Using an artificial star to adjust focuser tilt in a refractor?

[tim22]

I appear to have have an issue with tilt somewhere in my Apo  4 inch refractor which is fitted with a Moonlight focuser.  The issue does not appear to be sensor tilt (ASI 294MM camera) or anywhere in the optical  train from the reducer/ flattener to the camera since when I rotate this entire assembly through 180 degrees  realtive to the focuser  the 'tilt pattern'  of those sectors of the rectangular image that show elongated stars  also rotates accordingly.

So - the tilt presumably arises from some slight misalignment of the object lens within the tube and/or of the focuser to the tube.  I have tightened up the focuser - no evidence that there is any flex in the draw tube.  The Moonlite C2.5" focuser has four (effectively 3)  screws  that can be used to adjust tilt by slightly shifting the spacing between two flanges.  However I am a bit wary of adjusting these until I am clear exactly what I am doing  and also of the fact that it will be difficult to adjust at night under stars -- constantly imaging, analysing star shape across the frame (PI Abberation inspector etc)  and then adjusting again etc.    And of course so few clear nights and is that how you what to spend them?

So I am very interested to know whether anyone has experience of  using an artificial star for solving this sort of problem?  Browsing through the SL archives I saw one (I must say of many) rather brilliant posts from @vlaiv on this particular topic in 2018 in which (if I paraphrase correctly)  he suggested focusing on an artificial star  lined up dead centre of the image and then -- using Sharpcap's ability to measure and map FWHM values  --  track how far best focus apparently shifts as you shift the telescope to align with the four corners of the sensor.  Identify the 'worst'  - most shifted  corner.  Try  first adjusting  tilt to reduce the problem at this corner - then go back to the centre again and repeat etc.

I wondered whether anyone has actually tried this in practice -- or alternatively any other suggestions ?

Tim

 

[dweller25]

Is your Moonlight pointing to the centre of your objective ?

My Moonlight was 10mm off when I received it.

I made a paper template with a central hole to cover the objective, then put a collimated laser in the focuser - if its not central you can use the adjustment screws to correct it.

[windjammer]

Have you tried the conventional cheshire-crosshairs route ?  The idea is to have the lens cap on, illuminate the cheshire and look at the reflections of the crosshairs coming back from the interior objective lens surface(s).  The reflections should align with the original crosshairs.  Adjust focuser in the OTA tube if a primitive SW scope (like mine), or the lens cell bits if collimate-able, until reflections line up.

If you fiddle around with an eyepiece you can use your camera to view the action while you wield the spanners -makes life easier.  See pic.  Usual issue with a cheshire is making sure it fits properly in the focuser - no wobbles or tilts.  Worth a try if you haven't done it.

Simon

[tim22]

Thanks David!  Gosh 10 mm is a lot!  That is a good suggestion that I haven't tried.  It could be misalignment rather than tilt.  Will try it

Tim

[tim22]

15 minutes ago, windjammer said:

Have you tried the conventional cheshire-crosshairs route ?  The idea is to have the lens cap on, illuminate the cheshire and look at the reflections of the crosshairs coming back from the interior objective lens surface(s).  The reflections should align with the original crosshairs.  Adjust focuser in the OTA tube if a primitive SW scope (like mine), or the lens cell bits if collimate-able, until reflections line up.

If you fiddle around with an eyepiece you can use your camera to view the action while you wield the spanners -makes life easier.  See pic.  Usual issue with a cheshire is making sure it fits properly in the focuser - no wobbles or tilts.  Worth a try if you haven't done it.

Simon

 

Thanks Simon,  

I was completely unaware of the possibility of using a Cheshire in that way.  A very useful  suggestion and something to try.  But the thing that you really opened my eyes to was the fact that you can put a lens cap on and then look at the reflections back from the lens ! I am more used to collimating Newts and so already have a fairly well aligned Hotech laser as well.

So I just tried using the laser - which is a decent fit into the focuser -  as well as the Cheshire (needed to get quite a bright light to see it) and -actually the alignment looks really quite good

So unfortunately it looks like I am back to the tilt question again -  but thanks both excellent suggestions on something that I hadn't checked before.

Tim

Edited March 16, 2023 by tim22
new information

[John]

When I'm checking refractors I use a well collimated laser to check the focuser is aligned with the the optical axis and then (after making changes to focuser tilt if they are needed) use a cheshire to check the objective tilt. 

 

 

[Skipper Billy]

If you would like to post a single unprocessed sub in this thread I will happily run it through CCD Inspector which should identify the problem. 

[tim22]

3 hours ago, Skipper Billy said:

If you would like to post a single unprocessed sub in this thread I will happily run it through CCD Inspector which should identify the problem. 

Many thanks for the offer Skipper Billy  (I believe that you were the originator of the 2018 thread to which I alluded to above).  Yes please  (note --despite my reply above I  am not ruling out that there is some alignment error --- there is certainly some  there on more careful investigation )

Terrible images obtained in bad skies and LP between rain showers but they are what they are..

First  just a  cut and pastes of a Sharpcap stack -  no flat applied - of M44 -- and  the Aberration Inspector analysis of the same with the subjectively worst afflicted areas marked by me with a P.

Then .FIT files of a single sub  and firstly also of the stack (since the SNR of a single 10s sub is so poor)

Stack_16bits_60frames_600s.fits

 

thanks Tim

 

 

 

 

 

 

 

 

 

 

 

 

frame_00050.fits

[ollypenrice]

The artificial star is to be found in optical workshops everywhere. The simplest is the illuminated ballbearing. Only the part of the ball closest to the lens reflects light into it, so you get a near point-source. I used it successfully to re-align the front element of a TV Genesis but, in your case, I'd start by testing the orothogonality of the focuser. 

Olly

[windjammer]

Looking at the pic, it is noticeable that your vignetting is off-centre.  Have you tried adjusting focusser in the tube, or the focusser internals, so that vignetting is centred ? I have tried that on my scope with some success - do it in daylight with exposure and gamma turned down so all you see are the brighter regions.  It has the advantage of using the camera in the image train as it would be in use - so any sagging of the train can be examined.

Simon

[tim22]

20 hours ago, John said:

When I'm checking refractors I use a well collimated laser to check the focuser is aligned with the the optical axis and then (after making changes to focuser tilt if they are needed) use a cheshire to check the objective tilt. 

 

 

Hi John.  I've used the laser and also the Cheshire to confirm that the focuser draw tube alignment to the lense is good.  But I don't know how I should use the Cheshire for the objective tilt?  I can see no way of adjusting objective tilt so I was anticipating putting some tilt on the focuser to straighten the image out.  thanks Tim

[tim22]

4 hours ago, ollypenrice said:

The artificial star is to be found in optical workshops everywhere. The simplest is the illuminated ballbearing. Only the part of the ball closest to the lens reflects light into it, so you get a near point-source. I used it successfully to re-align the front element of a TV Genesis but, in your case, I'd start by testing the orothogonality of the focuser. 

Olly

Thanks.  Yes I have read about the sun reflection off of a ball-bearing method. Very clever idea. I could try that as my artificial star.

[tim22]

1 hour ago, windjammer said:

Looking at the pic, it is noticeable that your vignetting is off-centre.  Have you tried adjusting focusser in the tube, or the focusser internals, so that vignetting is centred ? I have tried that on my scope with some success - do it in daylight with exposure and gamma turned down so all you see are the brighter regions.  It has the advantage of using the camera in the image train as it would be in use - so any sagging of the train can be examined.

Simon

Thanks Simon.  You could be right - I had put the assymetry down to uneven light light pollution (street lamps and house on one side).  The Cheshire test that you suggested did  suggest that the alignment is near dead centre of the lense - at least with no camera in there.  But I will try what you suggest as a test with camera

[vlaiv]

For some reason, "at" mention did not trigger notification - and I was going to suggest the same method I already wrote about.

Something similar is done when collimating the RC scope primary mirror - one is looking to get same FWHM in each corner.

It is fairly easy to do - just fire up SharpCap - and check corners to see what sort of FWHM you will get.

CCD inspector really does the same thing - except it relies on star field to provide stars all over the sensor - and then it measures FWHM of them and uses their average.

You don't have bunch of stars - but you do have one that you can "move" around the sensor by simply slewing the mount in needed direction.

[Skipper Billy]

18 hours ago, tim22 said:

Many thanks for the offer Skipper Billy  (I believe that you were the originator of the 2018 thread to which I alluded to above).  Yes please  (note --despite my reply above I  am not ruling out that there is some alignment error --- there is certainly some  there on more careful investigation )

This is the result from CCDI - it is showing that there is some curvature and some tilt but nowhere near as much as I expected to see.

Hope it helps!

If you adjust anything and want the result running through CCDI again just ping me a message - it only takes me a couple of minutes.

Good luck.

[John]

5 hours ago, tim22 said:

Hi John.  I've used the laser and also the Cheshire to confirm that the focuser draw tube alignment to the lense is good.  But I don't know how I should use the Cheshire for the objective tilt?  I can see no way of adjusting objective tilt so I was anticipating putting some tilt on the focuser to straighten the image out.  thanks Tim

The view through the cheshire (no diagonal) when it's diagonal face is illuminated reveals either objective tilt or focuser tilt as per the illustration below. If your laser eliminates the latter then the former may apply if you see the misaligned cheshire circles:

 

[vlaiv]

I just examined the attached fits - and I'd direct my attention to field flattener.

There is minimal amount of tilt and I bet it is related to flattener and not focuser / sensor tilt.

How is flattener attached to the rest of the system?

Are there any spacers introduced to dial in distance, and if so, what kind and where?

[tim22]

On 17/03/2023 at 18:14, John said:

The view through the cheshire (no diagonal) when it's diagonal face is illuminated reveals either objective tilt or focuser tilt as per the illustration below. If your laser eliminates the latter then the former may apply if you see the misaligned cheshire circles:

 

Many thanks for the explanation John.  The left image is more or less what I see --actually confirming now that my system doesn't in fact have a big problem. I 'd  just been making basic mistakes with tightening things up properly.   At least I have learned some useful stuff from this thread though.

 

Tim

[tim22]

On 17/03/2023 at 18:51, vlaiv said:

I just examined the attached fits - and I'd direct my attention to field flattener.

There is minimal amount of tilt and I bet it is related to flattener and not focuser / sensor tilt.

How is flattener attached to the rest of the system?

Are there any spacers introduced to dial in distance, and if so, what kind and where?

 

Hi Vlaiv,   Thanks once again for your input.  It was your original suggestion about how to measure tilt using an artificial star that interested me and that I  and was all set to follow up.

But  - to my embarrassment -  and also relief -  in further investigating the apparent problem it has now been resolved.  A short window of clear skies last night allowed me to try a few more things.   Having already tried rotating the camera and flattener/ reducer relative to the focuser and Objective lens I thought it logical to try rotating just the objective lens relative to everything else 

Anyway to cut a long story short there were a couple of mechanical  issues.  Firstly I had not appreciated that the carbon fibre tube of my telescope is made up of a number of screw in sections.  In  trying out the experiment of  rotating just the objective  lens  I discovered that the tube was not fastened up tight.  Secondly I think that you were correct to highlight the flattener/ reducer connection  --  the connection to the Moonlite focuser is just a compression ring attachment  and while good as these things go it probably isn't consistent enough.    The third issue may possibly also be that when conditions are really very marginal  - as they were when I collected my original M44 data - it is possible to get such poor star shapes that it can look like tilt without really being that - so I probably tried to conclude too much from images that were just too poor.   So as you and  Skipper Billy both concluded from more objective tests- things were not as bad as they seemed.

At least as far as I can judge -  I am now getting performance not perfect but probably at least as good as I have any right to expect from quite an old and imperfectly matched set up.  Here is a 133 x 20s  image from last night  (M38 region at 1.54 arcsec/ pixel) - this time with flat and dark subtracted and ABE (background subtraction) and SPCC / SCNR colour correction applied (whole  image  and Aberration inspector view of the edges and middle)

 

To answer your queries about the equipment ..

The telescope is an old WO 110 F 7.0  apochromat with a TMB -designed lens and a black carbon fibre tube. fitted with a Moonlite  2.5 " focuser. 

The flattener/ 0.8X reducer is an adjustable WO FLAT3  - which although the same manufacture  is not a perfect optical or physical match to this old WO telescope .    The flattener  has an M63 thread which in my case is connected to the 2 inch compression ring  of the Moonlite focuser via an adapter (newer WO telescopes for which the flattener is designed attach directly  via a screw thread).   Extrapolating from the settings for very similar newer WO telescopes (F 6.9) the expected optimal backfocus distance is very close to 55 mm.  This connection to the AS1294 MC camera is rigid and firmly set using ZWO spacers (48.5 mm plus 6.5 mm internal camera distance to sensor) 

So you are quite right to ask about the nature of the attachment of the flattener.  The 2 " moonlite compression joint is good but can not be as consistent as a screw thread connection.  So I have now ordered the appropriate part from Ron at Moonlite  which should help. 

Thanks very much to all who contributed to this thread.  I learned a great deal  from you all that I did not know  -- as well as some things that I should have checked anyway - like to tighten everything up!

Tim

 

 

 

 

[tim22]

On 17/03/2023 at 18:01, Skipper Billy said:

This is the result from CCDI - it is showing that there is some curvature and some tilt but nowhere near as much as I expected to see.

Hope it helps!

If you adjust anything and want the result running through CCDI again just ping me a message - it only takes me a couple of minutes.

Good luck.

Many thanks indeed for this Skipper Billy.   I think that your analysis confirms that I didn't have the problem that I thought I had.  Please see my reply also  to Vlaiv  who made rather a similar point .   Anyway it is a relief for me that my set up is not so bad - and that there is room for a bit of improvement perhaps just be tightening up all the coupling and possibly further optimising the backspacing from the reducer/ flat?

 

Tim