Eggy stars

[Anthonyexmouth]

Got a full clear night tonight but first test subs are showing eggy stars, guiding looks flat but is this maybe a guiding issue? 

Only had a few nights with this new scope but from the outset its been producing nice round stars. 

How does tilt manifest? 

60sec sub. 

[pipnina]

Looks like tilt to me, but discerning the various misalignment optical effects from eachother has always been challenging for me.

Egg shapes on one side, and defocused and round on the other makes me think tilt at least.

This may manifest if the focuser has slopped somehow. Maybe if it's a crayford, the tension is not high enough or unbalanced, causing the camera to present to the objective lens at different angles?

I am unsure how to solve it if it appeared suddenly, but for manufacturing faults you either return the scope (if scope is at fault, i.e. lens cell collimation) or use a tilt adapter for the camera.

Good luck!

[Anthonyexmouth]

3 minutes ago, pipnina said:

Looks like tilt to me, but discerning the various misalignment optical effects from eachother has always been challenging for me.

Egg shapes on one side, and defocused and round on the other makes me think tilt at least.

This may manifest if the focuser has slopped somehow. Maybe if it's a crayford, the tension is not high enough or unbalanced, causing the camera to present to the objective lens at different angles?

I am unsure how to solve it if it appeared suddenly, but for manufacturing faults you either return the scope (if scope is at fault, i.e. lens cell collimation) or use a tilt adapter for the camera.

Good luck!

Focuser feels solid good, R&P focuser. It's a stellamira triplet. 

this was from my last outing about a week ago. 

[pipnina]

Seems like something has been knocked out of kilter then. Struggling with something like it on mine right now too : /

[Anthonyexmouth]

1 hour ago, pipnina said:

Seems like something has been knocked out of kilter then. Struggling with something like it on mine right now too : /

Have to have a look in the daylight. It's an all screwed image train, no compression fittings. Hope it's a simple fix. 

[Fegato]

Yes, I would think a bit of tilt and possibly backfocus not being perfect. The earlier image is better, but still has some issues in corners - so maybe you just had slightly different focus on the second one which exacerbated things?

Try ASTAP Image inspector, or if you use NINA, the Hocus Focus plug-in has a great Aberration tool.  But you need to be able to adjust your tilt and backfocus somehow.

[Anthonyexmouth]

1 hour ago, Fegato said:

Yes, I would think a bit of tilt and possibly backfocus not being perfect. The earlier image is better, but still has some issues in corners - so maybe you just had slightly different focus on the second one which exacerbated things?

Try ASTAP Image inspector, or if you use NINA, the Hocus Focus plug-in has a great Aberration tool.  But you need to be able to adjust your tilt and backfocus somehow.

I'll have a look at the Nina option. I do use hocus focus. Is the aberration tool easy to use? 

 

 

[Fegato]

8 minutes ago, Anthonyexmouth said:

I'll have a look at the Nina option. I do use hocus focus. Is the aberration tool easy to use? 

 

 

There's a lot of information provided, and some might find it too complex I suppose, but I really like it.  The basics are that it produces a focus curve for the centre and each corner of your image. So you can see where focus is falling for each corner. From there you can work out which corners are closer to the sensor than others, and then the idea is to adjust so that you get these curves reasonably close together. There are pretty graphics of tilted sensors and so on, but I largely ignore those and use the curves and the associated table of relative focus positions for centre and corners.

I've found it by far the easiest way to adjust my setup, which is horribly sensitive at F/2.2.  I do have the Octopi tilt / backfocus device on my RASA as well - so it's the combination of being able to see what's going on alongside a tool that allows for precise adjustments that has kept me sane! (at some cost, I might add....)

[Anthonyexmouth]

2 hours ago, Fegato said:

There's a lot of information provided, and some might find it too complex I suppose, but I really like it.  The basics are that it produces a focus curve for the centre and each corner of your image. So you can see where focus is falling for each corner. From there you can work out which corners are closer to the sensor than others, and then the idea is to adjust so that you get these curves reasonably close together. There are pretty graphics of tilted sensors and so on, but I largely ignore those and use the curves and the associated table of relative focus positions for centre and corners.

I've found it by far the easiest way to adjust my setup, which is horribly sensitive at F/2.2.  I do have the Octopi tilt / backfocus device on my RASA as well - so it's the combination of being able to see what's going on alongside a tool that allows for precise adjustments that has kept me sane! (at some cost, I might add....)

Do you have any hints on decyphering this?

[Fegato]

1 hour ago, Anthonyexmouth said:

Do you have any hints on decyphering this?

Key points are:

1. Work out which corner of an image from the camera matches which corner of your camera / tilt device. Not always obvious -  I had to block parts of the light path with a defocused image in day time to work it out.

2. Work out which direction your focus step changes are moving the backfocus. Not sure there's a failsafe way of doing this other than trial and error using the method below, but once you've worked it out, you will know where you're going.

3. Run the Aberration tool. The graph will show 5 curves (can be a bit hard to see, but the table below summarises the results). If say, top right corner focus position is way out from the others, you need to move this corner (this is where you need a tilt device) in the required direction (trial and error method - if it gets worse, go the other way!). If all corners are quite a way from the centre, this implies you need to move the whole sensor (i.e. backfocus distance isn't quite right). And so on...

Sorry, that probably makes it sounds even more complicated...   The beauty of it is that it's not based on one image alone. - all the focus steps are being analysed for each corner. But obviously the results produced are inevitably busy! 

[Anthonyexmouth]

5 minutes ago, Fegato said:

Key points are:

1. Work out which corner of an image from the camera matches which corner of your camera / tilt device. Not always obvious -  I had to block parts of the light path with a defocused image in day time to work it out.

2. Work out which direction your focus step changes are moving the backfocus. Not sure there's a failsafe way of doing this other than trial and error using the method below, but once you've worked it out, you will know where you're going.

3. Run the Aberration tool. The graph will show 5 curves (can be a bit hard to see, but the table below summarises the results). If say, top right corner focus position is way out from the others, you need to move this corner (this is where you need a tilt device) in the required direction (trial and error method - if it gets worse, go the other way!). If all corners are quite a way from the centre, this implies you need to move the whole sensor (i.e. backfocus distance isn't quite right). And so on...

Sorry, that probably makes it sounds even more complicated...   The beauty of it is that it's not based on one image alone. - all the focus steps are being analysed for each corner. But obviously the results produced are inevitably busy! 

I dont have a tilt adjuster so really glad it can detect backfocus issues too. If thats ok and it's tilt I guess I'll be abusing the credit card again and a bike ride to FLO. 

[Anthonyexmouth]

13 minutes ago, Anthonyexmouth said:

I dont have a tilt adjuster so really glad it can detect backfocus issues too. If thats ok and it's tilt I guess I'll be abusing the credit card again and a bike ride to FLO. 

Guessing this is tilt then. 

[Fegato]

Well, yes that shows a bit of tilt. However, the numbers aren't huge...   although it does depend how far a focus step on your system moves the focuser - do you know?  I worked it out on mine (the RASA documentation told me what a full turn of the focuser was in microns, and then I worked out how many steps did a full turn as well). This was 750 / 1000 = 0.75 microns per step. So on my system, your numbers would be pretty good! You can plug this figure into Hocus Focus and it will tell you the backfocus error in microns (but having said that, I do think you have to take the specific step value it gives with a pinch of salt - I've read that it will tend to suggest you need to push the sensor closer more than you need to... not totally sure yet, as mine is a continued work in progress).

Anyway, having said all that,  I'd play around with the backfocus first, particularly if you have an easier way of doing this. This looks worse to me - you have quite a big difference between inner and outer HFR  numbers - about 30%. Try moving backfocus distance around and see if you can improve this. Then when you've got it at as close as you can, see what the stars are like, and then focus on tilt if you need to.