angryowl

14 hours ago, FLO said:

Celestron U.K. (Hinds) have contacted Celestron in the US. for their opinion. Both are aware of this discussion. 

HTH, 

Steve 

Appreciate all the help on this Steve and look forward to hearing their opinions as to what may be causing this.

1 minute ago, steppenwolf said:

Thank you for the explanation and confirmation that on the one 600sec refractor image that you have, the stars are good. When trying to assess these sorts of issues, I like a level playing field wherever it is possible and this was a wide deviation, now resolved.

You’re absolutely right and many thanks for pointing that out.

This was a mistake on my part as I should have clearly mentioned that a 600s exposure also looked sharp on the refractor. I think I was a bit tired and frustrated when posting those tests, but still that’s no excuse.

13 hours 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.

Just tried this and the back aluminium mirror holder is rock solid with not even the tiniest movement detected when pulled and wiggled. Additionally, all screws are completely tightened and no play whatsoever there so I think this can safely be eliminated.

14 minutes ago, steppenwolf said:

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?

Well, for one the clouds were starting to roll in when I mounted the refractor so I thought I'd get as many in as possible and two, I couldn't fit any filter inside the imaging train and stupidly thought that 600s exposures on the frac would oversaturate stars, making results harder to interpret. Only then I had realised that I was imaging at F11 which is an entirely different ballgame from F2.2. I had taken one 600s exposure on the frac I think but not kept it, but I can tell you the stars were as pinpoint as those in the 300s exposures.

On the RASA I only took 600s exposures as that’s probably the longest I’m ever going to go and my latest astro images have all had 600s subs.

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.

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...

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.

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?

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.

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:

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.

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.

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.

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.

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.

2 hours ago, FLO said:

We posted our response earlier in this thread ? 

If you find your telescope is faulty we will of course swing into action and do whatever we can to help. But so far your experience is atypical for someone using a reflector/SCT style telescope (you confirmed this yourself when you tried another SCT telescope). It is not a warranty issue. Sorry. 

Steve 

So, the conclusion is that the drift I'm seeing cannot be caused by anything else but the mirror itself flexing. Could the mirror locking mechanisms themselves be faulty? And as this cannot be fully locked down there will always be movement in the mirror resulting in drift in longer exposures. When I purchased the scope, I was under the impression that the mirror can be fully locked down if need be. Also, 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? Could it be that the grease used on the mirror shaft degraded and now shifts a lot more than it did before?

So, the only solution for me at this point would be to use an OAG at the front of the scope which would cause havoc in my images.

If any RASA imagers chime in here with details of their setup and exposure lengths and how they deal with this mirror movement between exposures would be much appreciated.

A comprehensive email has been sent to FLO explaining the situation and we shall see what the response is and what the next steps may be.

Will regularly update this thread once I know more or as new developments arise.

Will also like to thank everyone who took the time to read through and provide advice and assistance., much appreciated.

1 minute ago, Davey-T said:

Not to confuse you I mean a motor focuser between the camera and scope not on the manual focus knob.

Dave

Ah, my bad entirely. Makes total sense now. Mind you, just had my morning coffee so still not up to full speed ?

1 minute ago, Davey-T said:

I've got a motor focuser on my SCT as I found it impossible to get sharp focus for imaging, once the focus is about right I lock the mirror and fine tune it with the motor focus.

Dave

Valuable piece of information right here! I've never done this as always thought once the mirror was locked any movement in the focusing knobs, even the fine one, might strip something inside the focuser or mess with other things.

This is very good to know as I currently focus with a Bahtinov mask and with the mirror unlocked it's a lot of going back and forth to reach proper focus with all that backlash in. I will definitely try this next time I'm out.

Hi Ron, appreciate the input on this

A very good mention indeed and have thought about this several times. I thought maybe with time the grease they used lost some of its properties and turned more liquid, or just performs worse and as you say allows the mirror to ever so slowly sag.

Again, you’re correct in saying that if it is indeed this, it would explain the varying in direction drift seen throughout different parts of the sky.

I always end all of my focusing runs pushing the mirror up and once in focus never go the other direction.

Your statement regarding racking the focuser in both directions to get the grease around both parts makes sense, but wouldn’t that still eventually cause the mirror to sag, albeit slower, but still sag? I may be wrong on this though.

The amount of backlash in the focusing mechanism/mirror mount when reversing focus is quite large, I would say. Never measured it in terms of pixels, but I think it’s considered common with all SCTs?

7 minutes ago, Davey-T said:

I don't use this if you want to try it 80mm f/9 so quite long focal length, good for testing and it's got a T2 thread on the focuser.

Eliminate those pesky SCTs at least 

Dave

 

Wow, this is such an incredible offer Dave. You're right, trying a refractor will surely eliminate any potential mirror movement or anything like that. Plus, my Atik has a T2 thread and would have plenty of adapters to reach focus if need be.

Just thinking what my options with the RASA would be if a refractor gives good results; Parallax rings, finding a way to mount it more sturdily or just smashing the thing to bits for the sheer joy of it.

12 minutes ago, souls33k3r said:

Not while i'm trying to finish this current project  after this, @angryowl you're more than welcome to borrow mine if you want mate. The least i can do.

Hehe, much appreciated mate, but you've done more than enough by borrowing me the OAG which has highlighted some interesting things.

10 minutes ago, FLO said:

We are never far away ? 

Differential flexure is frustrating because it can be difficult to pin down but with time and experience it can usually be overcome. It is always more likely when using a guide-scope vs an OAG (an OAG will pretty-much eliminate it). The amount of flexure depends on a telescope’s design and size. Reflectors add a further variable in the form of mirror shift, which particularly affects SCTs. A RASA is essentially a modified SCT with additional optics similar to those of the HyperStar system and although there are mirror locks, these do not totally overcome the risk of mirror shift. I am not surprised your experience was similar when you swopped your RASA with a friend's EDGE-HD. 

We like to help but differential flexure is not something covered by a manufacturer’s warranty and it is not easy for a retailer (or anyone else) to diagnose remotely. 

Steve 

I completely get your point of view Steve, but is differential flexure what we’re seeing here? I thought differential flexure was the main scope flexing in a different manner than the guidescope thus the latter not correcting the drift properly. However, I’m getting image drift over unguided relatively short exposures from the RASA alone. Would this not indicate something flexing/sagging in the scope/mirror/saddle assembly or mounting points?

I’m just trying to understand how other imagers using the RASA can image unguided, which is a main selling point for the scope, and get acceptable results. Is it due to them using CCDs with larger pixels resulting in larger image scales making the drift less apparent?

Well, I don't have anyone to borrow one off from so no, don't think that's an a option at the moment.

6 minutes ago, FaDG said:

Have you asked FLO directly for support? 

No, not yet, as I wanted to be as sure as possible it was something to do with the scope as I hate wasting peoples/suppliers time when in fact it may well be something else entirely.

Will reach out to them tomorrow, but given their reputation I'm sure they'll be more than helpful in resolving this.