Altair Astro ED-QR Curvature

[Aidan]

Could anyone assist in trying to solve this please?

ive got a 70mm quad, which produces some nice images.  However, I’m seeing slight elongation of stars along the bottom of the image, mainly towards the corners.  The other 3/4 of the image seems fine.

Another thing I am seeing is a constant 20% curvature when I run an image through ccd inspector.  Although I use a visual approach - Mk1 eyeball and ccd inspector as a confirmation, I’m surprised to see a consistent curve of this much considering it’s a quad scope.

[JimothyC]

Is your spacing correct? The Altair site states a backfocus of 66.78mm, although the image on their site implies it's more than that. 

Maybe worth playing around with some spacers. 

[Aidan]

As far as i am aware, spacing is irrelevant with a quad.  Being in focus should provide the flat filed.   Its one of the reasons i bought the quad so as no messing with spacing etc.   which, consequently i can achieve no problem at all with my image train.   The spacing should only really come into play to keep the focus draw tube as far into the scope as possible to allow you to still focus but mainly to reduce any chance of sag.

Edited November 11, 2018 by Aidan

[david_taurus83]

Similar to my other thread, I'm interested to know what acceptable curvature is on a flat field. I wouldn't expect it to 0% curved though. With CCDI i think you need to load in around 10 images or so for an average to beat the seeing and variable FWHM values. I'm hoping to do this tonight if it's reasonably clear to try and get the optimum back spacing on my flattener.

[Aidan]

I’ve done some good experimentation with ccdi, yes it gives better results with more frames, but in my experience, it only changes a  1 or 2 % over a single image.

So, a single image run through ccdi should give a good starting point.

On a quad or any scope for that matter with a flattener, something in my mind is clearly Isnt right when presented with a 3D plot like this!

Challenge is it find out how to resolve.  I don’t have the ability to change spacing ! 

 

Edited November 11, 2018 by Aidan

[vlaiv]

Two things come to mind that can cause this:

1. Being quad, or having front lens, and integrated dedicated flattener - just means that flattener is matched optically to front lens, at a correct distance - so you are right, when in focus you should also be at optimal distance for flattener. Question is are all elements in scope properly collimated? It could be that rear element is slightly tilted or something. Does manual mention collimation or adjustment of any kind for this? Refractors ought to be factory collimated and very rarely need additional adjustments, but sometimes they do (for refractors, often that adjustment needs to be performed by skilled individual rather than "general public", unlike collimating newtonian or other mirrored systems).

2. Sensor tilt. Is focuser adjustable? Can you compensate for tilt in any way? It might be that you need separate tilt element to achieve sensor orthogonality to principal axis.

No field flattener can provide "infinite" flat field, and usually there is technical spec on imaging circle, including sometimes just corrected field diameter and illuminated field diameter. More serious analysis and specification gives spot diagram over field and also illumination curve depending on distance from optical axis. Often if its just diameter of corrected field - it is more "acceptable" field rather than diffraction limited field, so you can start to see distortion near the edges of the field - something that spot diagram would clearly show. What is the size of sensor and does it match corrected field?

[Aidan]

As always, a very detailed diagnosis.

the focuser does have tilt correction grub screws, but they are so difficult to manipulate, it makes them very impractical.  

The image train is ZWO 1600mm Pro, zwo 5 position fw, zwo OAG and a 16.5mm spacer which attaches to the scope.  

I’ve just been out and done a full inspection of the scope & image train.  Made sure there is no slop and everything is tight.  I’ll give it a go later on and see what happens.

As said, I can nearly always crop out the bottom of the frame and still end up with a good image composition.   I guess reading how it should be vs how it actually is is not always true!

[vlaiv]

Just had a look, if your 70mm Quad is Altair Astro one, with ASI1600 you should have pin point stars across the whole field.

They have spot diagram on their product page and it shows that spot diagram is less than airy disk even at 15mm distance from center - ASI1600 has diagonal of about 23mm so center to corner is less than 12mm.

If there is dedicated tilt adjuster - it is there for a reason. Elongated stars in one corner or at one edge mean just that - tilted sensor. Above CCD inspector profile also suggests tilted sensor - just tilt your head to the right when looking at it and you will get much more acceptable looking field.

According to description, there is tilt mechanism:

"The Starwave 70 EDQ-R Quad APO is intended for astro photography and the focuser terminates in an M48 Male threaded rotator plate with locking thumbscrew. All screws are teflon tipped and grip on a flange for accuracy. The rotator plate includes a "push-pull" tilt adjustment system to enable you to square up the image sensor to the focal plane. "

You should try adjusting tilt until you get symmetric looking graph in CCD inspector as a start and then look into any further issues (if there is any left).

[Aidan]

26 minutes ago, vlaiv said:

 

If there is dedicated tilt adjuster - it is there for a reason. Elongated stars in one corner or at one edge mean just that - tilted sensor. Above CCD inspector profile also suggests tilted sensor - just tilt your head to the right when looking at it and you will get much more acceptable looking field.

According to description, there is tilt mechanism.

You are correct.  However, it’s not practical.  The focuser tube only has about 25cm of travel from fully in to fully out.  The problem is, at my focus point, it’s pretty much impossible to get a hex key between the gap to the grub screws. 

I would have to wind the focuser out to get enough space.   The only option I would have would be to remove the auto focus motor and wind the focuser out manually each time I needed to make an adjustment then re- focus.   

If that’s what is needed to get rid of the elongated stars, I guess I need to look into doing it that way

Edited November 11, 2018 by Aidan

[RolandKol]

On 11/11/2018 at 17:05, Aidan said:

You are correct.  However, it’s not practical.  The focuser tube only has about 25cm of travel from fully in to fully out.  The problem is, at my focus point, it’s pretty much impossible to get a hex key between the gap to the grub screws. 

I would have to wind the focuser out to get enough space.   The only option I would have would be to remove the auto focus motor and wind the focuser out manually each time I needed to make an adjustment then re- focus.   

If that’s what is needed to get rid of the elongated stars, I guess I need to look into doing it that way

 

Hi Aidan?
Have you managed to deal with it?
I am thinking about the same quad as a next possible scope and I also have the same ASI/EFW, - so a bit curious...

 

[Aidan]

Hi Roland,

Sorry for the lack of response... been kinda busy!    I eventually ended up sending the scope back to AA.  I didn't find out exactly what the problem was, they just replaced it.   New one is spot on.  I wouldn't be deterred by my experience..  You don't often hear folks having problems with AA scopes - and in my experience probelms are very quickly dealt with.

[RolandKol]

14 minutes ago, Aidan said:

Hi Roland,

Sorry for the lack of response... been kinda busy!    I eventually ended up sending the scope back to AA.  I didn't find out exactly what the problem was, they just replaced it.   New one is spot on.  I wouldn't be deterred by my experience..  You don't often hear folks having problems with AA scopes - and in my experience probelms are very quickly dealt with.

Thanks for update!

was not in a hurry anyway!