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Image tilt

Anthonyexmouth

By Anthonyexmouth
in Imaging - Discussion

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vlaiv

vlaiv

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30 minutes ago, Anthonyexmouth said:

Sucks that no testing can be done during the day. 

I think you can try something during the day. It's not going to be exact test, but it can give you indication of things.

Do you have any object at a distance that is highly structured? Maybe communications aerial / tower or similar? Brick wall can probably do as well - anything with high contrast pattern on it, and far away.

Test is to take an image of it with emphasis on center of the field as reference, and placing same texture / pattern in corners with different T2 adapters / spacing. You won't be able to get exact star shape (well, there might be a way to get that as well, but it's far more involved), but level of blur should be good indicator of FF/FR performance.

If you get the same clarity of image in center of the field and on the edges / in the corners - you have good correction. Any blur will indicate that there is distortion. It should be possible to compare levels of blur and best spacing / adapter will be one with the least of blur in corners.

On a separate note, if you wish to try to recover blur PSF, and get star shape out of image, you should take an object (or crop of texture, or whatever) placed in center of the field - region should be small. Shoot that same object / crop / region in corner of the frame and do deconvolution of corner image with central image - that should give you PSF of corner blur (mind you, this is easier said than done, since you need to be careful to get exact same crop of object texture in both cases).

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Thalestris24

Thalestris24

Posted
Posted

Hi Anthony
Hmm… That isn't a Baader 'protective' t ring in your pic is it? It's strange that they are so different. I have to say I can't vouch for the 'too near' vs 'too far' diagrams. They are just copied from much earlier threads. Anyway, in the first instance, what matters is the distance between the line of the sensor mark and the front of the t ring:

 Tring_to_sensor.jpg.bf51fb31aa3f6a49f418ff15d8f37c22.jpg

It should be close to 55mm, as above (ignore the red sticky tape patch - no idea why that's there!).

Louise

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Anthonyexmouth

Anthonyexmouth

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6 minutes ago, Thalestris24 said:

Hi Anthony
Hmm… That isn't a Baader 'protective' t ring in your pic is it? It's strange that they are so different. I have to say I can't vouch for the 'too near' vs 'too far' diagrams. They are just copied from much earlier threads. Anyway, in the first instance, what matters is the distance between the line of the sensor mark and the front of the t ring:

 Tring_to_sensor.jpg.bf51fb31aa3f6a49f418ff15d8f37c22.jpg

It should be close to 55mm, as above (ignore the red sticky tape patch - no idea why that's there!).

Louise

No it's just a standard baader t ring. That distortion diagram is identical to the one on the Altair page with the flattener. I was told the other day that someone else had the same pattern but extra distance was the cure so who knows. Will only really know on the next clear night but if a branded baader t ring doesn't sort it then the AA description for this FF is misleading at best. 

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Thalestris24

Thalestris24

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ps You might be able to test it during the day using an artificial star of some sort - maybe a distant spot created by a laser pointer (held close to the spot)? Never tried it myself but my brain says it could work for checking the corners. I can remember someone ( @ollypenrice ?) mentioning a ball bearing?

Louise

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Anthonyexmouth

Anthonyexmouth

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2 minutes ago, Thalestris24 said:

Well, under 10mm is too small... To get 55mm you need 44 + 11 or equivalent.

 

 

2 minutes ago, Thalestris24 said:

Well, under 10mm is too small... To get 55mm you need 44 + 11 or equivalent.

 

Just curious that the distortion pattern pic on the page for this FF indicted that it was too far away. Check the pic on this page. 

https://www.altairastro.com/lightwave-0.8x-reducer.html

 

 

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RolandKol

RolandKol

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I just checked that star distortion image at AA

And I can assure that with Newtonian, - it is 100% opposite! Not sure how it works with retractors, - never had any....

I have plenty of images with spacing 53.5, 54, 54.5 not reaching 55mm by any means and by AA image, - I am still way too far... :)

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Anthonyexmouth

Anthonyexmouth

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13 minutes ago, RolandKol said:

I just checked that star distortion image at AA

And I can assure that with Newtonian, - it is 100% opposite! Not sure how it works with retractors, - never had any....

I have plenty of images with spacing 53.5, 54, 54.5 not reaching 55mm by any means and by AA image, - I am still way too far... :)

I'm not gonna know for sure until I test it but what's annoying is being in contact with AA and they just point out the tilt and almost dismiss the spacing issue. A few on here said it was too far away from the camera and they could very well be right but another tells me they had advice with the same issue from Ian king and apparently he said straight away it was too close. Anyway, hopefully this better quality T ring will help. It does state clearly on the AA page that all it needs for correct spacing is a standard t ring. Fingers crossed for the next clear night. 

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Adam J

Adam J

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Yes this is a spacing issue for sure, correctors often produce something that looks a little like astigmatism at the edges of the field when not sufficiently spaced from the sensor.  

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carastro

carastro

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Quote

Just curious that the distortion pattern pic on the page for this FF indicted that it was too far away. 

Quote

I just checked that star distortion image at AA

And I can assure that with Newtonian, - it is 100% opposite! 

I can agree with that.  As I said in my PM to the original poster, my images were coming out indicating (from those pattern pics) that I far too far away, so I rang Ian King to ask about a spacer the correct size, and he told me I needed MORE space not less which is the opposite to what those pattern pics say.

Anyway, quite sceptical, I happened to have a spacer in the box that was near enough the size Ian King had said and tried it.                  Lo and behold, Ian King was absolutely right.  So I think we need to reverse those pattern pics.  

On the basis of the first image posted in this thread and what several of us have found, it looks like the OP need to add more distance to the spacing. 

Carole 

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Adam J

Adam J

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45 minutes ago, carastro said:

I can agree with that.  As I said in my PM to the original poster, my images were coming out indicating (from those pattern pics) that I far too far away, so I rang Ian King to ask about a spacer the correct size, and he told me I needed MORE space not less which is the opposite to what those pattern pics say.

Anyway, quite sceptical, I happened to have a spacer in the box that was near enough the size Ian King had said and tried it.                  Lo and behold, Ian King was absolutely right.  So I think we need to reverse those pattern pics.  

On the basis of the first image posted in this thread and what several of us have found, it looks like the OP need to add more distance to the spacing. 

Carole 

Yes the problem is that people naturally assume if something is appearing as being between a round star and the uncorrected star in shape then that it is as a result of under correction and so logically insufficient spacing, but in reality even if you reduce the spacing between the corrector and the sensor to zero what you see is not going to necessarily look anything like the original coma / spherical aberration at the edges of the field.

So the whole idea of over correction / under correction is actually a red herring all that you have in reality is a incorrectly spaced corrector that is creating a distortion at the edge of the field that is only indirectly related to the original distortion that it is designed to correct for. 

Adam

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Thalestris24

Thalestris24

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1 hour ago, carastro said:

I can agree with that.  As I said in my PM to the original poster, my images were coming out indicating (from those pattern pics) that I far too far away, so I rang Ian King to ask about a spacer the correct size, and he told me I needed MORE space not less which is the opposite to what those pattern pics say.

Anyway, quite sceptical, I happened to have a spacer in the box that was near enough the size Ian King had said and tried it.                  Lo and behold, Ian King was absolutely right.  So I think we need to reverse those pattern pics.  

On the basis of the first image posted in this thread and what several of us have found, it looks like the OP need to add more distance to the spacing. 

Carole 

That would concur with his t ring being thinner than normal. Probably with his new t ring everything will work ok, at least in terms of spacing - touch wood! :) 

Louise

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Anthonyexmouth

Anthonyexmouth

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19 minutes ago, Thalestris24 said:

That would concur with his t ring being thinner than normal. Probably with his new t ring everything will work ok, at least in terms of spacing - touch wood! :) 

Louise

It might even be machined better and get rid of the little bit of tilt. Maybe I shouldn't start counting my chickens. 

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Thalestris24

Thalestris24

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13 minutes ago, Anthonyexmouth said:

It might even be machined better and get rid of the little bit of tilt. Maybe I shouldn't start counting my chickens. 

Tilt could just be a small amount of flex, probably with the focus tube. But, one bridge at a time :)

Louise

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Anthonyexmouth

Anthonyexmouth

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23 hours ago, vlaiv said:

I think you can try something during the day. It's not going to be exact test, but it can give you indication of things.

Do you have any object at a distance that is highly structured? Maybe communications aerial / tower or similar? Brick wall can probably do as well - anything with high contrast pattern on it, and far away.

Test is to take an image of it with emphasis on center of the field as reference, and placing same texture / pattern in corners with different T2 adapters / spacing. You won't be able to get exact star shape (well, there might be a way to get that as well, but it's far more involved), but level of blur should be good indicator of FF/FR performance.

If you get the same clarity of image in center of the field and on the edges / in the corners - you have good correction. Any blur will indicate that there is distortion. It should be possible to compare levels of blur and best spacing / adapter will be one with the least of blur in corners.

On a separate note, if you wish to try to recover blur PSF, and get star shape out of image, you should take an object (or crop of texture, or whatever) placed in center of the field - region should be small. Shoot that same object / crop / region in corner of the frame and do deconvolution of corner image with central image - that should give you PSF of corner blur (mind you, this is easier said than done, since you need to be careful to get exact same crop of object texture in both cases).

Only clear long distance view I have is over a river to hills. Stuck in a horseshoe of houses. Just have to wait for clear skies. If it works I'll be happier about the flattener and will be note confident dropping some cash on a dedicated camera. That's if I can stop flip flopping on what one to get. 

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