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Out of focus stars across the FOV


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Evening guys.

I've spent the last 2 to 3 weeks trying to sort out, what I thought,  was a back focus issue.  I have out of focus stars all over the place, including centre view.  Ive used an Altair hypercam 1600 pro tec and now an Asi 183mm as i wanted to switch to asiair Plus.  Results are really no diferent.  I will attach images of the imaging train in the hope its something imediately obvious.

My scope is an Altair Starwave EDTF triplet and I have the dedicated 0.8 reducer attached.  I then have a 15.5mm adapter, an 11mm extension, a 2mm adapter, a 20mm filterwheel and the 6.5mm to the sensor on the camera.  Thats 55mm so I can only think of a few possibilities why I'm not getting sharp focus at least in the middle.  I would settle for that at this point........😪

  1. 55mm is wrong but I can't find anything online that says otherwise.
  2. The scope is damaged.  Altair say that's unlikely and the test report that came with the scope a few months ago is good.
  3. The reducer is the wrong way round but It didn't come with adapters to fit it the other way round and it's dedicated to this scope.
  4. Faulty cameras but I've tried 2 and they  both have the same problem.

If there's anything you can suggest I will be eternally grateful as I'm a tad frustrated as you can imagine.  Clear skies here are rare and I've already wasted so many this year.

Pete

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Edited by Astropedro
Wrong photo.
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The 55 mm back focus is required only to get all stars in the entire FOV focussed the same you still have to focus using the focusser by winding it in or out, so are you saying that from fully in to fully out the stars never get into focus ?
If this is true are they closer to focus when fully in or when fully out ?

I have never seen a reducer so long as that when the focusser is fully out as in your images the camera looks a huge distance from the OTA.

Also by the way you need to add 1/3 the thickness of your filters to the BF distance so if your filters are 3 mm thick the actual BF distance will be 56 mm.
BUT, this is not you primary issue so do not worry about that yet, the small error would only make the stars in the corners slightly elongated or teardrop shape.

Steve

 

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13 minutes ago, teoria_del_big_bang said:

The 55 mm back focus is required only to get all stars in the entire FOV focussed the same you still have to focus using the focusser by winding it in or out, so are you saying that from fully in to fully out the stars never get into focus ?
If this is true are they closer to focus when fully in or when fully out ?

I have never seen a reducer so long as that when the focusser is fully out as in your images the camera looks a huge distance from the OTA.

Also by the way you need to add 1/3 the thickness of your filters to the BF distance so if your filters are 3 mm thick the actual BF distance will be 56 mm.
BUT, this is not you primary issue so do not worry about that yet, the small error would only make the stars in the corners slightly elongated or teardrop shape.

Steve

 

Yes I'm saying exactly that.  From all the way in to all the way out, I can't get pinpoint stars either manually or with Eaf.  They are closer to focus arround 75% out.

I have some small 1mm and 0.5mm spacers to tweak the backfocus but nothing big if its way out.

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I really not sure but I would have thought that if you cannot achieve focus at all in the entire stroke of the focusser then the BF is not correct and by a long way.
I really cannot find very much info on this flattener and not even seen one exactly the same even on the Altair website.
One think that seems odd to me is that usually the back focus of 55 mm is from a shoulder on the flattener and I would have thought that was where I have marked below with the red lime so is there a chance that knurled bit at the end that looks to be around 10 to 12 mm long needs to come off and is only there when attaching a DSLR directly to the flattener via that T2 adapter ?

image.png.0155cb40ab8d82c49eda1bf5a0bf716f.png

But just a thought I could be talking rubbish it was just a thought.
 

Apart from that I do not think the issue is any of the four points you mention.
55 mm is pretty common for BF of most reducer / flatteners, not all I admit.
You can check if scope is faulty if you remove the flattener then you should still be able to get focus for most of the centre of the FOV.
I am pretty sure the flattener is the right way round.
If you have tried 2 cameras then would be pretty sure it is not the cameras.

Steve

.
 

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Have you tried it the other way around?

With my Z61 and the two flatteners I've got they're both 55mm (+1mm for filters) backspace, and I only need to rack the focus out around 20-25mm odd to reach focus, I have a 183mm also.

The one flattener I have the larger thread connects to focus tube with the smaller thread on the camera side, the reducer I have slides all the way into the 2 inch focus tube with only an m42 flange sticking out for the extension tubes to connect to.

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Okay I found this image on a USA website selling these and looks like the flattener is on correctly.
image.png.61e6569c5d9b201175f04eefef430559.png

But I still have a feeling that knurled bit on the end needs including in the 55 mm BF (or 56 with filter) so maybe you just have the BF too much.

Steve

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Posted (edited)

Ok ill have to take that piece out and order an adaptor as I'm not sure I can connect it all up without that.  I'll measure it and see if I can use it.  Even if it's without the filterwheel just to see if I can get a star to focus.

 

Thanks guys I'll report back but no clear skies tonight, tomorrow looks promising though.

Edited by Astropedro
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It might not necessarily need removing but just including in the BF distance of 55 mm, so that would be 55mm from the shoulder marked with red line.
And this is only a hunch, so could you remove the filter wheel and at least get somewhere 55 mm from that shoulder just to try and see if focus is any better ?

I have searched and searched and just cannot find another image of somebody with the same setup.

Steve

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I know you have found a diagram with the flattener on the way you have it, but I would try the other way round. On my Stella Mira FF for the 90mm ED I had the knurled part on the wrong end as the threads were the same at both ends. (My writing is the other way round too). Looking at the star shapes the exaggerated coma suggests it might be the wrong way.

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Actually I just found this image, its a different scope but it is an Altair and the flattener looks very similar to what you have EXCEPT you have that 11 mm and 2 mm adapter in the BF as well.
But this is also 55 mm back focus and from the back of that knurled adapter in this image we have:
15.5 mm M48 to T2 like you have.
17 mm Magnetic Filter holder
12.5 mm from flange of camera to image plane.

Total from rear of knurled adapter to image plane 45 mm.
So I reckon this knurled adapter is 10 mm and SHOULD be included in the BF of 55 mm.
image.thumb.png.43773194855a06b6c0d86ac66c65239f.png

Does that make sense ????

So I believe your actual BF is 65 which could be the issue, maybe ?

Can you just remove the 11 mm extension to try it if you do not have any 1 mm spacers just to try it if you are only 1 mm out then it should get much better focus at least as a test.

Steve

Edited by teoria_del_big_bang
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Posted (edited)

I've just put the knurled adaptor alongside the 15.5mm m48 to t2 adaptor and they're the same width.  No matter how hard I try, the reducer only threads into the rotator from the one side.  Another adaptor would be needed to reverse it.  So at this stage I'm thinking you're right.  Need to include the knurled part into the equation or swap it out for something else.  It not only provides 15.5mm of spacing but also reduces the thread to m48 from a much larger size and the interior thread is too big for t2.  Time to get the wallet out again......lol

Edited by Astropedro
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So.....in the pic from the reducer.....

  1. Knurled bit reducing 50something mm to 48mm......15.5mm
  2. M48 to t2 adaptor.........................................................15.5mm
  3. Filter wheel....................................................................20.0mm
  4. Camera sensor................................................................6.5mm

Total..........................................................................................57.5mm

I'm 2mm over........😪😪

Also, please tell me that the rotator between the focuser tube and the reducer isn't part of the 55mm?

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The 55 mm has to be after any optics in the flattener / reducer so if the rotator is on the end of the focusser tube then that cannot be part of the BF, it has to be measured from someting at the rear of the flattener.
Your filters will be 2 or 3 mm anyway so the true BF is either 55.7 or 56 mm so at less than 2 mm over the stars should be much improved and you should be in focus in the centre of your image with maybe tear drops around the edges.
It would at least show if we are going in the right direction or I am totally wrong.

Steve

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Not sure if this will help but as I use the same camera and filter wheel I thought I'd try and offer a possible solution.

From the end of the flattener to the camera body I have around 50mm distance and my stars are nice. My imaging train goes scope, flattener, M48 to t2 adapter, 21mm spacer, filter wheel, camera.

What distance do you have from camera to flattener?  From the image you've posted I wouldn't say your stars are out of focus, just alot of coma.

DSC_1773.JPG

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Posted (edited)

I've just ordered an m48 to t2 adaptor from harisons.   Hopefully get it Thursday.   If I've got clear skies tomorrow I'll try imaging with the 51mm (2 × 15.5mm adaptors and 20mm filterwheel) between camera and flattener and see what happens.

Thanks Elp for your insights and thankyou geordie 85 and Steve too.

 

I hope this problem goes away soon  after chatting with you guys. 😊

Edited by Astropedro
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23 minutes ago, Astropedro said:

I've just ordered an m48 to t2 adaptor from harisons.   Hopefully get it Thursday.   If I've got clear skies tomorrow I'll try imaging with the 51mm (2 × 15.5mm adaptors and 20mm filterwheel) between camera and flattener and see what happens.

Thanks Elp for your insights and thankyou geordie 85 and Steve too.

 

I hope this problem goes away soon  after chatting with you guys. 😊

You're welcome. Hopefully all will work out well for you.

Just making sure I'm clear, my 50mm is from camera body to flattener, not camera sensor. Don't want you having more troubles because of me being unclear.

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5 minutes ago, geordie85 said:

You're welcome. Hopefully all will work out well for you.

Just making sure I'm clear, my 50mm is from camera body to flattener, not camera sensor. Don't want you having more troubles because of me being unclear.

No problem.  I've got you.  I will have 48.5mm between camera body and flattener when the adaptor arrives.  I can manage 51mm at present.  So thats close to your spacing.  Just can't test it as its cloudy......🤔

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14 hours ago, Elp said:

Have you tried it the other way around?

With my Z61 and the two flatteners I've got they're both 55mm (+1mm for filters) backspace, and I only need to rack the focus out around 20-25mm odd to reach focus, I have a 183mm also.

The one flattener I have the larger thread connects to focus tube with the smaller thread on the camera side, the reducer I have slides all the way into the 2 inch focus tube with only an m42 flange sticking out for the extension tubes to connect to.

Altair Astro have got back to me this morning.........its on the wrong way round......🤣🤣

The rotator has to come off and the reducer attached directly to the focus tube.  So the rotator then becomes part of the back focus.  No idea what that means spacing wise with the filterwheel as well.  I'll have a look shortly.

Thanks again for your help guys....getting there....

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13 hours ago, Clarkey said:

I know you have found a diagram with the flattener on the way you have it, but I would try the other way round. On my Stella Mira FF for the 90mm ED I had the knurled part on the wrong end as the threads were the same at both ends. (My writing is the other way round too). Looking at the star shapes the exaggerated coma suggests it might be the wrong way.

It would appear you may be right mate.  Will know later tonight hopefully.

 

Thanks.

 

Pete

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10 minutes ago, teoria_del_big_bang said:

Oh well good news you at least know the issue even if that gives you other issues 🙂 
How deep is the rotator ? Is that extra width going to be a big problem ?

Steve

It might be.  Going to take it off now and see.  The photo shows the way it should be but that's an altair camera and the spacing will be diferent with  my asi 183.

received_466813905082048.jpeg

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