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Strange star elongation issue


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1 hour ago, angryowl said:

The scope was purchased from First Light Optics about two years ago and still have two months of warranty on it. I’d really like to know now if the scope is at fault here as if it is, I have no alternative but to return it

Have you asked FLO directly for support? 

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38 minutes ago, Davey-T said:

Have you tried with Ahmeds SCT yet ?

Dave

Yes, 12 posts up, the first image is of three 10 minute consecutive unguided exposures using his EdgeHD. Although I don't like the way the camera was mounted via the two flimsy thumb screws so not sure if that may be adding to the issue. Plus, the next post down explains the issue Ahmed had using the EdgeHD on his mount with very similar elongation issues and drift between exposures.

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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.

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27 minutes ago, angryowl said:

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.

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 

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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?

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12 minutes ago, souls33k3r said:

Not while i'm trying to finish this current project :D 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.

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Just now, angryowl said:

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

Nee bother mate. I'm only a shout away from you and you know it. If i've got something you need, you know where to find me :)

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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 :grin:

Dave

Nexstar-80mm.thumb.png.9aa485a1b91db0a5f1c7ad4abf89fabf.png

 

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.

 

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Don't suppose the movement that is causing star elongation could  be caused by mirror movement 
within the baffle tube. Could be a gradual sag perhaps due to lack of the lubricant used within that system.
The directional shift in different attitudes could be explained by this, If it were the case that is.
Just a thought, after all, SCT. mirror shift can be quite obvious during focusing.
Remedy was often to rack the focus mechanism in both directions to distribute the grease more evenly between the two surfaces.
Sorry guys, I'm sure this possibility may already have been thought of, and rejected.

Ron.
 

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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?

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24 minutes ago, angryowl said:

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?

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

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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.

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13 minutes 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

Dave, do you have something like a crayford focuser at the back of the OTA tube rather than using the stock focuser on SCT? I was under the impression that if you have motorised focuser on the stock focuser, you're not supposed to lock the mirrors OR have it got it all wrong?

Edited by souls33k3r
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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 ?

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12 minutes ago, souls33k3r said:

Dave, do you have something like a crayford focuser at the back of the OTA tube rather than using the stock focuser on SCT? I was under the impression that if you have motorised focuser on the stock focuser, you're not supposed to lock the mirrors OR have it got it all wrong?

A picture worth a thousand words :grin:

I can't imagine that a motor focuser fitted to the manual knob would be as good as locking the mirror after focusing must move it a bit, not that I've tried it.

Dave

Meade-focuser.png.ca396b8d7ee625dd6f1aaa57305c502b.png

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6 minutes ago, Davey-T said:

A picture worth a thousand words :grin:

I can't imagine that a motor focuser fitted to the manual knob would be as good as locking the mirror after focusing must move it a bit, not that I've tried it.

Dave

Meade-focuser.png.ca396b8d7ee625dd6f1aaa57305c502b.png

A picture does indeed make sense and that's exactly what i was implying when i said "Something like a crayford at the back of OTA". 

Well that's the thing, you don't lock the mirrors when you have the auto focuser attached to the manual knob.

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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.

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3 hours ago, angryowl said:

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.

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 

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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.

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