32 replies to this topic

[midwestastronomer]

I was given a meade 8 inch LX50 OTA but the image on the planets is really soft. 

There is virtually no detail to be seen. 

Collimation issue or something else? 

[deepwoods1]

How do stars look? Especially deliberately slightly out of focus. You'll see a donut. Is it symmetrical? 

[markb]

Collimation always first.

 

At the very least, get the Poisson Spot diffraction ring pattern, just off focus, concentric. Don't stop at the (in my opinion, very inadequate) centering of the secondary shadow. I use a free, cut from cardboard, Duncan Mask in place of the useless secondary shadow. My opinion  of course.

 

But if you still have soft images once collimated, someone may have cleaned the scope and reassembled it without using the factory 'spin alignment' marks made by the factory.

 

The point of the factory alignment, not related to collimation, is to place the secondary in the optical centerline of the primary, which should be coincident over the baffle axis. Collimation cannot fix it, only make the best of a bad alignment.

 

Once done, they mark the corrector and secondary.

 

The corrector may not be centered from the factory in recent scts  and it appears the factory uses intentionally off center secondaries (to the aluminum puck) at times to 'save' a poorly assembled tube (Meade epoxies theirs). That is the only explanation for my M8's parts.

 

My 11 had every alighment lost and was soft and useless. After restoration, great images. That required only restoring the original positions of the markings, and a slight adjustment of the corrector location (Celestron had stopped just centering them by that time).

 

My Meade 8 had the rear cell glued on at a huge angle, about 8mm at the corrector,  and had huge amounts of coma. Very extreme.  It needed the corrector flush to the cell, and use of an eccentrically glued secondary. A mess. But once rectified, a project far beyond the usual simple markings restoration, it is a great scope, pending a recoat.

 

Meade marks the corrector on the front by the edge under the retaining ring, with a wide whiteout splash, on corrector and cell, with a sharpie marker going across the cell to corrector margin. Both visible once the retainer was removed.. The secondary has a sharpie mark on the ground edgeof the secondary glass, that aligns to 3 oclock, viewed from the corrector end, with the tube writing right side up. That may require corrector removal. Flipped correctors don't matter according to DavidG, the best source I've read here. Besides, IIRC Meade figures both sides anyway.

 

My corrector, 2000 or so, had a chinese character at the top as well. IIRC it looked like a rectangle or square with a vertical line. I think I looked it up and it meant 'top', but it's been a while.

Edited by markb, 19 October 2021 - 05:28 PM.

[markb]

I was only skimming for photos, but this CN post shows the Poisson Spot diffraction pattern idea pretty well, defocus enough to see the diffraction rings, and make them concentric.

 

The final step is always an in-focus Airy Disc on a Polaris magnitude star. Always.

 

But atmospherics have to be great. So the Poisson Spot me this does fine in mediocre seeing. Pretty much all I ever get at home in a fully built major city suburb.

 

Duncan Masks can be sloppy and work fine. The look like a radiation hazard symbol, and you can skip making the window usually shown, a simple 3 arm works fine. My refractor likes 50% open window, my 11 likes 70% blog ked, 30% open. Ply to see what you like.

 

Or buy a pre-made Tri Bahtinov on Ebay.

 

I use the mask only for crude collimation after a disassembly.

[markb]

I was only skimming for photos, but this CN post shows the Poisson Spot diffraction pattern idea pretty well, defocus enough to see the diffraction rings, and make them concentric.

 

https://www.cloudyni...-a-photo-guide/

 

The final step is always an in-focus Airy Disc on a Polaris magnitude star. Always.

 

But atmospherics have to be great. So the Poisson Spot me this does fine in mediocre seeing. Pretty much all I ever get at home in a fully built major city suburb.

 

Duncan Masks can be sloppy and work fine. The look like a radiation hazard symbol, and you can skip making the window usually shown, a simple 3 arm works fine. My refractor likes 50% open window, my 11 likes 70% blog ked, 30% open. Ply to see what you like.

 

Or buy a pre-made Tri Bahtinov on Ebay.

 

I use the mask only for crude collimation after a disassembly.

[midwestastronomer]

Collimation always first.

 

At the very least, get the Poisson Spot diffraction ring pattern, just off focus, concentric. Don't stop at the (in my opinion, very inadequate) centering of the secondary shadow. I use a free, cut from cardboard, Duncan Mask in place of the useless secondary shadow. My opinion  of course.

 

But if you still have soft images once collimated, someone may have cleaned the scope and reassembled it without using the factory 'spin alignment' marks made by the factory.

 

The point of the factory alignment, not related to collimation, is to place the secondary in the optical centerline of the primary, which should be coincident over the baffle axis. Collimation cannot fix it, only make the best of a bad alignment.

 

Once done, they mark the corrector and secondary.

 

The corrector may not be centered from the factory in recent scts  and it appears the factory uses intentionally off center secondaries (to the aluminum puck) at times to 'save' a poorly assembled tube (Meade epoxies theirs). That is the only explanation for my M8's parts.

 

My 11 had every alighment lost and was soft and useless. After restoration, great images. That required only restoring the original positions of the markings, and a slight adjustment of the corrector location (Celestron had stopped just centering them by that time).

 

My Meade 8 had the rear cell glued on at a huge angle, about 8mm at the corrector,  and had huge amounts of coma. Very extreme.  It needed the corrector flush to the cell, and use of an eccentrically glued secondary. A mess. But once rectified, a project far beyond the usual simple markings restoration, it is a great scope, pending a recoat.

 

Meade marks the corrector on the front by the edge under the retaining ring, with a wide whiteout splash, on corrector and cell, with a sharpie marker going across the cell to corrector margin. Both visible once the retainer was removed.. The secondary has a sharpie mark on the ground edgeof the secondary glass, that aligns to 3 oclock, viewed from the corrector end, with the tube writing right side up. That may require corrector removal. Flipped correctors don't matter according to DavidG, the best source I've read here. Besides, IIRC Meade figures both sides anyway.

 

My corrector, 2000 or so, had a chinese character at the top as well. IIRC it looked like a rectangle or square with a vertical line. I think I looked it up and it meant 'top', but it's been a while.  

 

 

So if the alignment is off, do you have to send it back to the manufacturer, or can it be done at home?

Would Orion(new owner of Meade) be able to do something like that, and how much would it cost?

Edited by midwestastronomer, 19 October 2021 - 05:57 PM.

[markb]

A chromed ball bearing in the Sun, or lit by a red or (preferably) green laser, or even a focused flashlight (haven't tried that one) let's you collimate in daytime. The bearing can be secured by a magnet, and black paper backing helps the image.

 

Distance for collimation is forgiving, but star testing must be done a properly long distance.

 

I also have the super affordable Hubble Optics 5 star Artificial Star.  Worth it, but the ball bearing works until you get one. Christmas ornaments can work,but the silvering is often inside, leading to a doubled image, I can't use one.

[CHASLX200]

Look at a star at high power and see if it is round or flared on one side. Collimation is easy. But some SCT's just have bad optics no matter what. Collimate the best you can and go from there.

[markb]

Restoring the alignment marks can be done at home if you have the tools and are comfortable using them. 

 

A computer 50 bit screwdriver set usually has the Allen bits needed, and L wrenches are a pain, and can easily lead to destructive overtightening.

 

If the corrector needs to be significantly off center, extremely unlikely (mine was an outlier), PM me after trying the rest. Mine had horrendous coma, something you never hear reported. I am surprised they didn't trash the tube and retube the optics.

 

But a simple reorientation of the corrector marks should do it. Many scopes are built 'centered' and get away with sloppy reassembly. Not mine.

 

There are exceptions of course, but I think most optic sets are at least good to very good, if set up properly. QC was notorious defecient in the mid 80s, but I believe that was transient in nature. 

 

I am not a Meade fan (mechanicals and electrics), but meade optic sets seem to have a very good reputation. Even pending a badly needed recoat, my horribly assembled 8 has turned out to have very nice optics.

Edited by markb, 19 October 2021 - 06:18 PM.

[jgraham]

The two key factors for Schmidt Cassegrains is colimation and acclimation. Carefully check the colimation and give it time to acclimate as it gets dark. If those boxes are checked then seeing could be the culprit. For the past few nights Jupiter and Saturn have been decidedly soft due to poor atmospheric conditions.

[CHASLX200]

The two key factors for Schmidt Cassegrains is colimation and acclimation. Carefully check the colimation and give it time to acclimate as it gets dark. If those boxes are checked then seeing could be the culprit. For the past few nights Jupiter and Saturn have been decidedly soft due to poor atmospheric conditions.

Seeing has been bad for me for weeks living on the gulf. Can't wait for the planets to get higher in a few years.

[highfnum]

last few day seeing was so bad that even my 3 inch edmund showed  mush

i usually post planet images  with rv6 c8 and others

nothing worth showing this week so  far 

[midwestastronomer]

So anyway, I did a star test on this and found that the collimation is slightly off, but the image is blurry no matter what. 

 

I got this scope for free, and am wondering if I wasted money rehabbing it because there may have been a reason the scope was in the trash before I got it. 

[jgraham]

I guess that anything is possible, but the LX100s and LX50s have a reputation of very good optics. They were the last of the non-GoTo Meades and were popular with old-school photographers who didn't want the GoTo. I have had a couple of stubborn SCTs and the fix was the same; zeroing in the collimation. To get it spot-on I'd tweak it using increasing magnification and closer to focus with the final adjustments being made with the diffraction rings under good seeing. Still, it's possible that you've got a problem child.

[davidc135]

So anyway, I did a star test on this and found that the collimation is slightly off, but the image is blurry no matter what. 

 

I got this scope for free, and am wondering if I wasted money rehabbing it because there may have been a reason the scope was in the trash before I got it. 

Assuming collimation and aclimatisation aren't the culprits.

 

The star test should go much of the way to explaining why it's blurry eg if there's astigmatism or seriously under or over-corrected. Compare images exactly at equal distances inside and outside of focus. Just a little for astig and +/- 4.4ish mm for correction.

 

Using your 8'' Orion, if it's good, as a light source for a knife edge test is also easy and will give a more complete picture.

 

David

Edited by davidc135, 26 March 2022 - 04:22 AM.

[bob midiri]

So anyway, I did a star test on this and found that the collimation is slightly off, but the image is blurry no matter what. 

 

I got this scope for free, and am wondering if I wasted money rehabbing it because there may have been a reason the scope was in the trash before I got it. 

Hi if all this advice re collimation and star testing seems intimidating to you, seek out a local astronomy club in the area, I'm sure some member there would be more then happy to assist you in this regard. After you see how its done it will be easy for you to tweek when needed thereafter

[markb]

+1 on Bob's suggestion.

 

Collimation and acclimatization have been repeatedly mentioned, but many users have not suffered from the effects that result from misassembled SCTs (lost factory adjustments). I, sadly, have.

 

Miassembly can easily cause soft or even comatic images, even after careful collimation, of factory adjustments (sometimes even including intentional factory miscentering of correctors) if same are not restored.

 

Since the OP has soft images not resolved by collimation, that would be my next check. I'm not a Meade fan but the mirrors are usually very good indeed.

 

Finding and joining a local club will hopefully help in locating a mechanically competent member willing and able to restore at least the factory alignment marks.

 

My very sharp Meade 8, even though in need of a secondary recoat, went from an optical mess to a very good OTA.

 

It also started as a 'trash box' scope when I bought it for $75 shipped.

[midwestastronomer]

+1 on Bob's suggestion.

 

Collimation and acclimatization have been repeatedly mentioned, but many users have not suffered from the effects that result from misassembled SCTs (lost factory adjustments). I, sadly, have.

 

Miassembly can easily cause soft or even comatic images, even after careful collimation, of factory adjustments (sometimes even including intentional factory miscentering of correctors) if same are not restored.

 

Since the OP has soft images not resolved by collimation, that would be my next check. I'm not a Meade fan but the mirrors are usually very good indeed.

 

Finding and joining a local club will hopefully help in locating a mechanically competent member willing and able to restore at least the factory alignment marks.

 

My very sharp Meade 8, even though in need of a secondary recoat, went from an optical mess to a very good OTA.

 

It also started as a 'trash box' scope when I bought it for $75 shipped.

The collimation was only off at or near the zenith. I do know that SCTs are prone to mirror shift. I started messing around with the screws on the secondary and only made it worse, now I have no idea how to fix it. 

 

The corrector plate may need recoating. Does anyone know how to get that done? 

 

Thanks, 

MWA. 

Edited by midwestastronomer, 28 March 2022 - 01:39 PM.

[DAVIDG]

 Majestic Optical coating can do it  The problem is that you'll  going  to  have strip the old coating yourself and that requires hot sulfuric acid to remove it since they won't remove the old coating.  My guess is that it will cost over $300 to have it done and in reality it is not going to solve the soft image problem. 

   If your collimation is/was close and the scope was cooled down and the seeing was good and the image was still blurry then that points to poorly figured optics. It happens more then many want to believe. A critical star test looking at the both the inside and outside diffraction pattern using a 10mm  eyepiece or close to that focal length is what is needed or  to bench test the optics using double pass autocollimation to determine the quality of the system.

 

               - Dave  

[CHASLX200]

The collimation was only off at or near the zenith. I do know that SCTs are prone to mirror shift. I started messing around with the screws on the secondary and only made it worse, now I have no idea how to fix it. 

 

The corrector plate may need recoating. Does anyone know how to get that done? 

 

Thanks, 

MWA. 

I don't know of anyone that does that kind of coating for a corrector.

[Gil V]

If the main baffle and rear flange can be shimmed, it’s easy to align the baffle with the front cell. You do have to remove all the glass to do the job.

First step of OTA assembly, assuming the tube and front/rear cells are completely assembled.

Edited by Gil V, 18 April 2022 - 08:00 AM.

[tim53]

That it was rescued from the trash has me wondering:  Did someone replace a broken corrector with a sheet of float glass?

[davidc135]

The collimation was only off at or near the zenith. I do know that SCTs are prone to mirror shift. I started messing around with the screws on the secondary and only made it worse, now I have no idea how to fix it. 

 

The corrector plate may need recoating. Does anyone know how to get that done? 

 

Thanks, 

MWA. 

The corrector plate coating won't affect sharpness of images.

 

To get the collimation roughly back on track remove the ep and diagonal and, keeping one's line of sight centered along the primary baffle, look at the position of the secondary.

 

Collimated                                                                                                   

 

     

 

Well out of alignment

 

 

It's maybe easier to look along the inner edge of the baffle tube, top and bottom and both sides and extend one's view to the corrector and see how far off the secondary is in each case. (Actually, reflected images of corrector and 2ndry). It's then straightforward to adjust the secondary screws until all are the same. Fine adjustment then on a star.

 

David                                        

Edited by davidc135, 18 April 2022 - 01:37 PM.

[midwestastronomer]

I just collimated the scope. We'll see how it does on globs this summer. 

[jgraham]

I hope that it performs well for you, I do luvs mine. FYI... sometimes I will encounter an SCT that will not hold collimation and the alignment may even shift depending on where it is pointed; aka mirror flop, which is different than mirror shift that can occur due to backlash in the focusing mechanism. Mirror flop can occur if the support ring that holds the mirror to the light baffle is loose. This can occur in older SCTs dues to the lock ring shaking loose or the gasket taking a set. The fix is to remove the corrector and carefully reach in and snug the lock ring. This may involve turning the lock ring itself or the baffle tube. I have found that about 30% of my older (prior to 2000) SCTs had loose primaries.

 

Food for thought.