Jump to content

SkySurveyBanner.jpg.21855908fce40597655603b6c9af720d.jpg

Is this sensor tilt?


Recommended Posts

Am I experiencing sensor tilt here? This is a stretched single sub-frame. My guiding was good at around 0.6 px RMS. I have tried spacers in various configurations but they haven't helped. I am using an ATIK 460EX mono CCD, which doesn't have a large sensor: a  SONY ICX694AQG, 12.49mm x 9.99mm. Any thoughts would be very welcome.

 

WR134_300sec_1x1_WR134OIIILightsn2_0019 stretched.tif

Edited by Jezphil
Link to comment
Share on other sites

I suspect it's coma as there's evidence of elongation on all four sides. Less so top right but nonetheless it's there. Most elongation seems equal on the other three side's. I'm definitely no expert so I'll take my leave and leave it to much more experienced deepsky imagers, which no doubt will give a better understanding of what's going on. All the best. 

Link to comment
Share on other sites

10 minutes ago, Nigella Bryant said:

I suspect it's coma as there's evidence of elongation on all four sides. Less so top right but nonetheless it's there. Most elongation seems equal on the other three side's. I'm definitely no expert so I'll take my leave and leave it to much more experienced deepsky imagers, which no doubt will give a better understanding of what's going on. All the best. 

Thank you Nigella - useful and much appreciated. Yes, there is the effect on all four sides. Oriented in different directions though - wonder whether that still means it is coma? 

Link to comment
Share on other sites

17 minutes ago, newbie alert said:

Can you attach a JPEG of the image as I presume it's a fits file? Can't open on my phone.. also what's the scope and method of attachment, ie solid screw fittings ?  Any spacers like delrins?

 

Thank you - here you go. Scope is a SkyWatcher Evostar 120ED Pro refractor with a SW 0.85 flattener/reducer. There is also an ATIK EFW 2.2 filter wheel in the image train. I have tried the delrin-type spacers in various combinations but they don't help. The camera slots into the filter wheel. Image of this is also attached. 

WR134_jpeg version.jpg

IMG_4168.jpeg

Edited by Jezphil
Link to comment
Share on other sites

If I look at the stars in the corners, I see that in the upper two corners the stars are elongated in a direction perpendicular to the centre, while in the lower two it is in direction of the centre. I have drawn red lines in the direction of the elongation:

image.thumb.png.0adb7252fc3eda6647539b6456271eb7.png

This would indeed indicate a tilt issue, see this thread:

When the sensor is too close to the flattener, the stars all point towards the centre. When too far the point in a direction perpendicular to that. In your case the upper half of the sensor is too far, while the lower half is too close. In other words: yes, you have tilt.

Is that camera clamped to the filter-wheel? If so, you could improve that by using threaded connection. It could also be that the sensor is not perpendicular in the camera housing. You can check that by rotating the camera 90 or 180 degrees and take another shot. If the elongation stays on the same side, the issue is in the camera housing.

HTH,

Nicolàs

  • Like 1
Link to comment
Share on other sites

Ah - thank you so much guys. I really appreciate the time you have taken to respond to this issue, which has been bugging me for a long time.  What an amazing community astrophotographers are.

Regarding the red lines above showing the direction of elongation - yes, this makes complete sense and yes, the camera is clamped to the filter wheel. This had been a concern to me and I had tried to solve it by loading the image train vertically, and very carefully, but was always concerned that the clamping mechanism was still creating an issue. Obviously the electrical tape you can see in the image was a slightly desperate and vain attempt to keep things perpendicular.  I will definitely try a threaded connection now. 

Regarding the SkyWatcher Evo rotator link above - thank you for this really helpful guidance and I will now go down this route. I will need to make sure the back focus is correct.

 

Link to comment
Share on other sites

2 hours ago, Jezphil said:

Thank you - here you go. Scope is a SkyWatcher Evostar 120ED Pro refractor with a SW 0.85 flattener/reducer. There is also an ATIK EFW 2.2 filter wheel in the image train. I have tried the delrin-type spacers in various combinations but they don't help. The camera slots into the filter wheel. Image of this is also attached. 

WR134_jpeg version.jpg

IMG_4168.jpeg

A couple of points here flag up, the parts circled can show up as tilt... You don't need the nose cone on the reducer as it can screw directly onto the focuser with maybe an adapter... And the thumbscrew on the extension can throw things off line...

Btw, what's the spec on the spacing distance, yours looks quite a bit more than usual..

Good job that you're using solid baader shims rather than delrin, as they introduced tilt on my 460

Screenshot_20220702-221156.png

Link to comment
Share on other sites

11 minutes ago, newbie alert said:

A couple of points here flag up, the parts circled can show up as tilt... You don't need the nose cone on the reducer as it can screw directly onto the focuser with maybe an adapter... And the thumbscrew on the extension can throw things off line...

Btw, what's the spec on the spacing distance, yours looks quite a bit more than usual..

Good job that you're using solid baader shims rather than delrin, as they introduced tilt on my 460

Screenshot_20220702-221156.png

Thanks - yes I think there were indeed too many spacers. However, there are fewer spacers now than you see in the image - 2mm of spacers - as I removed a couple. It didn't make any noticeable difference to the issue. And that reduced amount was what I used for the test frame above. The cone on the reducer fits into a Starlight FeatherTouch focuser which I use as replacement for the original SkyWatcher one. It doesn't have a screw thread, just a clamp. But it's quite a high quality part with a wide opening/fitting for the wide cone, so perhaps the issue is more likely to be the camera with it's thinner cone going into the filter wheel clamp which doesn't seem to be as well engineered. It has just one screw operating the tightness for example. The star-shape issue was the same before I fitted the new focuser - i.e. when the reducer was screwed directly into the original SkyWatcher focuser. So this also makes me think maybe the camera into filter wheel junction could be the culprit. 

The distance between the camera with cone screwed off to the filter wheel is 35mm, so I think the rotator in the link above would need an added piece of tubing to keep the back focus correct. I will have to try to figure that out. 

Link to comment
Share on other sites

If you decide you need the rotator linked above, make sure to check that it will fit where you want it too. This rotator normally screws directly onto the standard Sky Watcher focuser draw tube and the reducer/flattener then screws onto the rotator. This configuration won't work with your Starlight focuser so it will have to be located somewhere else in the train. ;) 

Link to comment
Share on other sites

6 minutes ago, Budgie1 said:

If you decide you need the rotator linked above, make sure to check that it will fit where you want it too. This rotator normally screws directly onto the standard Sky Watcher focuser draw tube and the reducer/flattener then screws onto the rotator. This configuration won't work with your Starlight focuser so it will have to be located somewhere else in the train. ;) 

Thank you - yes, I need something to screw at one end to the ATIK 460EX camera and at the other end to the EFW2 filter wheel, keeping that distance 34mm so the back focus is correct. I think the rotator is less than 34mm, so I am looking into a solution. I think the weak point is likely to be the camera filter wheel connection, rather than the reducer/focus tube. 

Edited by Jezphil
Link to comment
Share on other sites

That looks like way too much between the camera and filter wheel. If the EF2 is 22mm and the Atik has 13mm to sensor, you should only have a 20mm spacer to make up the difference as the SW reducers are usually only 55mm requirement. That spacer looks more than 20mm to me. Also, you should have the camera threaded as close as possible to the filterwheel and the spacer between filterwheel and reducer if possible. 

 

Edit: You should have this much spacing between end of camera to the reducer.

IMG_4168.jpeg.dea0781386cd6d2653c8e919ef548aee.jpeg

Edited by david_taurus83
Link to comment
Share on other sites

8 hours ago, Budgie1 said:

This configuration won't work with your Starlight focuser so it will have to be located somewhere else in the train. ;) 

Oops. Missed the the Starlight focuser reference. Yes you are quite right - but a screw fitting system would still be a better option.

  • Like 1
Link to comment
Share on other sites

16 hours ago, david_taurus83 said:

That looks like way too much between the camera and filter wheel. If the EF2 is 22mm and the Atik has 13mm to sensor, you should only have a 20mm spacer to make up the difference as the SW reducers are usually only 55mm requirement. That spacer looks more than 20mm to me. Also, you should have the camera threaded as close as possible to the filterwheel and the spacer between filterwheel and reducer if possible. 

 

Edit: You should have this much spacing between end of camera to the reducer.

IMG_4168.jpeg.dea0781386cd6d2653c8e919ef548aee.jpeg

Hello David

Thank you for this. That total distance is: the filter wheel at 22mm + 55mm to the end of the camera housing as illustrated by your blue lines above. So that is 22+55=77mm, plus an extra 13mm to the sensor. So 90mm! And this should be 55mm? Blimey. Does this make sense? I am way, way out, right?

Link to comment
Share on other sites

32 minutes ago, Jezphil said:

Hello David

Thank you for this. That total distance is: the filter wheel at 22mm + 55mm to the end of the camera housing as illustrated by your blue lines above. So that is 22+55=77mm, plus an extra 13mm to the sensor. So 90mm! And this should be 55mm? Blimey. Does this make sense? I am way, way out, right?

Yes, I'd say that's a tad too much! Do you have a 20mm spacer?

Link to comment
Share on other sites

13 minutes ago, david_taurus83 said:

Yes, I'd say that's a tad too much! Do you have a 20mm spacer?

Correction: I measured to the wrong part of the camera, but it is still well over...

filter wheel: 22mm

sensor set back 13mm

cone to edge of camera housing 32mm

total 67

...so I am 12mm over.

I don't have a 20mm spacer but will get hold of one. I need to figure out the correct widths and thread sizes.

 

 

Link to comment
Share on other sites

On 03/07/2022 at 16:50, david_taurus83 said:

Yes, I'd say that's a tad too much! Do you have a 20mm spacer?

Okay, I ran test frames last night after switching in a 20mm spacer and screwing rather than clamping the camera to the filter wheel to avoid sensor tilt. Back focus was the correct 55mm. I have round stars in the margins at last!

Thanks so much for your help on this - you totally nailed something I hadn't understood and that's been bugging me for a long time and I am hugely grateful. I am sending a virtual beer to Birmingham which, apologies, I know isn't as good as an actual one.

I got back home to London, after testing in a field out of town, at 5am this morning and accidentally set off the house alarm, waking up the family. So if you also offer marriage guidance counselling... ?

 

  • Like 1
  • Haha 1
Link to comment
Share on other sites

On 02/07/2022 at 21:20, newbie alert said:

A couple of points here flag up, the parts circled can show up as tilt... You don't need the nose cone on the reducer as it can screw directly onto the focuser with maybe an adapter... And the thumbscrew on the extension can throw things off line...

Btw, what's the spec on the spacing distance, yours looks quite a bit more than usual..

Good job that you're using solid baader shims rather than delrin, as they introduced tilt on my 460

Screenshot_20220702-221156.png

Thank you - yes, you nailed it. Many thanks and I tested the corrected spacing last night in a field outside Edenbridge and I now have round stars. The camera now screws rather than clamping to the filter wheel which I am sure was indeed a dodgy connection. The other clamp you highlighted goes into a Starlight Feather Touch focuser which seems very finely engineered and snug and doesn't seem to create an issue. 

  • Like 1
Link to comment
Share on other sites

3 hours ago, Jezphil said:

Thank you - yes, you nailed it. Many thanks and I tested the corrected spacing last night in a field outside Edenbridge and I now have round stars. The camera now screws rather than clamping to the filter wheel which I am sure was indeed a dodgy connection. The other clamp you highlighted goes into a Starlight Feather Touch focuser which seems very finely engineered and snug and doesn't seem to create an issue. 

Great that you have it sorted..

Eden bridge isn't too far from me,. I'm in Ashford 

Link to comment
Share on other sites

22 minutes ago, newbie alert said:

Great that you have it sorted..

Eden bridge isn't too far from me,. I'm in Ashford 

Ah - it's within striking distance of London and Bortle 4(ish) in the countryside there. I'm an MKAS member, so another Kent astronomy link there.

Link to comment
Share on other sites

3 hours ago, Jezphil said:

Ah - it's within striking distance of London and Bortle 4(ish) in the countryside there. I'm an MKAS member, so another Kent astronomy link there.

Mid Kent is that the bredhurst one?

I'm a SEKAS member

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.