Filter position and back focus topic - Forums

Topic starter

Feb 10, 2024

Hi always,

I have few points kind of questions i want to discuss about, so here we go:

1. What is the difference in performance if any when placing the filter let's say before or after the corrector/reducer? So should it be placed after the reducer or corrector for example or before which means between telescope and flattener if possible?

2. Some correctors or reducers coming with a mount for filters mainly 2", so is it good enough to use that or place it elsewhere?

3. Will the back focus be changed if we placing the filter before the corrector or after it?

4. What is really the benefit or reason for having the back focus right?

5. If that #4 point answer is about stars, then is it really necessary to calculated back focus for setup that we will not use or bother using stars from say only Ha or SHO setup while using stars RGB from another setup? Because what is the point to calculate back focus for stars then we remove it and not using it.

6. Does the target itself say nebule be changed with different back focus distances or it is only about stars?

I posted above for using one filter only and not using filter wheels or even filter drawer, but let's say using one of those, is back focus also necessary when we will use stars RGB data from another setup well calculated or corrected already, so i know if i should spend time calculating back focus for ALL of my scopes no exception.

andrea tasselli

Frequent contributor

Feb 10, 2024

Tareq Abdulla:
Hi always,

I have few points kind of questions i want to discuss about, so here we go:

1. What is the difference in performance if any when placing the filter let's say before or after the corrector/reducer? So should it be placed after the reducer or corrector for example or before which means between telescope and flattener if possible?

2. Some correctors or reducers coming with a mount for filters mainly 2", so is it good enough to use that or place it elsewhere?

3. Will the back focus be changed if we placing the filter before the corrector or after it?

4. What is really the benefit or reason for having the back focus right?

5. If that #4 point answer is about stars, then is it really necessary to calculated back focus for setup that we will not use or bother using stars from say only Ha or SHO setup while using stars RGB from another setup? Because what is the point to calculate back focus for stars then we remove it and not using it.

6. Does the target itself say nebule be changed with different back focus distances or it is only about stars?

I posted above for using one filter only and not using filter wheels or even filter drawer, but let's say using one of those, is back focus also necessary when we will use stars RGB data from another setup well calculated or corrected already, so i know if i should spend time calculating back focus for ALL of my scopes no exception.

1. Essentially none.

2. It's practical thing if you don't plan to change filters often. So it's good enough.

3. Yes. If the filter is in front of the corrector there is only a modicum of focus shift. If placed after then it would move the focus point out therefore affecting effective backspace.

4. Field correction, in the main.

5. It isn't about the stars it is about everything, only is less obvious to the untrained eye.

6. See 4 & 5.

The filter thickness is in the main between 1 mm and 2 mm so, unless the corrector is exceedingly sensitive to the effective backspace you'll probably be well served with an average value for ALL your telescopes.

Tareq Abdulla

Topic starter

Feb 10, 2024

andrea tasselli:
Tareq Abdulla:
Hi always,

I have few points kind of questions i want to discuss about, so here we go:

1. What is the difference in performance if any when placing the filter let's say before or after the corrector/reducer? So should it be placed after the reducer or corrector for example or before which means between telescope and flattener if possible?

2. Some correctors or reducers coming with a mount for filters mainly 2", so is it good enough to use that or place it elsewhere?

3. Will the back focus be changed if we placing the filter before the corrector or after it?

4. What is really the benefit or reason for having the back focus right?

5. If that #4 point answer is about stars, then is it really necessary to calculated back focus for setup that we will not use or bother using stars from say only Ha or SHO setup while using stars RGB from another setup? Because what is the point to calculate back focus for stars then we remove it and not using it.

6. Does the target itself say nebule be changed with different back focus distances or it is only about stars?

I posted above for using one filter only and not using filter wheels or even filter drawer, but let's say using one of those, is back focus also necessary when we will use stars RGB data from another setup well calculated or corrected already, so i know if i should spend time calculating back focus for ALL of my scopes no exception.

1. Essentially none.

2. It's practical thing if you don't plan to change filters often. So it's good enough.

3. Yes. If the filter is in front of the corrector there is only a modicum of focus shift. If placed after then it would move the focus point out therefore affecting effective backspace.

4. Field correction, in the main.

5. It isn't about the stars it is about everything, only is less obvious to the untrained eye.

6. See 4 & 5.

The filter thickness is in the main between 1 mm and 2 mm so, unless the corrector is exceedingly sensitive to the effective backspace you'll probably be well served with an average value for ALL your telescopes.

Ok, i forgot about the filter thickness out there, thanks for pointing out about it.

Now for that point #3, if you said that putting the filter after that corrector will change the back focus or the focus point then why isn't that applied to the scopes with correctors built in, so that also should be there, or does that make it in a way that it doesn't matter anywhere to place the camera and filter it will be corrected precisely, kind of a mystery for me.

I think you mean with different back focus so the field overall isn't corrected mostly by edges whether it is stars or something else, because they are curved, although there is a settings in Photoshop to correct the curvature i think it is better to correct it by hardware or setup than in processing, assuming it can be corrected for astro results, now that i will change a lot of things thing, and in this case i have to make sure all my scopes are corrected then and not just RGB setup.

Thank you

andrea tasselli

Frequent contributor

Feb 10, 2024

Tareq Abdulla:
Now for that point #3, if you said that putting the filter after that corrector will change the back focus or the focus point then why isn't that applied to the scopes with correctors built in, so that also should be there, or does that make it in a way that it doesn't matter anywhere to place the camera and filter it will be corrected precisely, kind of a mystery for me.

But it does since it moves the focus out and in fact some professional designs have incorporated the filter in the optical budget as well as most high end lens that have a filter holder. As I said before, the effect is expected to be tiny and well within the tolerances of most correctors' design.

Tareq Abdulla:
I think you mean with different back focus so the field overall isn't corrected mostly by edges whether it is stars or something else, because they are curved, although there is a settings in Photoshop to correct the curvature i think it is better to correct it by hardware or setup than in processing, assuming it can be corrected for astro results, now that i will change a lot of things thing, and in this case i have to make sure all my scopes are corrected then and not just RGB setup.

Photoshop curvature correction does not correct for field curvature (and it couldn't) but corrects for field distortion which technically is considered an optical aberration but since it doesn't affect the PSF this isn't much a concern for AP, except for wide fields.

Tareq Abdulla

Topic starter

Feb 10, 2024

andrea tasselli:
Tareq Abdulla:
Now for that point #3, if you said that putting the filter after that corrector will change the back focus or the focus point then why isn't that applied to the scopes with correctors built in, so that also should be there, or does that make it in a way that it doesn't matter anywhere to place the camera and filter it will be corrected precisely, kind of a mystery for me.

But it does since it moves the focus out and in fact some professional designs have incorporated the filter in the optical budget as well as most high end lens that have a filter holder. As I said before, the effect is expected to be tiny and well within the tolerances of most correctors' design.
Tareq Abdulla:
I think you mean with different back focus so the field overall isn't corrected mostly by edges whether it is stars or something else, because they are curved, although there is a settings in Photoshop to correct the curvature i think it is better to correct it by hardware or setup than in processing, assuming it can be corrected for astro results, now that i will change a lot of things thing, and in this case i have to make sure all my scopes are corrected then and not just RGB setup.

Photoshop curvature correction does not correct for field curvature (and it couldn't) but corrects for field distortion which technically is considered an optical aberration but since it doesn't affect the PSF this isn't much a concern for AP, except for wide fields.

Ok, got it, thank you very much for all the answers and clarification, much appreciated.

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