Clarification please….🤔🤔

[Stuart1971]

Good Morning All,
Ok, I hope someone will clarify this for me as I just read a really confusing thread on CN
My flattener requires 56.2 physical backspace, and I use 2mm filters, so I have a physical backspace of 56.9mm as I added an extra spacer to add the 0.7mm as 1/3rd of my filter thickness….is this correct…??

As this guy on CN has said that if the backspace needed is 56.2mm and you use 2mm filters, then you already have 0.7mm backspace (due to the filter in the light path) and so you only need 55.5mm of physical spacing, to reach the correct 56.2mm…🤔🤔

[geeklee]

3 minutes ago, Stuart1971 said:

My flattener requires 56.2 physical backspace, and I use 2mm filters, so I have a physical backspace of 56.9mm as I added an extra spacer to add the 0.7mm as 1/3rd of my filter thickness….is this correct…??

This is how I understand it.

You always add the 1/3 filter thickness to your backspace don't you?

3 minutes ago, Stuart1971 said:

As this guy on CN has said that if the backspace needed is 56.2mm and you use 2mm filters, then you already have 0.7mm backspace (due to the filter in the light path) and so you only need 55.5mm of physical spacing, to reach the correct 56.2mm…🤔🤔

This is NOT how I understand it.

Always happy to learn something new/correct though.

[GalaxyGael]

If the refractive index of the filter is~ 1.5, then the refraction effect it causes to the light at 2 mm thickness can be compensated by adding approx. 1/3 the thickness value to the backspacing. Approx. 0.67 mm for a 2mm thick filter. NOrmally measured at a single wavelength though, but we assume that they are coming to focus together by the time they hit the filter and that any refraction difference from green to red is negligible for such a 2 mm thickness.

For cameras with either IR-cut or AR coated glass installed (sometimes for Ar-filled chambers it is quartz glass to pass certain UV wavelengths, which has a refractive index measured at a similar wavelength of ~1.55), compensation would be made also. But, I did read somewhere that for ASI cameras, say a 2600MC with IR cut filter installed as a working example, that the 17.5 mm backspacing to the sensor is the optical backspacing, i.e. the effect of the filter is included in the value. Only need to compensate with additional backspacing if you are adding additional filters into the optical train in that example.

Edited January 18, 2023 by GalaxyGael

[Stuart1971]

3 minutes ago, GalaxyGael said:

If the refractive index of the filter is~ 1.5, then the refraction effect it causes to the light at 2 mm thickness can be compensated by adding approx. 1/3 the thickness value to the backspacing. Approx. 0.67 mm for a 2mm thick filter. NOrmally measured at a single wavelength though, but we assume that they are coming to focus together by the time they hit the filter and that any refraction difference from green to red is negligible for such a 2 mm thickness.

For cameras with either IR-cut or AR coated glass installed (sometimes for Ar-filled chambers it is quartz glass to pass certain UV wavelengths, which has a refractive index measured at a similar wavelength of ~1.55), compensation would be made also. But, I did read somewhere that for ASI cameras, say a 2600MC with IR cut filter installed as a working example, that the 17.5 mm backspacing to the sensor is the optical backspacing, i.e. the effect of the filter is included in the value. Only need to compensate with additional backspacing if you are adding additional filters into the optical train in that example.

Thanks

so I have my set up correct then…?

[Stuart1971]

15 minutes ago, geeklee said:

This is how I understand it.

You always add the 1/3 filter thickness to your backspace don't you?

Yes, this is what I always have believed…🤔

Edited January 18, 2023 by Stuart1971

[bosun21]

I have always added the filter (1/3) thickness to the optical train and reduced my train by that amount to keep it at the prerequisite 55mm or whatever your set up calls for.

Edited January 18, 2023 by bosun21
Typo

[Stuart1971]

22 minutes ago, bosun21 said:

I have always added the filter (1/3) thickness to the optical train and reduced my train by that amount to keep it at the prerequisite 55mm or whatever your set up calls for.

Sorry am confused, you addd it and then reduced it…🤔🤔

[bosun21]

2 minutes ago, Stuart1971 said:

Sorry am confused, you addd it and then reduced it…🤔🤔

Not figuratively! I allow for the 1/3 thickness of the filter as part of my optical train. So I would still arrive at my desired backspace (55mm) in my case. So if my train was at 55mm without the filter, after adding it I would remove spacing shims to the magnitude of the 1/3 filter thickness.

[StevieDvd]

I thought you add the 1/3 to the back focus. I checked a few websites such as here

Scroll down to the  'I Use Filters in My Imaging Train. Does this Change My Back Focus?' section.

 

[ONIKKINEN]

If you need 56.2mm and have 2mm filters, you need 56.9mm so i think you have it set up correctly.

But in practice the 56.2mm stated by the flattener manufacturer wont be exactly that, nor will your extenders and shims be exactly the length they say they are, like for example an OAG might be 19.05mm instead of 19mm in optical length, a 10mm extender may be 9.95mm and so on. Best prepare and get a bag of shims of various sizes (if you dont have already) and spend a couple of nights fiddling with the kit.

I dont know how much difference there is in refractor correctors and newtonian ones, but so far of the 2 newtonian correctors i have tried so far neither were best at the stated backfocus.

[geeklee]

18 minutes ago, bosun21 said:

Not figuratively! I allow for the 1/3 thickness of the filter as part of my optical train. So I would still arrive at my desired backspace (55mm) in my case. So if my train was at 55mm without the filter, after adding it I would remove spacing shims to the magnitude of the 1/3 filter thickness.

This sounds like the opposite to what should be done.

As I understand it, the filter reduces the optical path by ~1/3 of its thickness, so you need to add to the physical path by the same amount to compensate.  So 55mm backspace and 1mm filter needs a 0.33mm spacer added to reach 55.3mm physical spacing.

Edited January 18, 2023 by geeklee

[Stuart1971]

22 minutes ago, bosun21 said:

Not figuratively! I allow for the 1/3 thickness of the filter as part of my optical train. So I would still arrive at my desired backspace (55mm) in my case. So if my train was at 55mm without the filter, after adding it I would remove spacing shims to the magnitude of the 1/3 filter thickness.

Hmmm, I think that is incorrect…this is what I am trying to clarify…you would be 1/3 of your filter thickness short in your train using that approach…🤔

[bosun21]

5 minutes ago, geeklee said:

This sounds like the opposite to what should be done.

As I understand it, the filter reduces the optical path by ~1/3 of its thickness, so you need to add to the physical path by the same amount to compensate.  So 55mm backspace and 1mm filter needs a 0.33mm spacer added to reach 55.3mm physical spacing.

I understood it that it adds 1/3 of the diameter of the filter glass. I reckon we all need clarification. Anyone seen @vlaiv?

[bosun21]

2 minutes ago, Stuart1971 said:

Hmmm, I think that is incorrect…this is what I am trying to clarify…you would be 1/3 of your filter thickness short in your train using that approach…🤔

Where’s @vlaiv when we need him 😂 Now I need clarification as well as the OP.

[Stuart1971]

13 minutes ago, bosun21 said:

Where’s @vlaiv when we need him 😂 Now I need clarification as well as the OP.

If you have a flattener and a sensor with a 55mm gap as recommended by most, you are all set …👍🏻

If you then add a 3mm filter into that 55mm gap, you are pushing the focus point away by 1mm, (1/3 of 3mm) so to get proper spacing again you now need 56mm of spacing to reach proper focus, my way of thinking to add the 1/3 filter thickness to the physical spacer

Makes sense to me….🤔🤔

Edited January 18, 2023 by Stuart1971

[bosun21]

4 minutes ago, Stuart1971 said:

If you have a flattener and a sensor with a 55mm gap as recommended by most, you are all set …👍🏻

If you then add a 3mm filter into that 55mm gap, you are pushing the focus point away by 1mm, (1/3 of 3mm) so to get proper spacing again you now need 56mm of spacing to reach proper focus, my way of thinking to add the 1/3 filter thickness to the physical spacer

Makes sense to me….🤔🤔

I agree with you, but I was told the opposite 🤔. I now want to know the actual truth in the matter. I’m starting to lean to your assessment. My query is which direction is it pushing the focus point? +/-

Edited January 18, 2023 by bosun21
Typo

[Stuart1971]

4 minutes ago, bosun21 said:

I agree with you, but I was told the opposite 🤔. I now want to know the actual truth in the matter. I’m starting to lean to your assessment. My query is which direction is it pushing the focus point? +/-

Further away from the flattener, hence needing a bigger spacer…so +

[bosun21]

1 minute ago, Stuart1971 said:

Further away from the flattener, hence needing a bigger spacer…so +

Makes sense.

[Stuart1971]

1 minute ago, bosun21 said:

Makes sense.

I am with you though, I want some definitive clarification, before tonight if possible as it’s clear tonight…😂👍🏻

Edited January 18, 2023 by Stuart1971

[GalaxyGael]

 

1 hour ago, Stuart1971 said:

Thanks

so I have my set up correct then…?

Yes, if you are adding an extra filter with 2  mm thickness. 56.2 mm is typical of takahashi scopes, but often people make the mistake of assuming you are using a normal flattener/reducer with 55 mm backspacing. They then assume that your 56.2mm is the standard 55 mm + 1.2 mm, or in some cases 55.5 mm + 0.67 mm. nbot the case if the stated backspacing for your particular system is 56.2 mm (i.e. not 55 mm like most flattener/reducers).

Is your scope a Tak with a 56.2 mm spacing? If so, and you are adding a new filter, it will increase to ~56.9 mm as you wrote.

 

EDIT: I should add that if you are not using a takahashi scope where the flattener back spacing is 56.2 mm, and you are using a 0.8x reducer that has a backfocus that depends on focal length, e.g. , https://www.teleskop-express.de/shop/product_info.php/language/en/info/p5965_TS-Optics-REFRAKTOR-0-79x-2--ED-Reducer-Korrektor-fuer-Apo-und-EDs.html, then you just need to add 0.67 mm to the recommended, and fine tune it from there. It all assumes that the manufacturer backfocus values have no wiggle room, and many of them do.

For tak, its 56.2 mm, and you add 0.67 mm with extra filter. For a 55 mm backfocus, increase to 55.7mm and fine tune from there. I dont know which reducer/flattener you are using

Edited January 18, 2023 by GalaxyGael

[Stuart1971]

5 minutes ago, GalaxyGael said:

Yes, if you are adding an extra filter with 2  mm thickness. 56.2 mm is typical of takahashi scopes, but often people make the mistake of assuming you are using a normal flattener/reducer with 55 mm backspacing. They then assume that your 56.2mm is the standard 55 mm + 1.2 mm, or in some cases 55.5 mm + 0.67 mm. nbot the case if the stated backspacing for your particular system is 56.2 mm (i.e. not 55 mm like most flattener/reducers).

Is your scope a Tak with a 56.2 mm spacing? If so, and you are adding a new filter, it will increase to ~56.9 mm as you wrote.

Correct…I just use the 55mm, as this was the norm for many flatteners, but yes, mine is the Tak with 56.2 needed, so I am at 56.9mm 👍🏻

Edited January 18, 2023 by Stuart1971

[Stuart1971]

4 minutes ago, GalaxyGael said:

Yes, if you are adding an extra filter with 2  mm thickness. 56.2 mm is typical of takahashi scopes, but often people make the mistake of assuming you are using a normal flattener/reducer with 55 mm backspacing. They then assume that your 56.2mm is the standard 55 mm + 1.2 mm, or in some cases 55.5 mm + 0.67 mm. nbot the case if the stated backspacing for your particular system is 56.2 mm (i.e. not 55 mm like most flattener/reducers).

Is your scope a Tak with a 56.2 mm spacing? If so, and you are adding a new filter, it will increase to ~56.9 mm as you wrote.

Just to clarify, 56.9mm “physical” spacing…

[bosun21]

2 minutes ago, Stuart1971 said:

Just to clarify, 56.9mm “physical” spacing…

Unless the diameter of the filter glass changes. Guess I’ll be opening up my spacing rings bag to reset my backspace. It’s going to be clear skies here tonight as well 🤞

[GalaxyGael]

3 minutes ago, Stuart1971 said:

Just to clarify, 56.9mm “physical” spacing…

Yes, and if you have a calipers (very useful) measure to confirm. I found that having many screw adaptions adds a tiny extension from mating of each screw component, so I always measure from sensor (the bottom of the black dot the side of ASI camera bodies for example) to the bottom of the part that eventually screws onto the flattener.

[GalaxyGael]

1 hour ago, bosun21 said:

Unless the diameter of the filter glass changes. Guess I’ll be opening up my spacing rings bag to reset my backspace. It’s going to be clear skies here tonight as well 🤞

Lucky you with clear skies😁. Murphy and his law like to come out at night...you could sneak a spacer on in the day and measure before they he gets wind of it! I'm hopeless with Allen keys and the like in the dark