Ovoids with Mesu and Tak FSQ 85

[sharkmelley]

Sounds like this could descend into one of those imaging train ? tilted sensor ? ??????????? threads, best sell it all and go back to EQ6 ED80, much less stressful 

Dave

My guess would be tilt. But sensor tilt or focuser tilt?

In this thread:  http://stargazerslounge.com/topic/197914-first-light-with-fsq-85-at-last/page-2  steppenwolf had an FSQ85 problem which looked like rotation but wasn't.

Mark

[gnomus]

I used the camera with my ED80 and got round stars in the corners - and this was using unscrewed connections. I believe steppenwolf eventually returned his Tak.

Steve

[gnomus]

I had a look back at some of my earlier images.  I can see the egginess in my Elephant Nebula shots from early October when I had pretty good PA (these are Tak 85, Atik 383 and Mesu) 20 minutes:

Link to FIT: https://www.dropbox.com/s/bbj874jco0ci58d/Elephant_1200sec_1x1_HA_frame5_c.xisf?dl=0

My Bodes Nebula image from a month earlier is significantly better to my eyes.  There is a slight issue top right, but I would not have noticed I think unless I had been looking for it.  The corners are certainly not as bad as on the Tak.  This is ED80, Atik 383 and CGEM (probably not as well polar aligned as the Mesu is now) - 10 minutes: 

Link to FIT: https://www.dropbox.com/s/wofjg4u8klv44va/Bodes%20Nebula_600sec_1x1_L_frame4.fit?dl=0

It seems that the Mesu is performing better than the CGEM so this must be down to some combination of Tak 85 and Atik 383L.  If this is a sensor tilt issue, then I might have expected to see it on the ED80 but I didn't.  If this is a focuser tilt issue on the Tak then I am not sure that I know how to fix this.  

[swag72]

If there's any thought that it may be a focuser tilt issue, then how about a test run of focusing at zenith? That way all of the weight will be pulling the focuser directly downwards and not to one side or another.

[gnomus]

If there's any thought that it may be a focuser tilt issue, then how about a test run of focusing at zenith? That way all of the weight will be pulling the focuser directly downwards and not to one side or another.

I can certainly give it a go.

[sharkmelley]

The image of Bodes Nebula is very much better than the rest.  Do as Sara suggested and also try taking a test image in different parts of the sky on the same evening.  For each image make a note of which part of the sky it is in.  Analysis of the results might show if some part of the imaging train (e.g. focuser, filter wheel, camera adaptor) is sagging.  It is worth do a careful inspection of the assembly to test if there is the slightest give in any of the components which might be causing the problem.

Mark

[ollypenrice]

Good ideas from Mark and Sara. A meridian flip on a southern target would turn the scope over but the target would be the same, so you'd see if the distortions reversed themselves as they ought to do if it were the focuser. (I'd give it ten minutes to re-sag, so to speak...) This is certainly becoming a candidate. If the sensor were not tilted in its previous scope then why would it be tilted in this one? If this doesn't happen then we'd be looking at slight mis-collimation which is not unknown.

I know that Per has discussed the Tak focuser and one place to ask for advice on adjusting is Texas Nautical Repair, who are very expert on all things Tak.

Olly

[gnomus]

Good ideas from Mark and Sara. A meridian flip on a southern target would turn the scope over but the target would be the same, so you'd see if the distortions reversed themselves as they ought to do if it were the focuser. (I'd give it ten minutes to re-sag, so to speak...) This is certainly becoming a candidate. If the sensor were not tilted in its previous scope then why would it be tilted in this one? If this doesn't happen then we'd be looking at slight mis-collimation which is not unknown.

I know that Per has discussed the Tak focuser and one place to ask for advice on adjusting is Texas Nautical Repair, who are very expert on all things Tak.

Olly

Thanks.  I will give that a go when next we get clear(-ish) skies.  I'm not at all sure that I am up for adjusting the collimation on a scope of this cost.  I would probably make the slight mis-collimation even worse!  In any event, I have only had it for a few weeks - it should have come to me properly set up.  

I spent a little time this morning looking through images on Astrobin that had been taken using the FSQ 85 and the Atik 383L.  It was a little difficult to tell because most images are resized to less than 2000 pixels on the long end.  Nevertheless, I do have to say that I found quite a few images (maybe the majority that I looked at) that appeared to show corner elongation similar to mine.  I began to wonder if I would find these issues if I were using a camera with a smaller sensor.  Is the design of the Tak such that it is particularly prone to orthogonality issues?  On the ED80 the camera was attached in the crudest possible way - a 2" nose piece and the brass compression ring in a Moonlite focuser.  I find it difficult to believe that this beats the out-of-the-box Tak set up.  

It is all a little frustrating since I bought this thing so that I could do some imaging, not spend what few decent nights I get 'testing'.   

[steppenwolf]

It is all a little frustrating since I bought this thing so that I could do some imaging, not spend what few decent nights I get 'testing'. 

Now, I do know how you feel here!

[ollypenrice]

Thanks.  I will give that a go when next we get clear(-ish) skies.  I'm not at all sure that I am up for adjusting the collimation on a scope of this cost.  I would probably make the slight mis-collimation even worse!  In any event, I have only had it for a few weeks - it should have come to me properly set up.  

I spent a little time this morning looking through images on Astrobin that had been taken using the FSQ 85 and the Atik 383L.  It was a little difficult to tell because most images are resized to less than 2000 pixels on the long end.  Nevertheless, I do have to say that I found quite a few images (maybe the majority that I looked at) that appeared to show corner elongation similar to mine.  I began to wonder if I would find these issues if I were using a camera with a smaller sensor.  Is the design of the Tak such that it is particularly prone to orthogonality issues?  On the ED80 the camera was attached in the crudest possible way - a 2" nose piece and the brass compression ring in a Moonlite focuser.  I find it difficult to believe that this beats the out-of-the-box Tak set up.  

It is all a little frustrating since I bought this thing so that I could do some imaging, not spend what few decent nights I get 'testing'.   

I wouldn't try to collimate it! It was the focus adjustment that I think you might be able to tweak painlessly if you find evidence of sag. If it's out of collimation in needs to go back.

Olly

[gnomus]

I wouldn't try to collimate it! It was the focus adjustment that I think you might be able to tweak painlessly if you find evidence of sag. If it's out of collimation in needs to go back.

Olly

Apologies - I meant adjusting the focuser (I thought that was called collimation too). I had early warning of my ineptitude in engineering matters when I tried adjusting the reticle in my polar scope using the three little grub screws. The advice on the internet (just like the advice in the old Hayes manuals) usually begins, "Simply adjust...." It always tickles me that phrase.

Steve

[strutsinaction]

Hi Steve

Elongated stars in the corners may also be due to play in the CAA (camera angle adjuster). I assume you have the CAA in your optical train? See here for more info and a possible fix (if you are brave):

https://groups.yahoo.com/neo/groups/UncensoredTakGroup/conversations/topics/43074

The following may help too:

https://groups.yahoo.com/neo/groups/UncensoredTakGroup/conversations/topics/44706

http://www.skyimager.com/FSQ-MMOAG/Using%20an%20Astrodon%20MonsterMOAG%20with%20a%20Tak%20FSQ-106N-2.pdf

I also have a Tak FSQ85 and have noticed elongated stars in one corner of my images and suspect the CAA, focuser slop or sensor tilt is the cause. It hasn't concerned me so far so haven't done anything about it but I may follow up with Ian King just in case this is a known issue.

Regards

John

Edited November 2, 2015 by strutsinaction

[gnomus]

Hi John

Thank you for those links. I do have the CAA in my train. I am going to try Sara's imaging at the zenith idea. That will tell me whether or not this is 'slop' of one form or another or field rotation. As I said, I thought I saw a lot of elongation in the corners of images of other users on Astrobin who had my scope/camera combo. My problem is that I don't really know what I should be prepared to accept when it comes to corner elongation. I must say that at this price point I was not expecting the corners to be worse than I had before.

I have contacted the supplier. I don't think that I should be having to dissolve glue and be making these fine adjustments on a brand new scope - assuming of course that that is what the problem turns out to be.

If you decide to take your own issues further, then it would be interesting to hear what Mr King has to say. I use a different supplier.

Steve

[gnomus]

I finally managed to get a brief window when the rain stopped and the skies cleared.  I did the test suggested by Sara - that is I took a series of pictures with the telescope pointed at the zenith.  Since the camera and focuser are hanging straight down, these images should not be affected by any play in the focuser or CAA (Camera Angle Adjuster).  In an attempt to rule out a significant polar alignment error, I took exposures of different lengths, from 5 seconds to 300 seconds.  If my problem was field rotation due to polar misalignment then one would expect ovoids not to be visible at all in the really short exposures and for any ovoids to get longer with increased exposure time.  Clearly there will be less stars in the shorter exposures.  The result is that I have ovoids present in at least 3 of the four corners of all images.  To my mind the worst affected area in all images is the bottom right corner.  I do not think that the extent of the ovoid distortion changes significantly from one image to the next (there may be some differences in 'brightness' due to the exposure times and my processing (which is just some stretching)).  

The distortions seem to form in a circular type pattern around the centre (all centre stars are round in my images). 

The final image is an exposure of M31 taken with the same camera and filter wheel but through my ED80.  The stars in the ED80 corners all look acceptable round to me.   

Incidentally, each of the corner panels is cropped in such a way that it shows just under 1000 pixels square - so that is a reasonable chunk of the sensor.

Any ideas as to what might be causing these issues with the Tak?  Is this coma?

(I suggest clicking on the image to get the full-size view):

[Owmuchonomy]

Have you consulted this thread?  On the second page Ian posts an excellent diagram of the consequences of incorrect spacing.

http://stargazerslounge.com/topic/225831-flattener-spacing/page-2

[Zakalwe]

Have you consulted this thread?  On the second page Ian posts an excellent diagram of the consequences of incorrect spacing.

http://stargazerslounge.com/topic/225831-flattener-spacing/page-2

Steve is using a Tak 85, which is a Petzval design. There is no reducer or flattener in use (outside the Petsval) and the design means that, AFAIK, spacing isn't that critical.

Having said that, it'd be worth playing about with spacing.

Is it worth playing with focus as well? If the focus is set bang-on in the centre of the imaging circle then there is a chance that it will be out at the edge, especially with a large sensor. After all, no focal plane is perfectly flat. What about setting the focus bang-on about 20-55% out from the centre? That might give the best balance of focus and minimize distortions? :icon_scratch:

Edited November 8, 2015 by Zakalwe

[gnomus]

Steve is using a Tak 85, which is a Petzval design. There is no reducer or flattener in use 9outside the Petsval) and the design means that, AFAIK, spacing isn't that critical.

Having said that, it'd be worth playing about with spacing.

Is it worth playing with focus as well? If the focus is set bang-on in the centre of the imaging circle then there is a chance that it will be out at the edge, especially with a large sensor. After all, no focal plane is perfectly flat. What about setting the focus bang-on about 20-55% out from the centre? That might give the best balance of focus and minimize distortions? :icon_scratch:

And I am not using the Tak reducer.

[steppenwolf]

When I had the same issue with my Tak 85, it was suggested to me that I should try offset focusing. Now, I was prepared to consider this on a camera lens or cheaper telescope but not on an instrument at this price point - that was not what I bought in to!

I expected a flat field on an 8300 sensor but that's not what I got so it went back. Perhaps my expectations were too high (?) but then, so too was the promise!

Offset focusing might be worth a try for you and if you are happy with the results - and many owners must be - then you have the solution. At least you can now rule out focus slop leaving you with CAA adjustment, sensor tilt or the plain fact that the field is not perfectly flat for your sensor size.

I hope this turns out to be an adjustment for you because the potential FOV of the Tak 85 makes it a very desirable instrument.

Sent from my iPhone from somewhere dark .....

[gnomus]

When I had the same issue with my Tak 85, it was suggested to me that I should try offset focusing. Now, I was prepared to consider this on a camera lens or cheaper telescope but not on an instrument at this price point - that was not what I bought in to!

I expected a flat field on an 8300 sensor but that's not what I got so it went back. Perhaps my expectations were too high (?) but then, so too was the promise!

Offset focusing might be worth a try for you and if you are happy with the results - and many owners must be - then you have the solution. At least you can now rule out focus slop leaving you with CAA adjustment, sensor tilt or the plain fact that the field is not perfectly flat for your sensor size.

I hope this turns out to be an adjustment for you because the potential FOV of the Tak 85 makes it a very desirable instrument.

Sent from my iPhone from somewhere dark .....

Thanks Steve. Like you I take the view that I was promised a flat field and I should not have to resort to such strategies. In any event I run an autofocus routine through SG Pro and I am not at all sure that I would know how to offset focus with this set up.

I have seen good reviews of the WO 71 and the TS Optics 65. These also give a wide FOV I believe. Does anyone know if these scopes are capable of giving round stars?

[Davey-T]

WO Star 71s have not been without their prob's but generally folks including me get round stars all over, I'm using QSI683 with built in filter wheel and 1.25 mounted filters, no problems, just alarming vignetting which is processed out.

For the price of the Tak you could have a dual Star 71 imaging rig and no adapters / reducers needed 

Dave

Edited November 8, 2015 by Davey-T

[steppenwolf]

The TS 65 had a bad start in life (especially when cold) but in fairness, the one that I had for a magazine review was very good indeed.

I have no direct experience of the WO 71 but will be watching with interest to see how well it is received over time.

Imaging should not be this fraught!! [emoji16]

Sent from my iPhone from somewhere dark .....

[sharkmelley]

Here's an off-the-wall thought. Were your latest batch of test images taken with the Ha filter? 

Have you taken a look at the spot diagrams published by Takahashi for the FSQ-85ED scope?   http://www.takahashi-europe.com/en/FSQ-85ED.optics.spots.htm

The spot diagrams show that stars at the Ha wavelength will become distinctly ovoid in shape 14mm away from the centre of the sensor. The corners of a KAF-8300 sized sensor are 11mm from the sensor so maybe it will be similarly affected.  The Ha wavelength is particularly affected by this ovoidness.  Maybe that is why your luminance image of Bode's Nebula looked almost perfect but the Ha images show the issue.

Perhaps the scope is performing more or less within the tolerances of its design and is unable to illuminate the 8300 sensor without distortion at the Ha wavelength?

Mark

Edited November 8, 2015 by sharkmelley

[swag72]

I like my FSQ85 and I would not change it. But here's me being totally honest ......... Would I buy a new one today? I doubt it. 

I've heard too many people having issues on the forum and had too many conversations with folks ........ It really does seem like pot luck as to whether you get a decent scope that is worth the best part of £4000 when everything is added in.

Now the WO Star 71 does seem interesting and I know that they have not been without issue either. But at under £1000 I would accept this more readily than with a £4000 Tak. Are you going to see a £3000 increase in image quality between the WO and FSQ85? 

I don't know if it's QC issues with Takahashi these days or whether unfairly I have only heard of the bad ones when there are thousands of great ones out there.

That's just my opinion as a very happy Tak owner

[gnomus]

Here's an off-the-wall thought. Were your latest batch of test images taken with the Ha filter? 

Have you taken a look at the spot diagrams published by Takahashi for the FSQ-85ED scope?   http://www.takahashi-europe.com/en/FSQ-85ED.optics.spots.htm

The spot diagrams show that stars at the Ha wavelength will become distinctly ovoid in shape 14mm away from the centre of the sensor. The corners of a KAF-8300 sized sensor are 11mm from the sensor so maybe it will be similarly affected.  The Ha wavelength is particularly affected by this ovoidness.  Maybe that is why your luminance image of Bode's Nebula looked almost perfect but the Ha images show the issue.

Perhaps the scope is performing more or less within the tolerances of its design and is unable to illuminate the 8300 sensor without distortion at the Ha wavelength?

Mark

Thank you for bringng this to my attention. These images were taken with the Lum filter though - I thought 5 secs through an Ha filter might be a bit optimistic!

[libraryman]

Late to this thread, but I was worried about my fsq106ed (new Q) and oval stars!

I decided to make a proper test using CCD inspector!

The advice is to take 10 or so 20 sec sub's..taken from a fairly rich field devoid of globular's and nebula...unguided .....stack them together in CCD inspector and then look at the curvature n tilt.

The logic being that guiding etc will distort the stars.

This should eradicate the sensor and the focuser tilt as being the cause of oval stars which it did for me, though I did also use a DTI to measure all sorts of movement, including the Captains wheel effect, which turned out to be negligible.

Aside from this, there are a set of grub screws on the side of the focuser that you can adjust to just stiffen up the focuser (on the 106 at least). Though friend of mine has loads of wobble on his FSQ 106 and has not been able to remove it by this means.

Frustrating issue with Taks !

Ray