Stars elongated and out of focus

[DeCosta]

Hi,

I am a relative newbie on the steep learning curve.  I have a William Optics FLT91 and a ZWO ASI294mm. with a WO 0.8 reducer.  Astrodon filterset and an EQ6R pro mount.  I am having trouble with my images.  They are not looking great at all. The stars seem to deteriorate in one direction to the corner of the image.  its not consistent in that its worse in one corner than others.  After some frames on NGC 6888 I rotated my camera the other night when imaging M31 and I noticed that the same issue occured but in a different area.  I have spent hours trying to understand PHD2 guiding.  I'm using PPEC in RA and Lowpass in DEC.   The guide assistant was complaining about Dec Backlash and as a result Dec backlash compensation was switched which tbh made the guiding worse.  I have tightened up backlash on the mount in  RA and Dec. - there was some backlash evident on both.  Guide scope is focused.  Tried 2,3,4 second guide exposures.  I have tightened up focus tube on main scope where there was a small amount of movement.  Autofocus seems to be running really well.  Very nice V curve.  My cable management is untidy, but I have taken any pressure off the imaging train with hooks and wire.  Scope is balanced accurately when everything is connected.   As a beginner I am learning a lot but am still way out of my depth on this stuff.  I'm wondering if there is a problem with the alignment of the camera/reducer/scope?  I have stripped the rig down cleaned everything and put it back together but no joy.  I've attached two not very good images and also a section of my M31 guide graph before the clouds started coming in.  I've got a feeling I'm doing something stupid, but any advice would be appreciated.

 2023-10-25_010155_M31_300sec_2x2_filter5_Ha_frame3_-14C_gain_200__.fit2023-10-24_215740_NGC6888_300sec_2x2_filter5_Ha_frame12_-15C_gain_200__.fit

 

Thanks 

[teoria_del_big_bang]

Well I am no expert and others will be along to assist more than I can do, but I just looked briefly at the NGC6888 and yest there is some elongation but for one thing generally it is a very good single frame and you should not think otherwise.
Personally I do not think it is particularly due to guiding and for a 5 min sub most of the stars towards the centre seem pretty round which they would not be if it were the guiding. Looks like the back-focus might need tuning in a bit hence why elongated in the corners and maybe some slight tilt hence why when you rotate the camera the biggest problem is in a different corner.

Anyway as I say I will not give too much advice as I do not want to send you down the wrong route and others far more experienced will help much more , I just wanted to give you some encouragement as from what I see I do not think you are out of your depth at all, or done anything stupid, in fact the list of things you have done seem very logical trying to solve this issue. 
It is a steep learning curve and we all had issues in the beginning, I I read lots of threads about this so a very common issue.

Steve

[vlaiv]

I'd say that there is some slight tilt and in general - you need to fiddle with spacing of field flattener to get the best result.

It is certainly not guiding - that impact whole field in the same way - so you would see same distortion in all stars in the field.

In the end, you might need to accept slightly astigmatic stars in the corners - as some field flatteners / focal reducers produce such results. It is more obvious in good seeing when stars are tight and sharp.

 

[DeCosta]

Wow thanks for the quick feedback.  So are the tilt and the backspacing the same issue or are they related?  I guess I can get some thin spacers to increase the back spacing and try to improve that way.   But tilt not sure how that gets addressed? 

[teoria_del_big_bang]

2 minutes ago, DeCosta said:

Wow thanks for the quick feedback.  So are the tilt and the backspacing the same issue or are they related?  I guess I can get some thin spacers to increase the back spacing and try to improve that way.   But tilt not sure how that gets addressed? 

The thin spacers are what's required to tune in the back-focus, Don't forget that the thickness of the filters affects the backspacing and you need to add 1/3 of the thickness to the back-focus, so a 3mm filter will add 1 mm to BF, but even then the theoretical FF may need tuning in to your actual setup to get the best scenario so still need these thin spacers.

Tilt is a bit different and not as easy to tune in.

Steve

[vlaiv]

1 minute ago, DeCosta said:

Wow thanks for the quick feedback.  So are the tilt and the backspacing the same issue or are they related?  I guess I can get some thin spacers to increase the back spacing and try to improve that way.   But tilt not sure how that gets addressed? 

Back spacing is just like you understood it - you need to either increase or decrease distance from sensor to FF/FR. Increasing is easy - you add some thin spacers. Decreasing distance is a bit more involved - you need to add shorter extension and again add some spacers.

Alternative is to get variable length extension that you can tweak / adjust.

Tilt on the other hand is much worse thing to deal with. It really means that something is not perpendicular to optical axis. It can be sensor or field flattener / focal reducer itself.

Often, cause of tilt is in the way everything is connected together. For imaging best type of connection is threaded connection. Clamping mechanisms are not very good (like when you insert eyepiece into receptacle and clamp it down with thumbscrew and possibly compression ring). I think that Baader click lock is exception as it centers the accessory.

As far as tilt is concerned - I'd first look into how things are connected.

Next thing to check is if camera already has tilt mechanism. Some newer models already have tilt plate in front that you can adjust.

If not - there is tilt mechanism that you can add to your optical train - but you must check to see if it's sensor, field flattener or perhaps even focuser that is not squared with the rest.

 

[DeCosta]

OK understood.  I'll give it a try.  I've moved my whole rig from the UK to my holiday apartment in Fuerteventura Canary Islands better skies and warmer.  Only trouble is there are no equipment retailers or Amazon delivery out here.  I'll have to get some spacers in the UK and bring them out in January when I'm back.  Thanks for the great information and it's a relief that things aren't as disastrous as I first thought.  Ive got several hours of data on both targets which I was considering ditching.  I might be able to get Pixinsight to do its magic and make the best of it.  Many thanks to both of you.

[alacant]

Hi.

eq6r, so...

eqascom

Set the mount guide speed to 0.9x sidereal for both RA and DEC, then recalibrate.

phd2

Set ppec to 480s.

If still subject to PE, post the guide log..

Guide DEC using Resist Switch.

Run the GA for at least 2 minutes. Take ALL the recommendations apart from backlash settings.

hardware

Move the FF a mm or so from the sensor. If matters worsen, return to the original spacing.

Tilt: remove the camera and FF assembly then loosen and wiggle the interface between the two before re-tightening and replacing. Reseating will usually help with the tilt before resorting to more drastic measures.

geografía

Fuerteventura? amazon.es will deliver anywhere in Canarias but you'll save quite a bit by using AliExpress via Correos. Leave a note to the seller to use an EU warehouse.

Cheers and HTH

Edited October 27, 2023 by alacant

[ollypenrice]

Sometimes tilt is improved by rotating the camera in the OTA, probably because a number of imperfections cancel themselves out. There might be a bit of tilt in one component which goes the opposite way to that of another in one particular orientation. It might be worth a try. If you find such a sweet spot, as we did with our RASA, you can still rotate to frame a target by rotating the entire OTA/camera in the tube rings.

Olly

[900SL]

What WO reducer do you have? If it is the adjustable model with a threaded adjuster, this can cause tilt. 6AIii? 

[DeCosta]

Hi, I think it's the 0.8 6aiii threaded reducer/flattener.   I'm guessing if my back focus needs increasing then that will increase the area of the image that is in focus?  I cropped my M31 by .65% so that the edges of the image were removed. The resulting zoomed in image came in at 0.42 eccentricity.  So I'm wondering if the biggest change would be back focus?  I've found a 0.5mm and 0.1mm spacer in my stuff that I'm going to try on next clear night.

[DeCosta]

Olly,Alacant,

Great feedback. Will try out your suggestions.  I'd love to get the eccentricity sorted.  Fingers crossed for some clear sky.  I'll feedback.

[AstroMuni]

On 27/10/2023 at 19:37, DeCosta said:

Olly,Alacant,

Great feedback. Will try out your suggestions.  I'd love to get the eccentricity sorted.  Fingers crossed for some clear sky.  I'll feedback.

You can use a tool such as Siril /ASTAP to work out the tilt. I have used Siril here on your M31 image.

 

[DeCosta]

Olly,Alacant,

Great feedback. Will try out your suggestions.  I'd love to get the eccentricity sorted.  Fingers crossed for some clear sky.  I'll feedback.

[DeCosta]

Hi AstroMuni,

Is that bad? I have some clear sky here forecast tonight so I am going to adjust the backfocus tonight on the adjustable 6aiii and test.  But if its purely down to the tilt then I will need to try and adjust the camera sensor I guess.  I have taken previous advice and reassembled the imaging train in case something is not seating correctly.  You mentioned ASTAP that's what I use for platesolving.  I'll have a look at that in terms of tilt.

 

Thanks for your help.

[DeCosta]

Hi all,

Thanks for your feedback.  I have taken your advice and made the changes.  I reseated the items on the imaging train and experimented with the  back focus with the adjustable 6aiii.  I have used ASTAP to get some tilt and HFD stats.  I have attached an Ha sub and an OIII sub with ASTAP pics and the PI subframe selector stats.   I think I've got some improvement.   Still some eccentricity but maybe a bit more work on guiding.  I made the guiding changes as recommended and then forgot to run the guide assist !!   I've attached the guide log.  The clouds rolled in so I only managed 7 frames. 

Any feedback would be great. 

As always thanks for your help.  Much appreciated.

2023-11-02_210444_NGC6888_300sec_2x2_filter5_Ha_frame1_-15C_gain_200__.fit 2023-11-02_211457_NGC6888_300sec_2x2_filter6_OIII_frame1_-15C_gain_200__.fit PHD2_GuideLog_2023-11-02_194527.txt

[alacant]

On 04/11/2023 at 18:49, DeCosta said:

attached the guide log

Congratulations.

Apart from a few gusts of wind/cat interference/adjusting the mount, the PE has all but gone and the field of view is super flat with perfectly controlled stars wherever you look. I'd recommend leaving it as it is. Make any minor adjustments on the night via the GA only.
You can now follow the golden rule to which few adhere: look at the images, not the graphs!
Cheers
 

Edited November 6, 2023 by alacant

[DeCosta]

Wow great feedback.  TBH it's been quite painful but I've learnt a massive amount.  I think my previous DSO projects have all been suffering the same problems so this hopefully is a big step up in quality of my image capture.  Thanks again !!!