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Focus issues


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Hi again team,

im looking for advice on some focus issues I’m having at the moment.

A while ago I had to repair my asi1600mm pro as I had fried the motherboard.

I replaced the board and used it again the other evening.

My image train is.... SW 150pds Newtonian , baader coma corrector Mark iii , zwo filter wheel, asi1600.

I could only just get a focus on the L filter with the focus hub all the way out , something which I had never struck before.

I could not get focus at all on the Ha or Oiii filters, just a blank image.

The only thing I can think of is that I’ve set up my back focus spacing wrong ? I’m pretty sure I’ve got 55mm between the baader and the camera chip.

Im totally at a loss to explain it ?! 

I have put a pic below of the image I got.

This is 240x30second subs on what should be the L filter....it’s actually blank as I don’t have a filter, it’s just nothing in front of the sensor.

Its 30 second subs as I am having a nightmare with guiding at the moment.....but that’s another story for later 🤦🏻‍♂️

I think for nearly 2 hours of data in short subs I should have gotten something rather better than this.

Any suggestions as to how to resolve this would be much appreciated as the whole subject of backfocus leaves me a jibbering wreck 🤪🤦🏻‍♂️🤷‍♂️

45904EFC-483E-4FC2-844B-C89764472536.jpeg

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10 minutes ago, Andywilliams said:

The only thing I can think of is that I’ve set up my back focus spacing wrong ? I’m pretty sure I’ve got 55mm between the baader and the camera chip.

Unless your backspacing is out by a few centimetres and pushing the focus point out past the focus travel, it shouldn’t affect this.

Can you post a photo of your focuser etc. 

When you say Focus Hub all the way out, what do you mean?  Is the the actual drawtube maxed out or do you have an autofocuser that thinks it’s all the way out?

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It makes no sense that you can get somewhere near focus with no filter but nothing with the Ha and OIII.
It probably is not good practice to use nothing for L as the back focus will be different when using the filters to using nothing. A filter changes the back focus by 1/3 the thickness of the filter, so usually by about 0.7 to 1 mm.

But that said you should still get some blurry image and not nothing.

What are you using to achieve focus, how long exposure are you using for Ha and OIII when trying to focus ?

Do you have other filters that will allow more light to pass such as RGB filters, what is the focus like with those.

As mentioned above the back focus when using filters will not be exactly 55 mm but 55mm + 1/3 thickness of the filters but even when this is wrong by a few mm you should still get focus on the middle of the image but the stars in the corners and outer edges may be elongated.

Steve

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5 hours ago, Andywilliams said:

I’m pretty sure I’ve got 55mm between the baader and the camera chip.

That's where you should check, "pretty sure" isn't good enough.

 

5 hours ago, Andywilliams said:

I could not get focus at all on the Ha or Oiii filters, just a blank image.

Filter wheel stuck on your opaque "Darks" Filter ?

Michael

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10 hours ago, tooth_dr said:

Unless your backspacing is out by a few centimetres and pushing the focus point out past the focus travel, it shouldn’t affect this.

Can you post a photo of your focuser etc. 

When you say Focus Hub all the way out, what do you mean?  Is the the actual drawtube maxed out or do you have an autofocuser that thinks it’s all the way out?

Hi there....and thanks for your reply 🙏

i mean to say that the draw tube is maxed all the way out.

i can’t send a pic of my image train at the moment as I’m away from home for work.

i can send one on Friday 👍

my focus at the moment is a manual focus.

with thanks,

Andy 

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37 minutes ago, Andywilliams said:

my focus at the moment is a manual focus.

So for the Ha and OIII do you take an image of several seconds and then keep adjusting the focus until you achieve focus?
I can see that with no filter you can just use live view and even a mask but I would be surprised if you get enough light passing an NB filter in live view.

Also did the same optical train work okay before you repaired the CCD ?
You haven't just added the coma corrector have you ?

Steve

Edited by teoria_del_big_bang
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9 hours ago, teoria_del_big_bang said:

It makes no sense that you can get somewhere near focus with no filter but nothing with the Ha and OIII.
It probably is not good practice to use nothing for L as the back focus will be different when using the filters to using nothing. A filter changes the back focus by 1/3 the thickness of the filter, so usually by about 0.7 to 1 mm.

But that said you should still get some blurry image and not nothing.

What are you using to achieve focus, how long exposure are you using for Ha and OIII when trying to focus ?

Do you have other filters that will allow more light to pass such as RGB filters, what is the focus like with those.

As mentioned above the back focus when using filters will not be exactly 55 mm but 55mm + 1/3 thickness of the filters but even when this is wrong by a few mm you should still get focus on the middle of the image but the stars in the corners and outer edges may be elongated.

Steve

Thanks for your reply Steve,

Im currently using a manual focus.

i didn’t try any of the rgb  filters .... that was a mistake.

I was using 20 seconds at gain 139 for focussing images. Too short for Ha ? 

I was always under the impression (yep, I don’t really understand the subject of backfocus 😬) ....that backfocus will not affect focus via the drawtube ? 

If it’s just totally a matter of backfocus then it should be easily resolvable !

i know that I’ve inadvertently changed something while my setup was stripped down for repair....it’s just a matter of figuring out what !

cheers,

Andy 

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1 minute ago, Andywilliams said:

Thanks for your reply Steve,

Im currently using a manual focus.

i didn’t try any of the rgb  filters .... that was a mistake.

I was using 20 seconds at gain 139 for focussing images. Too short for Ha ? 

I was always under the impression (yep, I don’t really understand the subject of backfocus 😬) ....that backfocus will not affect focus via the drawtube ? 

If it’s just totally a matter of backfocus then it should be easily resolvable !

i know that I’ve inadvertently changed something while my setup was stripped down for repair....it’s just a matter of figuring out what !

cheers,

Andy 

Andy,

For Ha and OIII I should think 20 seconds is easily enough if there ae bright stars for focussing. I think I only use 4 seconds or so.

As I said if the figure for your back focus is 55mm then because of the filters it will be slightly more due to the thickness of the filters, which means it is not good practice to have no filter at all for the L filter and is the reason filter manufacturers make a plane glass, or maybe an IR filter to use as the L and it is best to use filters of all same thickness so that pretty much when focus is achieved on one filter the others should be as near as damned it in focus.
But your filters should be between 2 and 3 mm thick I would think so the back focus will really be between 55.7 and 56 mm.
Does that extra 0.7 to 1 mm make a difference - well to a noticeable extent in the corners yes, the centre portion will be able to achieve round focussed stars but the edges will have some tear dropped shaped stars. This small error will be noticeable if you look closely, unless you do a heavy crop on the final image, but if more than this the effect could be well noticeable, and on your image although the whole image is not well focussed you can see that in middle the stars are out of focussed but roundish but in the corners they are distorted as well as mis-shapen so there is a problem with the back focus.

Start in middle of image
image.png.ef5d399465d3e93dc616d00fdb808b71.png

 

Stars in lower LH corner
image.png.fe6210c71787051dfa8d62683ec32005.png

 

Is this the reason for the inability to obtain focus - I don't think so.

So it could be you have two issues, one that means when focus tube is fully extended it still needs to go more to achieve focus and there is a small error on the back focus distance.

I know you are not at home but I really think when you are a picture of the setup might help us determine if there is an issue there.

Steve  

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On 04/01/2022 at 08:08, teoria_del_big_bang said:

Andy,

For Ha and OIII I should think 20 seconds is easily enough if there ae bright stars for focussing. I think I only use 4 seconds or so.

As I said if the figure for your back focus is 55mm then because of the filters it will be slightly more due to the thickness of the filters, which means it is not good practice to have no filter at all for the L filter and is the reason filter manufacturers make a plane glass, or maybe an IR filter to use as the L and it is best to use filters of all same thickness so that pretty much when focus is achieved on one filter the others should be as near as damned it in focus.
But your filters should be between 2 and 3 mm thick I would think so the back focus will really be between 55.7 and 56 mm.
Does that extra 0.7 to 1 mm make a difference - well to a noticeable extent in the corners yes, the centre portion will be able to achieve round focussed stars but the edges will have some tear dropped shaped stars. This small error will be noticeable if you look closely, unless you do a heavy crop on the final image, but if more than this the effect could be well noticeable, and on your image although the whole image is not well focussed you can see that in middle the stars are out of focussed but roundish but in the corners they are distorted as well as mis-shapen so there is a problem with the back focus.

Start in middle of image
image.png.ef5d399465d3e93dc616d00fdb808b71.png

 

Stars in lower LH corner
image.png.fe6210c71787051dfa8d62683ec32005.png

 

Is this the reason for the inability to obtain focus - I don't think so.

So it could be you have two issues, one that means when focus tube is fully extended it still needs to go more to achieve focus and there is a small error on the back focus distance.

I know you are not at home but I really think when you are a picture of the setup might help us determine if there is an issue there.

Steve  

Hi Steve,

many thanks for your reply. I am home now.

So this is the configuration as I had it on the night.

Focus tube fully extended and asi1600 spaced to the baader filter.

I could only just get focus and no focus with the Ha and Oiii filters.

Anything strike you as obvious ?

thanks so much for your help so far.

clear skies,

Andy 

E0738043-9E57-463E-93B4-CA298BA2DE67.jpeg

9BF631B8-5104-4BAA-83A5-6C8B0F4347C5.jpeg

B2AB7BAA-F4E4-49E6-ACA5-3CEEB9DBD0EF.jpeg

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11 minutes ago, teoria_del_big_bang said:

Where is the coma corrector, I cant seem to see it in those pics ?

Steve

Steve, to me looks like he needs to add the second extender after the Baader CC as from the picture looks like only one in use at the moment so thats. CC then 16.5mm extender then 11mm extender then t2-t2 (comes with EFW 2mm) then filter wheel thenCooled-Mono-Camera-solution.thumb.jpg.9fc57cf08aef25c84c62924df5196b0c.jpg camera  

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3 minutes ago, fozzybear said:

Steve, to me looks like he needs to add the second extender after the Baader CC as from the picture looks like only one in use at the moment so thats. CC then 16.5mm extender then 11mm extender then t2-t2 (comes with EFW 2mm) then filter wheel then camera  

Hi Andy, I couldn't see the corrector is it that I have ringed in red, that just looked like an EP holder to me.

image.png.a6333517ada73d31e6bab76e32f2007c.png

Steve

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1 minute ago, teoria_del_big_bang said:

Hi Andy, I couldn't see the corrector is it that I have ringed in red, that just looked like an EP holder to me.

image.png.a6333517ada73d31e6bab76e32f2007c.png

Steve

Steve, Yes thats the puppy in there with the retaining ring on Baader CC. So should be 2 spacers and looks to be only the one attached.  Oh Happy New year by the way (side Note).  I don't know if the OP is using the M42-M48  adaptor but probably so as would be hidden from view as this would attach to the CC then the first spacer and 0mm 

Andy

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@Andywilliams I agree with @fozzybear that there is a spacer missing between the coma corrector and the filter wheel, I would guess the one ringed in red below..

Yes you are right that the back focus (which is the back-focus from the coma corrector to the imaging plane) should be 55, BUT also to complicate things adding a piece of glass into the train such as the filters adds to this distance by 1/3 the width of the filter. Now filters do vary a bit but probably will not notice anything less than 0.5 mm out so filters generally are between 2 and 3 mm so 1/3 of this is as near as damn it 1 mm, hence why in the image below it states 56 mm back focus.

image.thumb.png.e6b53ecb71329ced029055c0721e4af3.png

I am confident with this you will achieve focus.

Tip I have learned is once you get a set up that works to take a few pics of it for reference later on 🙂 

Steve

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3 minutes ago, teoria_del_big_bang said:

Tip I have learned is once you get a set up that works to take a few pics of it for reference later on

Steve, Agree with that and other things as well. 

Andy

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Steve and Andy ..... Thankyou so much for this gold info 🤗🤗

I’m going to configure it as per your advice and see how I go later....

skies are looking good for tonight as well 👍👍

I will certainly let you know how I get on 

cheers and clear skies,

Andy 

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Maybe I'm missing something in the above posts.

Andy says he already has the ASI 1600 55mm back from the MPCC (let's ignore the tweak for the filter thickness for the moment).

So adding a further 11mm spacer between the MPCC and the ASI 1600 is going to make the spacing 66mm.

The 11mm spacer would be better placed between the MPCC and the focuser, so that the focuser can be moved inwards to get focus.

And just for clarity.

The diagrams that ZWO show on their website keep recommending 56mm Backspacing WITH NO CC.

Only CCs and FFs have Backspacing.

Cameras only have a Flange Distance, 6.5mm in the case of the ASI 1600.

So a Camera and EFW and no CC/FF can be placed anywhere in the range of the OTA's focuser.

The 56mm in this case is irrelevant and causes confusion.

Michael

 

 

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9 hours ago, michael8554 said:

The diagrams that ZWO show on their website keep recommending 56mm Backspacing WITH NO CC.

backspacing and back focus are two different things so back spacing on the camera without attachments ie flush to the flange is 6.5mm and the Baader CC requires 55mm. so we need to add 48.5mm between the Mpcc and the sensor and if you look at the OP photo's this does not look to be enough according to ZWO setup requirements albeit you end up with 56mm with supplied extensions and the EFW does this make sense9BF631B8-5104-4BAA-83A5-6C8B0F4347C5.thumb.jpeg.a2a97ba3f86189675404416c551ed294.jpeg.17a6a954f85f8ea7d11fcb49f5ce3a77.jpeg

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I admit we do not know that the missing spacer is 11 mm as that would only depend on whether the spacer being used is 16.5 mm, which we do not know.
But the spacing is not correct which you can see form the shape of the stars in the corners, even though the image was out of focus.
image.png.fe6210c71787051dfa8d62683ec32005.png

So to get 55 mm if using only one spacer between the cc and fw it would need to be 27.5 mm and as the fw is only 20 mm I would say that spacer does not look to be 7.5 mm wider than the FW although measuring anything from a photo is very deceptive.
image.png.6bd031afc8b9c2879f0a8f3a94c42c81.png

We can only offer advice to what we can see and cannot say for definite what is wrong only the OP can do that.

@Andywilliams can you confirm what size he spacer between the FW and CC is ?

 

Steve

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7 minutes ago, michael8554 said:

Steve, your diagram is more conclusive than mine is (though 20 + 27.5 + 6.5 only makes 54mm)

Andywilliams, this 55mm Back Spacing is not difficult to measure.

Use Steve's diagram and post the result.

 

There should be 2 mm in the adapter I think on the scope side of the FV so this is still aiming for the 56 mm shown on ZWO website but either way 55 or 56 mm should give proeet good images that can be focussed.

Steve

Edited by teoria_del_big_bang
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