Image tilt

[Anthonyexmouth]

Ok, so got a little image tilt problem. I've kinda eliminated everything but the focuser, but I could be very wrong. It's the stock SW ED80 focuser, is there anything I can do to test this? I'd buy the upgrade Crayford but don't want to buy it to find I've still got the problem. Any ideas out there? 

 this and a slight flattener spacing issue are stopping me dumping some cash into a new cooled camera, would really like to get this sorted first. 

[Anthonyexmouth]

Here's a 60 sec sub

[RolandKol]

I would try without flattener first...

My unexperienced eyes say, - spacing problem only...

I can be wrong...

Below are all 4 corners

[RolandKol]

And the stars in the center, look OK for me...

Some merged double-stars, nothing special

[Anthonyexmouth]

10 minutes ago, RolandKol said:

I would try without flattener first...

My unexperienced eyes say, - spacing problem only...

I can be wrong...

Below are all 4 corners

A few people in another thread said there was a bit of tilt. Also sent it off to Altair to get advice on the spacing issue as I've got it bang on 55mm and they said tilt problem. Really frustrating. 

[RolandKol]

Tilt would be one directional... and visible in central stars also... isn't it?

As per 55m, - once you fit all rings and etc into the train, you will have +/- 0.5mm for sure...

After I bought OAG, I played with my spacings for last 2 months (was cloudy...) I had such a star distortion with my PDS and it was approx 1mm = at 56mm or 54mm in total (sadly do not recall the direction of 1mm)

I am leaning towards 56mm...
 

[Davey-T]

Agree with Roland, definitely looks like spacing,

Dave

[Anthonyexmouth]

1 minute ago, RolandKol said:

Tilt would be one directional... and visible in central stars also... isn't it?

As per 55m, - once you fit all rings and etc into the train, you will have +/- 0.5mm for sure...

After I bought OAG, I played with my spacings for last 2 months (was cloudy...) I had such a star distortion with my PDS and it was approx 1mm = at 56mm or 54mm in total (sadly do not recall the direction of 1mm plus or minus)

I am leaning towards 56mm...
 

This is the setup. It's only the t ring directly onto the flattener. 

[RolandKol]

yep... Sadly, we haven't got much clear skies to play around and test all possibilities...

As per tilt,

if focuser's drawtube does not wobble and your setup does not wobble in it, should be no tilt.

Not sure how your focus locking screw works, - mine, on PDS, always shifted focus a bit, but it have not distorted stars by any means, it shifted focus only.

In your case, I would go for ASI anyway...

You will have some spacing problems with ASi (probably, - hope not) - so no need to fix the same problem twice.

[vlaiv]

In my opinion it's both spacing and tilt.

Corners show that you have spacing issue. Depending on quality of FF/FR, you need to check if it will correct required sensor size. You might end up with little cross shaped stars in corners anyway (astigmatism) with even proper spacing.

Tilt is visible in lower corners. With proper spacing, even if FF/FR does not fully correct sensor diagonal, you will have small crosses, that should be have spikes about the same in length.

This is the place where sagittal and tangential astigmatism are about the same. By racking focuser in / out you will have one stronger and other weaker.

In bottom corners you have dominant tangential astigmatism which means that those corners are either "racked" in or out a bit from the rest of the frame - which means tilt.

You can have "loose" or "firm" tilt. Try rotating your ota and taking another sub for comparison (or just orienting it another way - another part of the sky). If affected corners change - you have a loose tilt. This means that camera is tilting under gravity and that your imaging train is not firmly held together. Adjust any focuser slop to see if it will help.

If you have firm tilt, this means that camera is not affected by gravity and it is always in the same position - tilted with respect to optical axis. For this, try squaring focuser, or using tilt adjuster before camera to correct for tilt.

[Anthonyexmouth]

Next clear night I'll take a sub, rotate 180° in the rings and take another. As for tilt adjuster, there isn't the space to place one. Would a smaller sensor be a benefit in this case? 

[Oddsocks]

13 minutes ago, RolandKol said:

Tilt would be one directional... and visible in central stars also... isn't it?

No that's not quite correct, tilt will be symmetric about some reference point of the image. The reference point can be at one corner, in which case star distortion will appear to increase linearly across the image with one corner sharp and getting progressively worse as the opposite corner is reached, equally, tilt can be symmetric about the centre of the image and will result in sharp stars in the central area and distorted stars at either ends of the image as the sensor plane is either above or below the focus plane at the ends of the sensor. It is equally possible that the tilt axis, and direction, can be anywhere on the image.

To test the OTA - focuser alignment, the easiest way is to place the OTA in a vee block wooden cradle on a bench, place a (collimated) newtonian collimation laser in the focuser, wrapped with paper to ensure a snug fit in the focuser then point the OTA in the vee block cradle at a wall as far away as practical, but at least 10 meters, mark the laser spot on the wall and have someone rotate the OTA in the vee block cradle, if the focuser draw tube is aligned with the OTA and the objective collimated then the laser spot will remain stationary as the OTA is rotated, if the laser spot wanders around in a circle then either the focuser draw tube is not aligned with the optical axis of the tube or the objective is out of collimation.

I looked at some of the earlier posted images in a different thread from the same OP using CCD Inspector and that seemed to show spacing was the fundamental issue though using site forum jpgs is not really of high enough quality to be positively diagnostic, neither is having large diffuse galaxies and nebula in the image, nevertheless, there were obvious changes in the shape of the field as different spacings were tried.

The way to move forward with this issue is to cary out the vee block laser test first to ensure focuser draw-tube alignment and objective collimation is correct, then, if ok, download the CCD inspector and use it for the free evaluation period while increasing the spacing between camera and flattener in ~0.1mm to 0.2mm steps using Delrin "T" mount spacing rings (available from FLO), or home made rings cut from the wall of an aluminium drinks can, point the telescope at a rich star field with no large scale galaxies or nebula and evaluate the raw image, or fits file, in CCD inspector, add a single spacer at a time between "T" mount and flattener and evaluate which way the flat field is changing, there most likely will be a "sweet" spot within one or two mm of where it is now. If the flat field only gets worse as you add spacers then the flattener would be suspect.

HTH

William.

[Anthonyexmouth]

Is the vee block test a reliable one for errors in the focuser as the laser is substantially less weight than the camera. Can you distinguish between alignment and collimation with the vee block test? 

[Oddsocks]

25 minutes ago, Anthonyexmouth said:

Is the vee block test a reliable one for errors in the focuser as the laser is substantially less weight than the camera. Can you distinguish between alignment and collimation with the vee block test? 

Yes, generally the test is sound, nothing to stop you adding some weight to the end of the focuser draw tube since it will be at maximum extension for the test and overhanging the end of the vee block cradle.

If you find during testing that the laser spot is indeed wandering then, depending on the telescope model, the way to distinguish between focuser and lens cell misalignment is to unscrew the dew shield, slide it back to reveal the lens cell and just unscrew the entire lens cell to remove it from the OTA, this does not in any way affect the lens collimation since the entire cell is unscrewed in one piece, then repeat the laser test just with the focuser.

Looking at your earlier posted images in your other thread with CCD inspector the collimatiion/focuser alignment looked pretty good to me TBH, just the distance spacing distance was wrong, but as I mentioned before the only images available were jpgs from the forum which are not reliable for CCD inspector because of the compression artefacts that exist with jpgs and the problem with the large diffuse objects in the middle of the image field.

The important thing with any issue such as this is to start at the beginning and work through the problem logically rather than jump from one approach to another as it is easy to miss something valid. Since Altair has said tilt is the problem, and you may need to return the flattener then you will have to carry out a tilt test of some kind to progress the issue further with them.

HTH

William.

[carastro]

I agree it is primarily a spacing problem.  If you get this right and you still have a bit of tilt you can use on the SWED80 a Baader click lock which screws onto the focusser and holds the camera on by twist and squeeze, instead of using just two focusser bolts to hold on your camera.  It sorted my tilt out and also grips the camera more safely.

I think some-one posted a photo of it above. 

Carole 

[RolandKol]

as I mentioned before, my eyes are not experienced....

but the more I look at this image... I see nothing but spacing problem...

If all experienced memebers, Altair and CCD inspector tells there is a tilt, - I must agree even if my eyes say opposite...

Maybe the tilt is so slight, that is almost invisible and the spacing problem hides it completely from me....
 

[Anthonyexmouth]

6 minutes ago, carastro said:

I agree it is primarily a spacing problem.  If you get this right and you still have a bit of tilt you can use on the SWED80 a Baader click lock which screws onto the focusser and holds the camera on by twist and squeeze, instead of using just two focusser bolts to hold on your camera.  It sorted my tilt out and also grips the camera more safely.

I think some-one posted a photo of it above. 

Carole 

Yeah, that's my setup. I got the click lock to enable easier framing. 

[carastro]

Quote

as I mentioned before, my eyes are not experienced....

but the more I look at this image... I see nothing but spacing problem...

If all experienced memebers, Altair and CCD inspector tells there is a tilt, - I must agree even if my eyes say opposite...

Maybe the till is so slight, that is almost invisible and the spacing problem hides it completely from me....

That's why I said get the spacing right first and see if that cures it. 

Carole 

[Thalestris24]

Hi Anthony

Sorry you're still having problems . I just wondered about your t-ring - can you say which make it is? Can you measure its thickness, just to double check? Fwiw, one of mine from my box of bits, measures ~10.6mm with a digital caliper. As far as I know, it's classed as a 'standard' ring. One other thing, has your camera been modded? The pattern is a too-far-away one, as before... Did you have any more dialogue with AltairAstro? It's an annoying problem.

Louise

[Anthonyexmouth]

35 minutes ago, carastro said:

That's why I said get the spacing right first and see if that cures it. 

Carole 

I've got a low profile t ring. Just looks like it's gonna be expensive getting enough spacers in various thicknesses to try and adjust it. Damn these pricy hobbies.

[carastro]

Spacers are a right PITA, I hate them and they don't even come with the same threads quite often.

Carole 

[RolandKol]

@Anthonyexmouth

And there is quite a high possibility, you will be sorting the same problem again once you move to astrocam... 

And by the way,

it does not look you started imaging yesterday... How and why you have not managed to notice it before?

On another had, have you changed something recently? Introduced ClickClock maybe? 

Maybe had two T-Rings initially and forgot which one is On?

[Anthonyexmouth]

2 hours ago, Thalestris24 said:

Hi Anthony

Sorry you're still having problems . I just wondered about your t-ring - can you say which make it is? Can you measure its thickness, just to double check? Fwiw, one of mine from my box of bits, measures ~10.6mm with a digital caliper. As far as I know, it's classed as a 'standard' ring. One other thing, has your camera been modded? The pattern is a too-far-away one, as before... Did you have any more dialogue with AltairAstro? It's an annoying problem.

Louise

It's an unmodded camera. Had a few more emails but he seemed mainly concerned with the tilt. I've got a baader t ring on the way to try. 

[Anthonyexmouth]

1 hour ago, RolandKol said:

@Anthonyexmouth

And there is quite a high possibility, you will be sorting the same problem again once you move to astrocam... 

And by the way,

it does not look you started imaging yesterday... How and why you have not managed to notice it before?

On another had, have you changed something recently? Introduced ClickClock maybe? 

Maybe had two T-Rings initially and forgot which one is On?

I've only just got a flattener so kinda accepted the distortion until now. It's only when I started questioning it after the flattener that tilt was mentioned by others

[RolandKol]

Hmmm 

is this the one you've got?

https://www.altairastro.com/altair-planostar-0.8x-flattener-reducer-for-starwave-80ed-doublet-refractor-only.html

very strange description...

"Spacing from rear flat shoulder of the PlanoStar to the CCD film plane is approximately 51-56mm and one can use the Altair Astro M48 variable spacer system to fine-tune the optimum distance for their particular chip and filter setup."