Strange star elongation issue

[michael8554]

Well that must be disheartening for you.

You can check if the mirror is locked or not:

With the mirror locked, gently rotate the manual mirror focus knob while looking for star movement in short exposure download images.

If you feel that the knob won't move obviously don't force it, that and lack of star movement would indicate the mirror is well locked.

Michael

[Davey-T]

I would doubt that the mirror locks are faulty, my Meade only has one mirror lock and can slew all over without it moving, well as all over as a fork mount can 

Why don't you come over and take the refractor to at least eliminate the mount from the equation.

I'm only 10 minutes off junction 3 of the M25.

Dave

 

[michael8554]

i agree with Davey -T, my 8" Meade SCT doesn't have a mirror lock and flop has never been an imaging issue.

But the RASA is a different beast, there are some corrector lenses inside the camera port, perhaps these are loose?

Michael

Edited February 25, 2019 by michael8554

[angryowl]

11 minutes ago, michael8554 said:

Well that must be disheartening for you.

You can check if the mirror is locked or not:

With the mirror locked, gently rotate the manual mirror focus knob while looking for star movement in short exposure download images.

If you feel that the knob won't move obviously don't force it, that and lack of star movement would indicate the mirror is well locked.

Michael

That's a good test to perform next time I'm out and I remember once I forgot I had the mirror locked and tried to turn the fine focus knob a bit and it only rotated like three quarters before it became apparent that something was stopping it and would not go further.

[michael8554]

5 minutes ago, angryowl said:

That's a good test to perform next time I'm out and I remember once I forgot I had the mirror locked and tried to turn the fine focus knob a bit and it only rotated like three quarters before it became apparent that something was stopping it and would not go further.

Well it would be interesting to see if there is any star movement during that small range.

Michael

[FLO]

39 minutes ago, angryowl said:

So, the conclusion is that the drift I'm seeing cannot be caused by anything else but the mirror itself flexing. 

Unfortunately I cannot offer a conclusion. A number here have suggested you try a refractor on your mount. I think it is good advice. 

41 minutes ago, angryowl said:

... how can it be explained that I had been getting round stars in 5 minute exposures just after purchasing the scope in roughly the same parts of the sky that I now get elongated stars? 

Consider what has has changed since then. It might lead you to the source. You need a detective mindset when tracing these things. 

If the conclusion is your telescope is faulty we are here for you but I think currently we'd struggle to persuade Celestron's team of that. A little mirror shift is not unusual for a telescope like yours. 

Steve 

[angryowl]

27 minutes ago, Davey-T said:

I would doubt that the mirror locks are faulty, my Meade only has one mirror lock and can slew all over without it moving, well as all over as a fork mount can 

Why don't you come over and take the refractor to at least eliminate the mount from the equation.

I'm only 10 minutes off junction 3 of the M25.

Dave

 

Appreciated Dave and by the looks of it I might have to take you up on that as this may be a definite way of excluding the mount as being the culprit here.

[angryowl]

14 minutes ago, FLO said:

Unfortunately I cannot offer a conclusion. A number here have suggested you try a refractor on your mount. I think it is good advice. 

Consider what has has changed since then. It might lead you to the source. You need a detective mindset when tracing these things. 

If the conclusion is your telescope is faulty we are here for you but I think currently we'd struggle to persuade Celestron's team of that. A little mirror shift is not unusual for a telescope like yours. 

Steve 

Yes, it looks like the next step is trying a refractor on the mount and see where that puts me.

As to the changes made since I was getting good stars with the scope, there are none, except for an aluminium dew shield and routing the cables round the scope differently.

I completely understand that it’s hard to pinpoint defective/faulty equipment and I most certainly don’t want to jump to the conclusion that the scope is at fault here, but I’ve been looking at this for months now and all that’s left in the equation is the OTA itself. That being said, my desire is not to return or replace the scope as it’s fantastic otherwise and simply outstanding optically, I just want to get to the bottom of this and find a fix so I can get back to imaging.

Edited February 25, 2019 by angryowl

[angryowl]

41 minutes ago, michael8554 said:

i agree with Davey -T, my 8" Meade SCT doesn't have a mirror lock and flop has never been an imaging issue.

But the RASA is a different beast, there are some corrector lenses inside the camera port, perhaps these are loose?

Michael

This had crossed my mind, but I think it may well have shown in the stars in the image as weird aberrations.

Edited February 25, 2019 by angryowl

[angryowl]

On Tuesday @Davey-T generously lent me his 80mm 900mm FL refractor to test and when got home I set up as usual.

Done a proper three-point balance as I do with my RASA and got it spot on. Polar aligned within 30 or so arc seconds using SharpCap, and got Stellarium and SiTech up and running, and after a one star alignment I was up and imaging. I didn’t have a Bahtinov mask for the scope so hand focusing for testing worked fine.

Here are the results:

Ignore the triangular-shaped stars the frac as they're a result of the retaining rings holding the lens.

Scope on East side of pier pointing SE roughly 50 deg elevation 300s UNGUIDED (two consecutive exposures presented)

 

Scope on East side of pier pointing South roughly 50 deg elevation 300s UNGUIDED

 

Scope on West side of pier pointing South roughly 50 deg elevation 300s UNGUIDED

 

Zenith 300s UNGUIDED

 

Took the refractor off the mount and without changing ANYHTING I mounted the RASA, these are the results for the same parts of the sky:

RASA on East side of pier pointing SE roughly 50 deg elevation 600s UNGUIDED

 

RASA on East side of pier pointing SE roughly 50 deg elevation 600s GUIDED with the 60mm scope

 

RASA on West side of pier pointing South roughly 50 deg elevation 600s UNGUIDED

 

RASA on West side of pier pointing South roughly 50 deg elevation 600s GUIDED with the 60mm scope

 

Apologies for the lengthy post, but I figured images of more parts of the sky might help with better understanding what’s happening here.

A report of lest nights’ results will follow shortly.

Edited February 28, 2019 by angryowl

[angryowl]

Last nights’ results:

All of the usual stuff, then quickly mounted the refractor on to test I was still getting pinpoint round stars in 300s exposures, which I was. Mounted the RASA similarly to the night before with no changes WHATSOEVER and here are the results:

Pointed near Procyon, here’s an unguided 600s exposure with the mirror unlocked:

 

Pointed near Procyon, here’s an unguided 600s exposure with the mirror fully locked:

 

Pointed near Procyon, here’s a guided 600s exposure with the mirror unlocked:

 

Pointed near Procyon, here’s a guided 600s exposure with the mirror fully locked:

[angryowl]

An interesting one I thought was this single 300s exposure last night with the scope pointing NW at a high-ish elevation. 600s unguided mirror locks fully engaged.

[angryowl]

Oh and a question I've always been meaning to ask; on the EdgeHD/RASA scopes, do the mirror locks lock the mirror itself in position or just the focuser?

Reason I'm asking is I've read somewhere that they only lock the focusing mechanism in position and not really affect the mirror at all?

Edited February 28, 2019 by angryowl

[michael8554]

Found an online cutaway diagram of the Edge HD, the mirror locks engaged the mirror. 

What are the conclusions of your comprehensive tests?

Mirror locks only make a small but worthwhile difference?

Much heavier RASA is flexing on the mount ?

Michael 

 

[angryowl]

9 minutes ago, michael8554 said:

Found an online cutaway diagram of the Edge HD, the mirror locks engaged the mirror. 

What are the conclusions of your comprehensive tests?

Mirror locks only make a small but worthwhile difference?

Much heavier RASA is flexing on the mount ?

Michael 

 

Thanks for that Michael, hopefully the RASA lock mechanisms work in a similar fashion.

I agree, looking through the images, engaging the mirrors only make a small difference it seems and I'm not sure what to think of this.

As to a conclusion I simply don’t know at this point, as you put it may be a combination of the heavy OTA hanging off the mount sagging due to gravity and the mirror itself sagging and flexing. But then again could be something else.

Hoping someone more knowledgeable than me might see something or suggest something I may try that I haven’t before.

One thing I know for sure is that the Mesu 200 mount can be eliminated from the equation as it can track splendidly and provides fantastic results even at a relatively long 900mm FL as the tests showed.

[Davey-T]

Hi Alex, I've been doing a bit of browsing around and I don't think it can be mirror flop, in fact one guy suggested that if using the Celestron motor focuser the locks can be left slightly loose to allow software auto refocusing during an imaging run with no ill effects.

I can find no one with any problems with this scope in any of it's sizes and some superb images taken with it.

The supplied dovetail is being used by others with no problems even on 14" models.

One possibility could be a fault with the actual 4 element front lens assembly, if that's loose in some way then it could cause your problems I think.

One faint possibility, are you using a dew shield ? maybe it could flop around and affect the image but doesn't seem very likely but if you are using one it's worth a quick try without it.

I really think you should contact Celestron for their take on it, now that you've tested it extensively, maybe they are aware of the problem but not broadcasting it.

ATB

Dave

[Davey-T]

Mark Ackerman ( the "A" in RASA ) is a member on Cloudy Nights and the guy that designed the thing, maybe he can shed some light, if anybody knows how it's put together he should.

Dave

[vlaiv]

I think that there are two "obvious" points of failure here, and it's not tube, nor dovetail. For me it's either - mirror or front corrector.

It might not be lock mechanism at all that is at fault - it might be that mirror cell is not secure and whole assembly, together with lock is loose and tilting under gravity. Lock would secure mirror to mirror cell, but if mirror cell is not secure in place - it would not help much. Second possibility would be front corrector plate - it holds camera and any movement in that element would also move camera.

I think we can do quick analysis of what would happen if front corrector was at fault - it would produce sensor tilt - and you should be able to tell from images - star shapes would be very distorted in one of the corners due to this. I don' think I'm seeing that in test images. Due to this, it is very likely that this is not source of fault.

This really leaves mirror cell as being the main problem. If this is the case, it might be even possible to tell by inspecting back of the ota - see if back side of the scope is secured in place or if it has some slack - just use your hand and try to wiggle it a bit and see if you can feel it move (even tiny motion - looseness). If there are screws holding back cell on tube - see if any of them is loose and tighten them up with screwdriver if needed. It might be something simple as that and I think tightening loose screw would not void warranty.

[Davey-T]

1 hour ago, vlaiv said:

This really leaves mirror cell as being the main problem. If this is the case, it might be even possible to tell by inspecting back of the ota - see if back side of the scope is secured in place or if it has some slack - just use your hand and try to wiggle it a bit and see if you can feel it move (even tiny motion - looseness). If there are screws holding back cell on tube - see if any of them is loose and tighten them up with screwdriver if needed. It might be something simple as that and I think tightening loose screw would not void warranty.

Difficult to see where it might be loose.

Dave

 

[vlaiv]

I'm guessing that it has to be bolted to tube somewhere:

I've noticed that most tubes don't have round holes there but elongated ones - and if screw is loose it can whole mirror cell / focuser assembly can move in relation to tube - which has dovetail bolted on it.

[vlaiv]

Maybe this angle is a bit better:

[michael8554]

9 hours ago, vlaiv said:

lt might be that mirror cell is not secure and whole assembly, together with lock is loose and tilting under gravity.

The mirror is the blue item in the cutaway.

It should be securely locked at the factory onto the metal carrier that slides to give focus.

The mirror locks bear onto tbe metal carrier, but if the mirror is loose on the carrier, it will flop.

Not unheard of on Meade SCTs.......

Michael 

[steppenwolf]

I may have missed something earlier in the thread but I notice that your RASA exposures are twice as long as those of the refractor - what was the reasoning behind not having exactly the same unguided image lengths?

[angryowl]

Much appreciated all of the input here...

@Davey-T – Trust me, I’ve too been doing a lot of reading regarding SCT mirror flop and it should only ever be as apparent as what I’m seeing during a Meridian flip, but not during a normal exposures. That bit you said leaving the locks slightly loose to allow tiny focus motions during imaging runs I think I heard somewhere else and is very interesting.

A loose element inside the 4 element secondary was suggested a few posts up by @michael8554 and I had considered this before, but as I said that would have, I think been easy to spot as it would have produced all sorts of optical aberrations in my stars. Still at this point in time this could be a possibility, but how to test for this without taking the corrector plate apart? hmmm.

The issue presented itself with both a flexible Astrozap, DIY aluminium dew shield and no dew shield at all on the OTA. The dew shields were all secured in place properly so I doubt that’d be it, but a good shout indeed.

I have been told David Hinds would be the person to contact in such situations regarding Celestron issues within the UK. Is this correct?

My thinking was that FLO being the distributor and place I purchased the OTA from, are the first point to go to when one feels something is off about equipment, but I don’t mind directly contacting the Celestron folk in trying to better understand what I’m dealing with here.

 

@vlaiv– I completely agree with your points on the mirror cell about it not being secured in place, that makes a lot of sense. Regarding the front corrector plate, I could never find/hear/feel any movement of the corrector plate whenever I screw/unscrew my camera. I realise you’re referring to the smallest movements here, but as you say this would result in the camera/secondary no longer being orthogonal with the primary thus resulting in odd stars at one corner and not the others.

I can definitely check the screws going round the aluminium mirror block to see if any are loose or have any play. Will do so today and report back...

Edited March 1, 2019 by angryowl

[angryowl]

1 hour ago, michael8554 said:

The mirror is the blue item in the cutaway. 

It should be securely locked at the factory onto the metal carrier that slides to give focus.

The mirror locks bear onto tbe metal carrier, but if the mirror is loose on the carrier, it will flop.

Not unheard of on Meade SCTs.......

Michael 

Just thinking about this gives shivers down my spine...

Really do hope it's not this and something else easily fixable, but that's just wishful thinking at this point.