Potential severe problems with 16" Meade OTA

[PeterWar]

Hello everyone,

 

A couple of months ago I bought a 16" Meade ACF F8 OTA from a dealer in Europe to be used in my observatory. I'm now concerned that this OTA has a severe optical issue, as stars won't come completely to focus, I've tried time and repeatedly and the lower FHWM I've been able to measure in Maxim DL is 8.0.

 

I've also tried to use it visually at magnitude 90x and I could not get sharp stars at all. This week I used a newly acquired artificial star and a ronchi ocular to do some tests an to check for collimation. Using the Atik Infinity camera, I've detected what seems to be a rough edge with its own diffraction pattern. This asymmetry is present both in focus and out focus on opposite edges respectively.

 

All testing has been done with the scope acclimated to the outside temperature, no diagonals nor other optical elements are present.

 

Also, the ronchi test shows potential spherical aberration and some zone aberrations, although we lack the experience to make a definitive diagnostic.

 

I'm attaching some pictures below in the hopes that someone could tell me what's going on:

 

 

 

 

 

Some pictures of the aberration infocus and outfocus.

 

 

A picture using Maxim DL of a star at the "sharpest" point of focus, notice the tails of the star on the right hand side.

 

 

A picture of M13 showing the sharpest point of focus.

 

As always your input is much appreciated.

[Chris]

Hi Peter, I'm struggling to think? Any issues with achieving thermal equilibrium? I'm just looking at the wave effect rolling over the defocused star. Having said this there does seem to be a flat spot on the disc? I hope someone can shed some light for you.

[ronin]

Is the scope the only thing in the optical chain before the camera and so the images you have obtained?

Was a odd one I read of but would rather you give an answer first.

[PeterWar]

5 minutes ago, Lockie said:

Hi Peter, I'm struggling to think? Any issues with achieving thermal equilibrium? I'm just looking at the wave effect rolling over the defocused star. Having said this there does seem to be a flat spot on the disc? I hope someone can shed some light for you.

Hi Lockie,

The scope should have been in thermal equilibrium as those pics where done last night at 3am and I opened the dome at 11pm. Oddly enough, this wave effect that you describe switches from lower left to upper right when I move from infocus to outfocus.

 

ronin, we ended up performing all testing at primary focus, without other optical elements to narrow down the error.

[Chris]

Hmmm? it could be a touch of astigmatism? I think that takes the roundness out of defocused stars. Hopefully an optics expert will chime in.

[Chris]

7 minutes ago, PeterWar said:

Hi Lockie,

The scope should have been in thermal equilibrium as those pics where done last night at 3am and I opened the dome at 11pm. Oddly enough, this wave effect that you describe switches from lower left to upper right when I move from infocus to outfocus.

 

ronin, we ended up performing all testing at primary focus, without other optical elements to narrow down the error.

Ooh! is the mirror tilting? i.e. mirror flop?

[ronin]

OK that removes the amusing one I met about 2 years ago. Someone had added or used their existing coma corrector which thay had always used in a previous scope but in an ACF. The ACF was already coma corrected so all they managed was to add some back in.

Just easily done, and it was a passing thought. People will add in the normal optical items, CC, reducer etc, and if one is a CC in this case then it causes problems not reduces them.

I suspect you will need someone like Steve at SCT to give an answer.

[Astrobits]

Go back to the dealer. If they can't fix it then a replacement OT should be offered. It should not be up to you to sort out a faulty instrument within the first few months from new.

Nigel

[John]

6 minutes ago, Astrobits said:

Go back to the dealer. If they can't fix it then a replacement OT should be offered. It should not be up to you to sort out a faulty instrument within the first few months from new.

Nigel

I agree with Nigel - let the dealer sort this out.

 

[Peter Drew]

I have two 16" LX200 SCT's, theorerically inferior (?) to an ACF but neither exhibit the star test images you have posted. I'm struggling to offer a diagnosis but in view of the sensitivity of large SCT's to temperature issues my leaning would be in this direction. Having opened the dome well in advance is no absolute guarantee that the optics have reached thermal  equilibrium and there could possibly be other inside and outside influences. I have to say that I'm not impressed with the intra and extra focal images compared to the ones I normally see with mine. 

[PeterWar]

4 minutes ago, Peter Drew said:

I have two 16" LX200 SCT's, theorerically inferior (?) to an ACF but neither exhibit the star test images you have posted. I'm struggling to offer a diagnosis but in view of the sensitivity of large SCT's to temperature issues my leaning would be in this direction. Having opened the dome well in advance is no absolute guarantee that the optics have reached thermal  equilibrium and there could possibly be other inside and outside influences. I have to say that I'm not impressed with the intra and extra focal images compared to the ones I normally see with mine. 

Peter what cable are you using to power your fans? the OTA came without instruction manual and without a cable to power the fans.

[Merlin66]

Peter,

It would be interesting to see the Ronchi results...

Over the years I've owned many Meades, 8", 10" and 12", I did find on a couple of occasions when testing "new" s/h Meades that the correction plate was "nipped" - the retaining screws tightened tooooo much. After making the screws finger tight and resting over night I found a dramatic improvement in the Ronchi. May be worth checking....

[pete_l]

I see you've done what I was going to suggest, post the issue on CN.

There are many more Meade customers in the USA as the supply chain is so much shorter. They will have more experience of this sort of thing. Though personally I would get in touch with whoever you bought it from. Send them your test results and get them to comment - possibly with some input through their channel back to Meade.

But the first action is to get the seller to admit there is a fault. Only then, send it back. I would NOT try fixing anything myself and I absolutely wouldn't disassemble it to look for problems - that is a guaranteed way to void your warranty.

[Davey-T]

Mine is only 10" but in the recent hot, for the UK, weather despite being permanently in an observatory, it still took until midnight to settle down but the view was never as bad as yours.

I'm not sure whether it's better to leave the roof open or closed in hot weather.

Dave

[Chris]

Hi Pete, sorry I completely missed the bit where you bought it from a dealer. Yes as above, send the dealer the star test and get them to comment! 

[newbie alert]

I'm no expert..and own a sct of half of what you have but does look like thermal distortion waves..a 16 inch is going to take a good few hours to reach equilibrium ..by that time the temp might be shifting back up again so might not actually get there..i take it the seeing conditions were fairly good on the night the images were taken? Might be worth steve from sc telescopes take a look or worth a phone call..collimatation looks a little out but that flat area don't look right at all,and it switching sides either side of focus is the same as my one did so I'd say it's normal..maybe the collimation screws are pinched..sometimes one will need to be slackened off to tighten the other...hope you get it sorted..

[triton1]

Yup as what's been said send your pictures and results(email)to the dealer and see what they suggest you don't really want to be messing around with an expensive scope like that especially if it's still under warranty.There was a web site for Meade users not sure if it's still running.Finding a cable for the fans might be a step in the right direction see if the dealer can send one out.

[PeterWar]

The book page 27 of the book Star Testing Astronomical Telescopes states that a common tube current appearance is the squeezed or herniated lobe on one side of the disk, and a flattened look on the other.  In this case unfortunately the lobe is herniated on both sides of the focus, as seen per the images, I don't thinks it's a tube current issue.

[Peter Drew]

1 hour ago, PeterWar said:

Peter what cable are you using to power your fans? the OTA came without instruction manual and without a cable to power the fans.

@peter  We actually never use the fans!, one 16" is in a 9metre dome and the other in a 3metre dome, they are usually at or around ambient temperature and good to go at the outset. Always difficult to tell whether any subsequent improvement is due to further cool down or gradually better seeing conditions. When there are thermal problems they do disturb the intra and extra focal rings on star images but they are still concentric rings either side of focus, unlike the ones you've shown. I recall that fans can introduce vibration which could cause the diffraction pattern to deform, might be woth trying without.  

[Peter Drew]

I don't claim to be an optical expert but I do have considerable experience as a user and former dealer with SCT's. I find it hard to believe that a recent Meade 16" would have significant optical errors. They are their flagship aperture and cost enough to ensure a good quality. The manufacture of Schmidt-Cassegrain optics is well established and reliable, there is little opportunity for errors. Is the OTA new or used, if used it would be good to know its history. 

[PeterWar]

2 minutes ago, Peter Drew said:

I don't claim to be an optical expert but I do have considerable experience as a user and former dealer with SCT's. I find it hard to believe that a recent Meade 16" would have significant optical errors. They are their flagship aperture and cost enough to ensure a good quality. The manufacture of Schmidt-Cassegrain optics is well established and reliable, there is little opportunity for errors. Is the OTA new or used, if used it would be good to know its history. 

The OTA is brand new.

[Peter Drew]

1 minute ago, PeterWar said:

The OTA is brand new.

Thanks Peter. Then based on my previous comments I still suspect some form of temperature problem. Before we fitted a heater band to remove dew from the corrector we used a hair drier, this of course warmed the corrector and for 10 minutes or so afterwards the star images were somewhat like yours. I also think it worth checking that the corrector retaining screws are not too tight as already suggested but I would have thought that a pre-delivery check at the factory would have revealed this. Clutching at straws now but being as it was a new OTA they are usually shipped with the mirror lock on, has this been released fully?  

[PeterWar]

1 minute ago, Peter Drew said:

Thanks Peter. Then based on my previous comments I still suspect some form of temperature problem. Before we fitted a heater band to remove dew from the corrector we used a hair drier, this of course warmed the corrector and for 10 minutes or so afterwards the star images were somewhat like yours. I also think it worth checking that the corrector retaining screws are not too tight as already suggested but I would have thought that a pre-delivery check at the factory would have revealed this. Clutching at straws now but being as it was a new OTA they are usually shipped with the mirror lock on, has this been released fully?  

Thank you for your input! we tried both locking and unlocking the mirror to no result. The scope is mounted inside a 4M scopedome, come to think about it, yesterday we opened the dome shutter much before 11pm, it was actually 9pm, so naturally the scope should have cooled down by 3am, even without fans.

[Chris]

If it was just the outer ring looking rough I'd perhaps think of a turned down edge, but all the rings look a bit rough which makes me still think of thermal issues, has the temp dropped quicker than the scope can handle??

Still not sure about that flat bit on the outer edge, but revising up I see astigmatism gives an oval appearance so it's probably not that, and wouldn't pinching show several flat bits?

Is the outer ring equally as bright intra and extra focus? bearing in mind that lots of scopes show some degree of under or over correction. 

It's hard to know hwat to say other than if it's new speak to the dealer. Have you done this, Peter? 

 

[michael8554]

As Merlin suggests, overtightened screw holding the corrector plate.

Similarly the collimation screws mustn't be too tight either.

If you're not returning it to the dealer, I would take the corrector plate securing ring off and check that Meade's alignment marks on the corrector and ota are coincident, not rotated wrt each other.

Michael