gnomus

During the phone call the retailer kept repeating that the tilt was only 0.02 mm (he must have said this about a dozen times) and that this amount of tilt would be difficult to eliminate - it being so small. My response to him was that I was looking at an image that had ovoids that encroached almost to the centre of my images and that this was unacceptable to me. Would 'tilt' give me the radial pattern that I am seeing - the ovoids go in different directions in each corner? The retailer said I only had tilt in one axis (I think the Y axis, but I'm awaiting the report from Tak).

I think you will find that the Sale of Goods Act affords consumers considerably more protection than this, especially in cases where the item is not of satisfactory quality, as described (pinpoints out to 22 mm from optical centre) and/or not fit for purpose. The worst case scenario is the County Court, but I would prefer to avoid that (although I have found with people who have not paid me that it is effective). What I found most disappointing was the supplier's attitude.

A rather strange development. Having said that he would get me a new FSQ 85, the supplier has now reneged on this. He tells me that Takahashi have examined my FITS files and determined that I have a very slight tilt somewhere in my set up. This is of the order of 0.02mm. The supplier tells me that this is somethng that Takahashi may not be able to correct (they could correct a larger error). The supplier could not tell me how a sensor tilt could give rise to a radial distortion pattern. He went on to tell me that the problem was exaggerated by my having a camera with such a small sensor pixel size. I suggested that my sensor pixel size was not especially small at 5.4 microns and that there were many cameras with smaller pixel sizes than this. "In which case they would show the problem more", was the response. It was suggested to me that I should have a pixel size of 7 or even 9 microns. When I pointed out that I had made it clear from the outset that I intended to use an Atik 383L with the scope, he responded that it was I who had decided to purchase the scope despite having this camera. I said that it seemed unlikley that we were going to be able to resolve the matter and I said that I wished to return the scope for a refund. The retailer refused to take the scope back and refund me. He did offer to send the scope to Takahashi so that they could issue me with a "certificate" confirming that there was nothing optically amiss. I am now having to consider the best way forward. Has anyone been in this situation before? Any advice? Am I right to assume that I would not get radial distortion if this was a tilt somehere in the imaging chain?

Oh dear. I really am sorry.

I believe that that chip is physically smaller than the Kodak, correct? Whether or not you return your Tak is, of course, a matter for you. Are you happy with what it is doing in your corners? Have you tried the 5 second pointing at the zenith test?

I had perfectly acceptably (I suppose nothing is perfect) round stars with the same camera on my ED80. I've spent enough time on this particular scope and it is going back. I hope that I had a bad copy, but you and I are not the only people who have had radial distortion is our corners. Are you using a KAF 8300 sensor?

Already done. The supplier is getting a new FSQ85 in for me. No need to be sorry ..... although ..... on reflection .... this is all your fault!!!! I hope the new one works....

Thanks Olly. I have taken a crop from the same region of the 5 second and 300 second exposure and upscaled it a little. I hope this helps. There are clearly less stars on the 5 second image. On the 300 second image the stars are brighter and a bit more 'swollen'. Nevertheless, the ovoids look to be identical in proportion. I've circled a few just to give an idea. What do you think?

Takahashi state: As on the FSQ-106ED, the built-in field flattener, perfectly [my emphasis] designed and manufactured, gives an almost homogeneous flat field of 44mm diameter. At prime focus, at the edge of field (22mm from the optical axis), stars remain pinpoints [my emphasis again] ... My view is that if the scope does not function as promised then it should be returned.

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!

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?

And I am not using the Tak reducer.

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):

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

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

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 can certainly give it a go.

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.

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

Was that your attempt at a Cockney accent Stephen?

No need to sell it Dave - if it ain't right it's going back. (You weren't hoping to pick up a bargain were you?)

Thanks again for looking at these Mark. I can imagine a circle drawn around the yellow lines in your analysis of my 1200 second exposure (except for perhaps the middle left panel). The 600 second analysis appears to show random errors. I'm not sure that your figures for the 600 second exposure marry up with what I am seeing on screen. If I understand your data correctly, your script is suggesting that my roundest stars are in the middle left panel. This is not what I see in the processed image. I should add that last night was quite horrible. Visibility was only 4 miles according to 'Cloudy Outside'. Not only that but temperature and dew point were pretty close. The equipment and observatory walls were dripping wet again after only an hour or so. When I looked into the objectives at the end of the session, I wondered if I had my dew heaters turned up high enough. Certainly the PHD2 image seemed to be out of focus by the end of the exposures. The neighbours decided to set off a number of fireworks and from time to time firework smoke drifted across the field of view. I do recall also that PHD2 reported one of those "guide camera unresponsive" messages during the 5 min exposure (although I reconnected everything pretty quickly and the guide trace seemed unaffected). Finally, one disadvantage of having an observatory is that you can get set up and going too quickly. I knew I only had limited time last night so whipped the roof off and got going immediately without leaving any time for the gear to cool down. The 10 min exposure was taken first and started before astronomical darkness. Thanks again Steve

I am not using the reducer. I believe that the Tak has an inbuilt flattener. I am using the correct Tak rings as specified in the manual.

Thank you. As I said in my probably too lengthy post I also shot a 10 min Luminance sub and I couldn't make out any significant ovoid-osity in the corners. Field rotation it is then.I will never post any smug 'look-at-how-flat-my-guide-trace-is' images again after your AstroArt screenshot! How are you doing that? BTW, I am now using a 600mm guide scope (the ED80) and it does not seem that it is possible to set my ZWO guide camera to bin - there is an option under the PHD2 brain, but it is greyed out. So I think that gives me 1.29 arcseconds per pixel. I intend improving my PA by looking at the video coming from my guide scope - that will work won't it? I don't need to be looking through the imaging scope do I? (I really don't fancy unscrewing all those Tak adapter rings again.)

Congrats Steve