Tak FSQ85 radial star elongation

[SyedT]

Hi all,

I recently acquired a Tak FSQ85-EDX secondhand, and I'm using a QHY163M to image with it; there is notable star elongation in the corners and on the sides in a radial pattern, rather like when flattener spacing in a triplet scope is too much. However, since this has a built-in flattener, this shouldn't be the issue from what I've been told. The previous owner did note some slight elongation at the corners which I was ok to accept since I got the scope at a very good price. I've read around and opinions seem to differ as to the cause of this, ranging from replacing the focuser to sending it back to the factory for recollimation. Would appreciate any thoughts here. Images from a single session: https://drive.google.com/open?id=1H6JPMMc8C3C6iUxHBGIhcFquPnEtmzhl

Cheers

[steppenwolf]

I had two examples of this instrument both of which had this issue. Unfortunately, it is the nature of the some of the beasts, however, there are some examples out there that apparently don't exhibit this flaw. I was advised to focus on an off-axis star to spread the defocus evenly across the FOV - a solution that I didn't find acceptable!

[Ben the Ignorant]

Wolfgang Rohr knows how to adjust a Takahashi quadruplet, here's the job he did on a 106:

http://r2.astro-foren.com/index.php/de/12-beitraege/04-zweispiegel-systeme-astrofotografie/674-d109-tak-quadruplet106-530-sand-im-getriebe

He charges 80€ or 120€, I read it somewhere but I don't remember the right sum. This is his email adress:

wolfgang.rohr@t-online.de

I had a few questions about a friend's triplet and Rohr replied quickly for free, he likes to cooperate.

[Vixen4eva]

I`ve read of similar issues with these...... Taks used to be the dogs!!! My old FS78 was stunning. Seems everything is built to a cost now or QE is lacking at the factory.

Alan

[kirkster501]

2 minutes ago, Vixen4eva said:

I`ve read of similar issues with these...... Taks used to be the dogs!!! My old FS78 was stunning. Seems everything is built to a cost now or QE is lacking at the factory.

Alan

In excess of £3000 for a small telescope is hardly built to a cost Alan.

[Vixen4eva]

I know what your saying £3000 is alot of money to shell out you`d think they`d get them right before they leave the factory. Can`t be doing there reputation much good..

[kirkster501]

Here is a raw L 10 minute sub - just stretched - of M31 through my FSQ85/reducer on a 8300 chip from October this year.  This is *with* the reducer (so at F3.9 so very exacting).  I too have some slight radial distortion if you zoom in, and some subs are worse than others.  However, I can live with this I think in my example (that I bought from IKI in Nov 2013).

[kirkster501]

Delete post don't want to thread hijack OP's post.

[SyedT]

5 hours ago, steppenwolf said:

I had two examples of this instrument both of which had this issue. Unfortunately, it is the nature of the some of the beasts, however, there are some examples out there that apparently don't exhibit this flaw. I was advised to focus on an off-axis star to spread the defocus evenly across the FOV - a solution that I didn't find acceptable!

Absolutely not acceptable! 

4 hours ago, Ben the Ignorant said:

Wolfgang Rohr knows how to adjust a Takahashi quadruplet, here's the job he did on a 106:

http://r2.astro-foren.com/index.php/de/12-beitraege/04-zweispiegel-systeme-astrofotografie/674-d109-tak-quadruplet106-530-sand-im-getriebe

He charges 80€ or 120€, I read it somewhere but I don't remember the right sum. This is his email adress:

wolfgang.rohr@t-online.de

I had a few questions about a friend's triplet and Rohr replied quickly for free, he likes to cooperate.

Thanks Ben! Have emailed him, will hopefully have some luck.

2 hours ago, Vixen4eva said:

I`ve read of similar issues with these...... Taks used to be the dogs!!! My old FS78 was stunning. Seems everything is built to a cost now or QE is lacking at the factory.

Alan

Yeah, it's only recently that they've gone downhill I see...

58 minutes ago, kirkster501 said:

Delete post don't want to thread hijack OP's post.

Not to worry at all! Please continue posting, I'm all for discussion. The more opinions and experiences I can get on board the better to be honest, would like to sort this. I think with the reducer and larger field of view you're at least able to crop out a fair amount. I expect adding a reducer to my train will only make things worse?

[Skipper Billy]

Just for your info I ran some of your subs through CCD Inspector - expecting them to show horrible tilt or curvature - surprisingly - they don't!

I cant explain that.

I tried a few at random - later I will run them all through and see if it sheds any light on the problem.

 

[Skipper Billy]

Update - just ran all your subs through and they are all almost exactly the same.

I guess that rules out anything moving around or loose !

Good luck getting it resolved - I have to admit I would be really upset!

[SyedT]

4 hours ago, Skipper Billy said:

Update - just ran all your subs through and they are all almost exactly the same.

I guess that rules out anything moving around or loose !

Good luck getting it resolved - I have to admit I would be really upset!

Thank you for running it through, much appreciated! Yes, I'm getting the feeling that this is a QC issue from Tak; quite a poor effort on their part considering the price really!

[nicoscy]

I asked re Herr. Rohr in the past and got a quotation of EUR 120 + shipping costs for adjusting a scope. Posted on CN about that.

However, this is something that Remi in France (authorized service center for Takahashi) can do for you and probably at no charge as the scope is under guarantee? 

[nicoscy]

Sending you Remi's email via PM

[SyedT]

Can I just say, that yes I'm having a major issue with the Tak, but the responses I've received on this thread and via PM have been very helpful, going above and beyond to help out with sorting this. This is exactly why I've never needed to go to another forum! ?

[nicoscy]

For those who do not want to go to the expense of paying for CCD Inspector, a free tool is ASTAP: http://www.hnsky.org/astap.htm

It has many functions but I just use it for tilt and curvature 

[nicoscy]

I used the software on one of the images in Google Drive provided by Syed and it shows the same thing as CCD Inspector. However, what CCD Inspector calls curvature, I would call tilt as it is more pronounced to one side significantly.

I have read many reports of FSQ85 owners just giving up and replacing the stock focuser for a FeatherTouch focuser. However, this may be premature since first the imaging train must be examined component by component for tilt and then possibly using a shim to correct.

@SyedT given the orientation of your scope when taking the images and the position of the curvature / tilt, can you try imaging again the same object well before and well after a meridian flip? An inspection of the two images should provide a clear indication if there is flexure / focuser sag / other component sag in the imaging train. Quite possibly, the optics are just fine and it is a mechanical issue.

[SyedT]

1 minute ago, nicoscy said:

I used the software on one of the images in Google Drive provided by Syed and it shows the same thing as CCD Inspector. However, what CCD Inspector calls curvature, I would call tilt as it is more pronounced to one side significantly.

I have read many reports of FSQ85 owners just giving up and replacing the stock focuser for a FeatherTouch focuser. However, this may be premature since first the imaging train must be examined component by component for tilt and then possibly using a shim to correct.

@SyedT given the orientation of your scope when taking the images and the position of the curvature / tilt, can you try imaging again the same object well before and well after a meridian flip? An inspection of the two images should provide a clear indication if there is flexure / focuser sag / other component sag in the imaging train. Quite possibly, the optics are just fine and it is a mechanical issue.

Thanks Nicos, I can give that a try. I did try imaging pointing straight up, and still got the same result with star elongation in all 4 corners and the sides. I suspect it's a combination of curvature and tilt?

[nicoscy]

There may be some lateral pressure inside the focuser due to screws etc which may produce the same result even if scope is pointed up.

Same object, say one hour before and one hour after meridian flip should provide a very clear indication

[Skipper Billy]

Deleted post - the link to the free version of CCD Inspector no longer works. - Sorry !

 

 

[steppenwolf]

3 hours ago, SyedT said:

I did try imaging pointing straight up, and still got the same result with star elongation in all 4 corners and the sides. I suspect it's a combination of curvature and tilt?

This would rather indicate optical rather mechanical issues. I would be wary of changing the focuser at this point until you have ruled everything else out. I was able to prove conclusively that my issues were optical - i.e. the filed is not as flat as I was expecting. Of course, like me, you also have the option of accepting that the FSQ 85 does produce very fine images except at the field edges and enjoy it with this limitation and just crop your images but for me, the whole reason for the purchase was for a very wide field of view using a sensor the same size as your but with larger pixels than yours. I have no doubt that you achieve some great images with this telescope.

The focuser is a potential weak-point but in fairness, I felt that mine was a pretty good example.

[Davey-T]

According to someone who should know that I was talking to a few years ago when thinking of purchasing one the FSQ85 is an old optical design from before amateur images got hold of big sensors and became very obsessed with pixel peeping so the QC hasn't changed from the acceptable at the time standard and Tak don't seem inclined to alter that.

The 106 on the other hand has later design optics and hopefully better flat field to the corners.

I ended up with a good WO Star71 Mk1 but that's another can of worms 

Dave

[nicoscy]

Did a quick analysis of each image. The tilt changes between 10% (best) and 14% (worse) between images, which to my simple mind means something is not tight enough. 

Yes, there is also a collimation issue, because the direction of the tilt is consistent, but I think the focuser is not playing a fair game as tilt would be constant if there was no play.

So, it seems to me both an optical and mechanical issue.

 

[AngryDonkey]

Maybe this would be throwing good money after bad but if you don't use a reducer you could consider adding the new flattener for the scope. Not sure if this would cure it though, maybe someone else has experience with it? And yes I agree, it shouldn't be required for your sensor size, not for that kind of money!