Secondary Holder should have a Center Screw / Bolt -- you may have to remove an exterior Cap to get to it.  Loosen that screw just a bit to slide & center it.  I use a Digital Caliper, set to Metric.  First I measure the inside diameter of the Corrector Cell -- taking several measurements, checking for out-of-round, too.  Divide that result in 2 to get the radius.  I set & lock my Caliper at the value.  Then, I loosen the Secondary Mirror Holder, move as needed to center, then tighten it.  You don't have to take the whole corrector cell off the scope.  I normally do, as there's almost always haze, dust, etc. on the inside face of the corrector, on these Old Scopes that I buy & restore.

Edited by Bomber Bob, 19 February 2024 - 11:55 AM.

[StarryEyedSwede]

I must admit that I didn't pay close attention to how the secondary holder was mounted on the corrector. But I did take some photos and I can't see any visible mountings. Unless the baffle around the secondary can be removed?

 

0.5 mm precision sounds very challenging, considering gravity and the cork spacers. Also, I suspect this is really 0.5 mm with respect to the axis of symmetry of the primary mirror and not just 0.5 mm with respect to the center of the housing. So if the primary is not centered with the same precision AND precisely perpendicular to the axis of the housing it won't be of much use to adjust the secondary? Doesn't the primary shift a little bit when focusing?

 

 

Though it does seem like a fun project to add a bunch of adjustment screws for moving and tilting the different optical elements. I guess that might be how the professionals do it for scopes that are to be used for actual science.

[DAVIDG]

 The baffle tube on the inside of the corrector unscrews and top section can be removed. The secondary is glued to a metal plate and the three screws thread into the plate to allow it to be tilted.

  The hole in the corrector is larger then the secondary holder so there is play in getting it centered but many times the hole in the corrector is not precisely centered so you don't want to use having the secondary cell centered in the hole in the corrector has having the secondary centered in  system. 

As for the primary it's optical axis needs to be aligned with the mechanical axis the OTA.  When that condition is meet  you can use the mechanical reference points like the inner surface of the corrector cell  as a reference surface. There is an easy way to test this. With the corrector removed  you place string through the screw holes in the corrector retainer ring so they form a X across the front the tube. With a laser collimator is the focuser, the laser should hit the exact center of were the strings cross.  Now stand back a couple of feet and look into the primary. You'll see the reflection of the strings in the primary. You move  your head so  the reflection of the strings are exactly behind the string. If the mirror is correctly aligned the reflection of the primary will be exactly in the center of tube 

  Here is a drawing that should make it clearer what I'm describing  If the primary is off it needs to adjusted if not you'll never get the optics correctly aligned.

 

   

  Yes the primary can flop around when you focus and it will throw off the collimation which will have a negative effect on the image quality. So what is done is  an external focuser is added and the mirror  locked in place.

 

                     - Dave

 

 

 

    

 

   

[StarryEyedSwede]

Thanks, that is a very clever way of checking the primary axis. Your drawing doesn't show the central hole in the mirror but I guess the principle works anyway. I am sorely tempted to check this, including seeing how much shift the focusing and varying OTA orientation induces.

 

Ok, so if I embark on this journey (big if... I believed cleaning the inside of the corrector was akin to open heart surgery just a couple of days ago...) would this be the right way to go about it?

 

1. Check the alignment of the primary mirror using the string reflection test, including the stability of the alignment during focusing and rotation of the OTA. If alignment is acceptable, proceed to 3. If not, do 2 first
2. (Lock primary mirror in place, align it, and add external focuser) - need to read up on ways to do this
3. Center the secondary (and the corrector?) with respect to the mechanical housing. Did I understand correctly that centering the secondary is the important part and that centering of the corrector is less critical?
4. Tilt the secondary using the collimation screws, regular star test should be done here
5. Hopefully enjoy the best possible views this scope is capable of

 

Btw, I am curious why the standard SCT design uses an integrated focuser rather than an external one. There has to be some advantage that is supposed to compensate for the primary flop drawback? Is it just to make the OTA more compact so that the fork arms can be made shorter?

[DAVIDG]

 The SCT is designed to be  an "all purpose" telescope and mainly for photography.  Moving the primary mirror allows for larger range of focus but with a problem that there is only one spacing between the primary and secondary that results in the best optical correction.  The newer models come with or have the option to add an external focuser on the back. The scope usually comes with a mirror lock or a place to insert a bolt to lock the movement of the mirror and then ones uses the external focuser.

  The corrector plate has almost zero optical power. It is not focusing light but distorting the in coming wave front. So it is not as critical to getting it perfectly centered but it is not difficult to get the spacing between the outer edge of the corrector and the inner edge of the corrector cell uniform.

   The secondary has 5x optical power and no matter what the "experts" say up in the Cassegrain form that have never made an optical surface, the secondary has aspheric figure and not spherical. An aspheric as an optical center so it needs to be aligned with the optical center of the primary since and with 5x power any misalignment causes large errors in the final image. 

   This is  why you want to get the primary aligned so the mechanical axis of the tube and optical axis of the primary are aligned. With that criteria meet, you can get the secondary mechanically aligned on it's mounting plate and installed in the secondary holder so the optical axis and mechanical axis of the secondary are aligned. Now you can get the secondary correctly aligned with the primary by measuring it's position in relationship to the corrector cell. 

 

 

                    - Dave 

[DAVIDG]

 Here is  a link to thread were I helped the author swap out a complete set of SCT with new ones and get them aligned 

 

                https://www.cloudyni...p#entry12731444

 

       - Dave 

[StarryEyedSwede]

 The baffle tube on the inside of the corrector unscrews and top section can be removed. The secondary is glued to a metal plate and the three screws thread into the plate to allow it to be tilted.

  The hole in the corrector is larger then the secondary holder so there is play in getting it centered but many times the hole in the corrector is not precisely centered so you don't want to use having the secondary cell centered in the hole in the corrector has having the secondary centered in  system. 

As for the primary it's optical axis needs to be aligned with the mechanical axis the OTA.  When that condition is meet  you can use the mechanical reference points like the inner surface of the corrector cell  as a reference surface. There is an easy way to test this. With the corrector removed  you place string through the screw holes in the corrector retainer ring so they form a X across the front the tube. With a laser collimator is the focuser, the laser should hit the exact center of were the strings cross.  Now stand back a couple of feet and look into the primary. You'll see the reflection of the strings in the primary. You move  your head so  the reflection of the strings are exactly behind the string. If the mirror is correctly aligned the reflection of the primary will be exactly in the center of tube 

  Here is a drawing that should make it clearer what I'm describing  If the primary is off it needs to adjusted if not you'll never get the optics correctly aligned.

mirror alignment string test.jpg

 

   

  Yes the primary can flop around when you focus and it will throw off the collimation which will have a negative effect on the image quality. So what is done is  an external focuser is added and the mirror  locked in place.

 

                     - Dave

I had a go. Looks like the primary well aligned but not perfect. The image on the right is the same as the left but I have superimposed a perfect circle and four diameters with exactly 45 degree spacing. These match up very well to the holes and the strings but for the center there is a tiny offset of about 1 mm. Maybe difficult to see in the attached figure but the full resolution image shows this better. When I moved the scope in and out of focus (near and far) I couldn't see that the alignment shifted.

But I realize that the way I pulled one contiguous string through all the holes causes it to be a bit offset in some of the holes. Also, I put the strings in the plastic retaining ring which adds maybe a mm of uncertainty. Will probably try again to see if I can improve the precision.

 

Since I am this close to alignment, is it good enough to just determine where the optical axis of the primary mirror is and make sure that the secondary mirror is located as close as possible to this point?

[deSitter]

I had a go. Looks like the primary well aligned but not perfect. The image on the right is the same as the left but I have superimposed a perfect circle and four diameters with exactly 45 degree spacing. These match up very well to the holes and the strings but for the center there is a tiny offset of about 1 mm. Maybe difficult to see in the attached figure but the full resolution image shows this better. When I moved the scope in and out of focus (near and far) I couldn't see that the alignment shifted.

primary_alignment.jpg

But I realize that the way I pulled one contiguous string through all the holes causes it to be a bit offset in some of the holes. Also, I put the strings in the plastic retaining ring which adds maybe a mm of uncertainty. Will probably try again to see if I can improve the precision.

 

Since I am this close to alignment, is it good enough to just determine where the optical axis of the primary mirror is and make sure that the secondary mirror is located as close as possible to this point?

There is a geometrical way to locate the center without relying on fine measurements. Get an accurate right-angle carpenter's tool (everyone needs one anyway). Put the vertex of that on the periphery of the cell. Then regardless of the angle, the two legs will cross the periphery in diametrically opposed points. Here is a picture. If omega is a right angle, then A and B are diametrically opposed. This if done carefully will allow you to locate the exact center. Once you have a reliable diameter, put a length of blue tape around the periphery and mark the midpoints to construct the perpendicular diameter.

 

-drl

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[DAVIDG]

 I'm trying to understand what tests you did and the results.  I see a red dot in the right image. Is that from a laser collimator in the focuser ? If so it looks to be hitting almost the center of the strings. That test shows  that the mechanical axis of the focuser is aligned with the mechanical axis of the tube. What did you see when you stood back a couple of feet and moved your head so the reflections of the string in the primary were behind the strings ? That test shows if the primary is tilted or not and aligned or not aligned with the mechanical axis of the scope.  These are two separate tests  so you need to check both conditions  You should check  the aligned of the primary when you focus all the way in and also all the out to be sure that the mirror is staying aligned.

 

         

                        - Dave 

[StarryEyedSwede]

Ah, I knew I was not being very clear on what I did. So I tried to follow your advice, using some white thread to make the strings that I observed from roughly one tube-length. The left photo (actually a single frame from a video - I selected the frame with the best alignment between the strings and the reflection of the strings in the primary mirror) shows the strings and their reflection in the primary mirror. It is very difficult to see the reflections in the photo since these almost exactly behind the strings. But they are indeed almost perfectly aligned. The right image is just a copy of the same photo but with some red help lines I have made to try to figure out whether the optical axis is indeed centered.

 

So based on this I estimated that the optical axis is at most 2 mm offset from the center of the tube at the corrector, probably less. I will try to eliminate some uncertainties I had in the first try and make a refined estimate.

 

PS. There are some additional off-center white string reflections in the photo as well. These confused me at first until I realized that these are actually double reflections in both the primary mirror and the smartphone camera.

[DAVIDG]

 Thanks for the explanation. I would try the reflection test by standing back abut 3 or 4 meters. The sensitive increases at the a longer distance but it sounds like the mirror is aligned or very close to it 

    If you have a laser collimator I would also check that the focuser/baffle system is also mechanically align since the mirror can be aligned but the focuser baffle system not.

   I hope it is starting to become clear what the rotation issues is really about. If the primary is slightly out of alignment and the secondary/corrector assembly is also out, by rotating them, you can get the optical centers close to center but not perfect. So that is good enough for Meade but if you can them centered now you can them correctly aligned, have better optical performance and it doesn't matter how you rotate them. 

 

               - Dave