field rotation?

[Galaxyfaraway]

A question regarding field rotation: is this what I am seeing for this image in the corners? (mainly top right corner)

Snapshots of corners below (100% crop). Is this considered mild or severe?

I haven't done any drift polar alignment (only basic polar alignment). This is for WO GT81 at around 380mm at f4.7 (6x20mins subs).

Also, how can you tell if a problem comes from bad polar alignment or wrong backfocus distance? I have to use a reducer and my backfocus distance may be off by a millimeter or so.

thanks, gfa

 

Top right corner:

[ultranova]

Hi gfa,I am no expert but to me it looks like your optical axis is not

quite right, plus overall it looks like you may be out on your chip distance by a little

Your bottom picture which I think maybe top left on the main pic is the best corner,

I ran it through ccd inspector, The picture is not ideal for analysis  due to the Nebula, 

and CCD inspector can be fooled by this,,

the cross in the middle is middle of axis,,, the cross higher up to the right is actual,,

as I say take this with a pich of salt  as may be a bit off.

I don't think its field rotation but could be a amalgamation of a few things,

unless your kit ie, camera filter wheel reducer flattener etc. is  screwed into the end

of the scope your going to struggle to get the optical axis good repeatedly,

Is this the first time you have had this problem, with 20 minute subs

nice Pic by the way

Paul

 

 

 

[Galaxyfaraway]

Thanks Paul, I have only just started taking images with this kit and only noticed this problem when enlarging the image.

How can I correct the optical axis (sorry, this is a little over my head). Being 'out on the chip distance' means I have to move the sensor closer to the reducer? I have a 1mm delrin spacer which I could remove and try without. But this wouldn't fix the optic axis issue, would it?

Everything is screwed in, except for the WO reducer, which slots into the back of the scope (and the Delrin spacer, that goes over the thread).

[Davey-T]

If you put two subs taken some time apart into P'Shop or similar, layer one on top of the other and alter the opacity you can see if it's rotation.

Dave

[Galaxyfaraway]

Probably not field rotation as stars point towards middle (in right top corner). Must be backfocus distance.

[Davey-T]

1 hour ago, Galaxyfaraway said:

Probably not field rotation as stars point towards middle (in right top corner). Must be backfocus distance.

Wrong back focus should give the same effect in all four corners.

Dave

[Galaxyfaraway]

"Wrong back focus should give the same effect in all four corners."

Would it not also apply to field rotation? 

What could it be? I will try the photoshop suggestion.

[Thalestris24]

Tilt?

[ultranova]

I have a similar problem with me little  WO zenith star 71, with all the weight

of the camera, filter wheel etc. all the weight is being held up by  the r/f 

on the zenith star 71 there is a screw on the underside where you tighten the

R/F that's used to try and get your kit  lined up with the optical axis, its never going to be

perfect every time especially if you break your kit down every time, I keep my camera and filter attached

to the scope all the time,  which help a little once you get it zoned in,

I used to get a flat and stretch all ways so I could see it clearly,,, your flat should look

brighter in the middle and darker on the edges , but even all round if that makes sense.

here's a HA flat showing what I mean, you can see its not even, it will flatten the image out but

will not help if your optical axis is out, as long as you are using a good even light source it should show

what way out you are, as it shows in this flat mine was out also

Paul  

[ultranova]

this is the same flat with ccd inspector which shows what I mean

[ultranova]

11 minutes ago, Thalestris24 said:

Tilt?

That what I was thinking

[Galaxyfaraway]

This looks very similar to this thread: 

 

I wonder if Steve @steppenwolf managed to resolve it after tightening the focuser? (how does tagging work?)

[Davey-T]

I don't think I've ever had bad enough field rotation to show up in one sub, it usually shows when I do an hours worth of Ha then an hours worth of O111 then I have to use Registar to align them.

Dave

[Galaxyfaraway]

I actually have data of a sub before meridian flip, and after (below). Don't know if it helps pinpoint the issue.

Both subs seem to be worst affected in bottom left corners. Does it say anything about the tilt?

[ollypenrice]

I don't think it's field rotation either. I'd come to this opinion before seeing Ultranova's helpful CCD Inspector analysis. The elongations don't go in the right direction so far as I can see.

15 hours ago, Galaxyfaraway said:

"Wrong back focus should give the same effect in all four corners."

Would it not also apply to field rotation? 

What could it be? I will try the photoshop suggestion.

The way field rotation affects the image depends on where the guide star is. I'd expect to see 'four corner roation' in a rig with a central guide star but not in one with a guide star well off axis.. (With good PA you don't need an on-axis guide star. People used to use guide rings to go in search of them. Modern guide cams find stars anywhere, usually.)

Ragarding the effects of the flip: If the OTA were pointing towards the zenith before and after the flip then sag in the focuser might not change much. If the OTA were aiming closer to the horizon then we'd expect the sag to change sides relative to the object in the image. So if you put the pre- and post- flip images side by side just as they left the camera than the bad corner would be the same in both. If you rotated the post-flip one and then made them layers in Ps you might expect to see the bad corner alternate when one layer was blinked on and off.

I remember Steve FLO suggesting a handy way of testing the collimation of the focuser but I can't remember how he did it. I think Sara may have worked on this problem. I'll ask her. The miscollimation could be either focuser or lens cell. We'd all prefer the former!

Olly

[Galaxyfaraway]

Hi Olly, thanks for this. My last post shows images before/after flip and the misshape of stars is both times worse in the same corner (bottom left), but relative to the image, this is not the same corner. Does it mean it's the focuser? The scope just came back after being fixed (weird diffraction spikes). I have also installed a Lakeside focuser and perhaps didn't tighten everything up under the bracket. It might need a longer screw now that the bracket is there.

This is an image before I installed Lakeside of the California and the shapes are still not 100% round. Although I can't see which particular corner is worse (I think it was closer to zenith). I will try tonight zenith, west and east (can't go close to horizon as I have trees). Anything else I can do to test if it's focuser or lens? Don't want to send it back again...

Would be good if @swag72 or someone could advise how to test whether it's lens or focuser collimation.

ps: The California stars seem definitely different to my eye. Not 100% round but more uniform across the whole image rather than specific to a corner, so perhaps bad PA here.

 

[swag72]

Blimey, this is is going back a little way!!!

As far as I recall, FLO suggested that I use the Hotech laser collimator (which they kindly let me borrow), place it in the focuser and then see where the laser point exits the scope at the end. 

I could NEVER get my focuser to sit squarely, and in the end the only answer was to resign it to the rubbish heap. It had 3 silly screws in a flange that was IMPOSSIBLE to get accurately aligned .... I trued shimming with masses of coke tin, but nothing ever worked I'm afraid....... I guess that's not what you wanted to hear......

 

[ultranova]

Do You have any Flats we could have a look at, a recent one would be good

or if you have the time, take a flat with the scope pointing east almost level then west

,I think what Olly said make sense, if something is moving around it might show

up on the flats

Paul 

[Thalestris24]

4 hours ago, swag72 said:

Blimey, this is is going back a little way!!!

As far as I recall, FLO suggested that I use the Hotech laser collimator (which they kindly let me borrow), place it in the focuser and then see where the laser point exits the scope at the end. 

I could NEVER get my focuser to sit squarely, and in the end the only answer was to resign it to the rubbish heap. It had 3 silly screws in a flange that was IMPOSSIBLE to get accurately aligned .... I trued shimming with masses of coke tin, but nothing ever worked I'm afraid....... I guess that's not what you wanted to hear......

 

Dion (Astronomyshed ) has a YouTube video tutorial for aligning the focuser on a Quattro. It does involve undoing the spider and focuser, and measuring where the centre spot should be. This is then marked with a sticky page reinforcer thingy. The focuser can then be aligned with a laser. I've been meaning to do it with my 130pds but keep putting it off... Dion makes it all look so easy...

Louise

[Galaxyfaraway]

Hi Paul @ultranova

Very kind of you; I have just taken a round of flats:

do you need fits files? In which case they are uploaded in this folder.

1. With no filter: pointing East (towards horizon really)

2. With no filter: pointing West

3. With no filter: pointing up

4. With HA filter: pointing East (in case no filter is no good)

5. With Ha filter pointing West

I hope the flats are ok: I used SGP flats calibration wizard to end up roughly around 30000 ADU. I used a white t shirt and laptop screen with a white page open (sorry, I don't yet have a flatbox; it's on my order list).

About my focuser: I have installed a Lakeside focuser. It is possible I messed something up; when i tightened the focuser (the screw under the focuser that now also holds the lakeside bracket), it did make some strange noises when I tried to use the motor moving in/out of focus. I think the screw underneath cannot be too tight otherwise the motor cannot move the focuser in or out properly remotely. Does this make sense?

1.

2.

[Galaxyfaraway]

3.

4.

[Galaxyfaraway]

And the last one, 5.

Continuous troubleshooting seems to be one of the main aspects of this hobby it seems.

 

[swag72]

12 hours ago, Thalestris24 said:

Dion (Astronomyshed ) has a YouTube video tutorial for aligning the focuser on a Quattro. It does involve undoing the spider and focuser, and measuring where the centre spot should be. This is then marked with a sticky page reinforcer thingy. The focuser can then be aligned with a laser. I've been meaning to do it with my 130pds but keep putting it off... Dion makes it all look so easy...

Louise

Not sure that this will be useful for someone with a refractor ..............

[Thalestris24]

5 hours ago, swag72 said:

Not sure that this will be useful for someone with a refractor ..............

True...

 

[ultranova]

Hi gfa, sorry for the delay

I have checked you flats out, firstly your HA flat pointing east takes me to a dangerous website

according to Norton's lucky it blocked it.

I am not sure how you take your flats, but it looks like the illumination field of what ever

you are using is not even, espeacially noticeable on your ha flat pointing west, very un even

if the light source is even it should produce concentric rings working out from the middle,

ccd inspector works by looking at the difference in light intensity over the whole frame,

its very accurate but In what it does, but can be fooled by light leakage, for instance if like mine your

ha flats are several second long if any deflected or retransmitted light is leaking in from your set up

it  will not give a true reading of what it is seeing.

your normal clear filter flats are the best compared to the HA one

as long as you used the same light source and it was evenly illuminated you do seem to have some sort of movement

somewhere, we are not talking about a lot , but the main concern is that your optical axis is not true with your camera

in fact the HA flat was so bad the ccd inspector could not work out where your real axis is hence no other cross

on the screen..

below HA flat

HA flat

 

 

below clear pointing east

 

below clear pointing east

 

 

below clear pointing up

 

up appears to be the best one of the lot

a reasonable flat should look something like this

 

you don't really need ccd inspector to check your flats, just use your levels in photo shop

or another art package and use levels to stretch it if it look even in the corners you wont be far out.

same flat as above done in an art package,,, below

 

 

 

but by the information you have given me there does seem to be something out of axis / tilt

Paul