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MartinFransson

Filters - add or subtract spacing?

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Help me out here guys... let´s say I have a flattener that needs 123mm spacing and I solve that with spacers, filter wheel etc. Then I add my filters, for example a 3mm Astrodon filter. I understand that this affects the spacing by approximately 1mm. BUT... does the filter add 1mm spacing or does it require me to add an extra 1mm?

Two alternatives here to reach 123mm back focus distance:
1. 122mm spacers + Astrodon filter
2. 124mm spacers + Astrodon filter

Help appreciated 🤯

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It's a question that seems to cause a lot of confusion.

Here is the example I gave which I hope explains it: 

For my FSQ85 reducer I need a spacing of 72.2mm.  If I have a 3mm astrodon filter in the image train it pushes the image cone outwards by 1mm so the spacing requirement is increased by 1mm to 73.2mm.  You are ADDING to the back focus requirement.

So to make that 73.2mm I have:

Atik OAG: 24mm

Atik EFW2:  21.8mm

Atik 490: 13mm

Spacer:  14.4mm

=73.2mm - the impact of the astrodon filter has been taken into account by increasing the required back focus distance.

Now alternatively you can include the impact of the filter on the imaging train calculation by including it as follows:

Atik OAG: 24mm

Atik EFW2:  21.8mm

Astrodon filter: -1mm

Atik 490: 13mm

Spacer:  14.4mm

=72.2mm

In this instance you are SUBTRACTING the impact of the filters on the elements contributing to your backfocus.  It might help the penny drop if you calculate the spacer required in both of the scenarios above.

More on this thread: 

 

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If you are using a filter wheel as described above its internal to the wheel and doesn’t effect the distance

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2 minutes ago, Astrosharkey said:

If you are using a filter wheel as described above its internal to the wheel and doesn’t effect the distance

I don't believe that is the case as it's refraction due to the filter that pushes the light cone out.

Cheers, Ian

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40 minutes ago, x6gas said:

For my FSQ85 reducer I need a spacing of 72.2mm.  If I have a 3mm astrodon filter in the image train it pushes the image cone outwards by 1mm so the spacing requirement is increased by 1mm to 73.2mm.  You are ADDING to the back focus requirement.

This is correct

10 minutes ago, Astrosharkey said:

If you are using a filter wheel as described above its internal to the wheel and doesn’t effect the distance

This is incorrect - as 'Scottie' would say, 'Ye cannae change the laws of physics'

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So to recap if the filter comes between the flattener and the image plane you need to add 1/3 the thickness of the glass in the filter to the back focus and that is true even if inside a filter wheel.

There are thin spacers, such as 1 mm, 0.5 mm, 0.3 mm etc available in T2 and M42 available in order to get just the right distance without resorting to anything custom made.

If your filters are not all same make and so different thicknesses then just work on an average thickness of glass, not ideal but the best you can do and most filters are 2 to 3 mm so the difference is very small and will have very little affect (I would think - honestly no real idea how much 0.3 mm error in back focus would make to the image and whether it would be that noticeable).

Steve

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Theres physical distance and there's optical distance. If you add a filter, you increase the optical distance. If you follow that train of thought, you would reduce your spacers. I think its a case of trial and error tbf...

 

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3 minutes ago, david_taurus83 said:

Theres physical distance and there's optical distance. If you add a filter, you increase the optical distance. If you follow that train of thought, you would reduce your spacers. I think its a case of trial and error tbf...

 

I don't  agree as per my diagram in the linked thread you need to increase  the back focus distance by 1mm.

Regards Andrew 

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19 minutes ago, andrew s said:

I don't  agree as per my diagram in the linked thread you need to increase  the back focus distance by 1mm.

Regards Andrew 

I agree with you Andrew.

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52 minutes ago, andrew s said:

I don't  agree as per my diagram in the linked thread you need to increase  the back focus distance by 1mm.

Regards Andrew 

If the optical distance is increased, by say 1mm, in the case of Astrodon filters, the optical distance between flattener and sensor becomes too far. So is my own experience..

Ive had better results by reducing the distance.

https://stargazerslounge.com/topic/340444-flattener-spacing-does-it-work/?tab=comments#comment-3707128

 

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1 hour ago, andrew s said:

I don't  agree as per my diagram in the linked thread you need to increase  the back focus distance by 1mm.

I agree with you, Andrew - the optical distance dictates the physical distance

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Posted (edited)

@david_taurus83 I don't  doubt you experimental results.

If you change the distance from the objective to the reducer then you will change its performance. If a filter is thin enough not to add aberration then it will just push out the focus. I don't think optical v physical distance is the issue it is the simultaneous move ment of focuser and back distance change due to the filter that confuses the situation.

If you have a built in corrector (I.e. before the focuser) then I belive you would have to push our the camera to obtain focus with a filter compared to without.

However, I would always recommend experimenting to find what works with your particular set up. Without detailed ray tracing of the entire system it is impossible to predict exactly what will happen if you change the  objective  to corrector and correctror to CCD distance. 

Regards Andrew 

Edited by andrew s
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2 minutes ago, MartinFransson said:

...and there I´m lost again just when I thought I had it down 🤣

My advice would be to start by adding 1mm. If this proves unsatisfactory experiment about +/-  2mm of the optimal distance

Regards Andrew 

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2 minutes ago, andrew s said:

@david_taurus83 I don't  doubt you experimental results.

If you change the distance from the objective to the reducer then you will change its performance. If a filter is thin enough not to add aberration then it will just push out the focus. I don't think optical v physical distance is the issue it is the simultaneous move ment of focuser and back distance change due to the filter that confuses the situation.

If you have a built in corrector (I.e. before the focuser) then I belive you would have to push our the camera to obtain focus with a filter compared to without.

However, I would always recommend experimenting to find what works with your particular set up. Without detailed ray tracing of the entire system it is impossible to predict exactly what will happen. 

Regards Andrew 

Perhaps I came across as too staunch. I can only comment on my own findings, rightly or wrongly. I see often that it is suggested you should always add spacers to account for filter thickness but i would always advocate experimenting either side of the proposed optimum spacing. A frustrating endurance anyway. I think I will always stick to Petzvals in future!

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14 minutes ago, david_taurus83 said:

Perhaps I came across as too staunch. I can only comment on my own findings, rightly or wrongly. I see often that it is suggested you should always add spacers to account for filter thickness but i would always advocate experimenting either side of the proposed optimum spacing. A frustrating endurance anyway. I think I will always stick to Petzvals in future!

I think you are absolutely right to suggest experimentation, David - just because the 'logic' suggests one route does not necessarily mean that it is a 'one size fits all' as your own experimentation has clearly shown! My own experiments confirmed the 'physics' as I see them but even QSI whose camera I use are not clear on this issue ............

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4 hours ago, MartinFransson said:

Help me out here guys... let´s say I have a flattener that needs 123mm spacing and I solve that with spacers, filter wheel etc. Then I add my filters, for example a 3mm Astrodon filter. I understand that this affects the spacing by approximately 1mm. BUT... does the filter add 1mm spacing or does it require me to add an extra 1mm?

Two alternatives here to reach 123mm back focus distance:
1. 122mm spacers + Astrodon filter
2. 124mm spacers + Astrodon filter

Help appreciated 🤯

I am pretty new to all this but because I too had the same issues with back focus on my first setup as far as I understand :-

A back focus of 123mm with a 3mm glass filter would require spacers to make 124 mm (In Theory). And that is what seems to have worked for me in two imaging rigs.

So I would say that is your starting point (and hopefully end point 🙂 )

But after saying that from many of these threads I have read then @david_taurus83 may well be right as many have said to get it 100% perfect some people do fiddle about with small spacers to get the field absolutely flat. To be honest I just dont think my eyes are that good, despite adding a 1 mm spacer to get the distance the theoretical correct value I am hard pushed to say that I could see any difference, and I think it is the outer edges that suffer and usually I crop some of this anyway so not sure how close you must get this,

Steve

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Nice diagram. I think it's got to be easier to think in terms of first principles.  If the filter pushes the focal plane further away (from the objective lens) then you need to add to the spacing requirement. 

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