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Would using a Bahtinov Mask to confirm different Filters Change Focus?

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#1 cengell

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Posted 23 January 2021 - 07:03 PM

Hi all, I was thinking that I have 2 brands of filters brand are Optolong L-Pro, L-eXtreme and L-eNhance and the SVBONY LRGB Imaging Filter Set and I was thinking that inside using a Bahtinov Mask on a artificial star and putting say the L-Pro filter on and focus with a astro camera get it the perfect spikes and then switch to the the eXtreem and then to the eNhance then SVBONY LRGB Imaging Filters and see if there is a focus change of the Bahtinov Mask spikes? I understand that if these filters are the same thickness say 2mm thick then the focus with the Bahtinov Mask spikes should not change?

 

Also thinner or thicker glass of the filters will change the focus ie the Bahtinov Mask spikes, so why won't this work? Any feedback would be appreciated.

 

Thanks TIA

Christopher



#2 Scott Mitchell

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Posted 23 January 2021 - 09:52 PM

Sounds reasonable to me. I would have to imagine that there would be a shift between filters that weren't intentionally designed to be parafocal? Certainly worth testing though. 



#3 cengell

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Posted 24 January 2021 - 01:32 AM

Thank you Scott Mitchell for your reply. I did have a chance tonight so setup my Tak FS-78 and my ZWO-ASI178MM camera and took my LRGB filters and my artificial star and about 50 feet away focused with the L and all with the Bahtinov Mask and used the ASI studio software and zoomed in of the 3 spikes and got the center spike dead center and then used the R, G and those were dead center but when I went to the B filter is was not longer centered and of course I can fix that but this would tell me that using the same scope in the same temp and same set focus that the B filter was off, a tab bit high like in between the center and the top spike.

 

FYI just for fun I also tested the L-eNhance & L-eXtreme and all 5 filters were dead centered spike. I tested if I could use the same filter by inserting it in the 2" ext tube and I can repeat with the same center Bahtinov Mask spike, so I think this is a valid test right?

 

I think I understand Der_Pit that "other factors affects the focus position" but if I use the same OTA and temp and repeating the filter focus position and only the Blue filter is not centered with the Bahtinov Mask spike then in the real world I should have the same slightly out of focus imaging with my LRGB & Optolong filters?

 

In a perfect world I would use a filter wheel so no parts being removed but I think this is a valid test? Also I can just have a different focus mark of value for the Blue filter, but I am sure that high end LRGB filters would all be perfect Parfocal as that what you are paying for.

 

I also would expect that focused at infinity a real star with be the best test but this should prove my thinking right? Appreciate any feedback or thinking?

 

I did use ASi Studio ZWO camera software and was viewing the Bahtinov Mask spike at 300-400% magnified, to see the spikes better. 

 

So my test eq is

1. Tak FS-78

2. ZWO-ASI178MM

3. 6 2" filters LRGB & Optolong L-eNhance & L-eXtreme

4. Inside with a Bahtinov Mask on a tripod and artificial star.

5. ZWO EAF focuser motor.


Edited by cengell, 24 January 2021 - 01:37 AM.


#4 james7ca

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Posted 24 January 2021 - 10:02 AM

It's possible that the reason the blue filter seemed "off" is  that the FS-78 may have a bit of chromatic aberration. That's not uncommon even in scopes that are described as being apochromatic. Look very closely at the Bahtinov pattern with your luminance filter, do the spikes look perfectly straight and not at all curved (even slightly)? The latter would indicate some amount of chromatic aberration. I see this kind of problem with my Tele Vue NP127is and when I use a focal reducer on my Stellarvue SV80ST2 (the latter a pretty good triplet APO). 



#5 cengell

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Posted 29 January 2021 - 09:48 PM

3-UV-IR Oplong small.png

It's possible that the reason the blue filter seemed "off" is  that the FS-78 may have a bit of chromatic aberration. That's not uncommon even in scopes that are described as being apochromatic. Look very closely at the Bahtinov pattern with your luminance filter, do the spikes look perfectly straight and not at all curved (even slightly)? The latter would indicate some amount of chromatic aberration. I see this kind of problem with my Tele Vue NP127is and when I use a focal reducer on my Stellarvue SV80ST2 (the latter a pretty good triplet APO). 

Thanks james7ca, I will look at that very closely. I did take a shot with the Optolong UV/IR filter and what do you think, as to me looks great right? I did use a ZWO-ASI178MM Mono camera.

 

Thanks again

Christopher


Edited by cengell, 29 January 2021 - 09:52 PM.


#6 fetoma

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Posted 30 January 2021 - 01:15 AM

Why don't you just use autofocusing software, then you won't have to worry about it. Just set it to autofocus every filter change.


Edited by fetoma, 30 January 2021 - 01:16 AM.


#7 james7ca

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Posted 30 January 2021 - 01:40 AM

Well, that image of the Bahtinov mask is a little off focus, but it doesn't look bad in terms of the chromatic aberration (as far as I can tell give the pixelization in the image you uploaded). I think I can see a little shift in the blue (closest to the center), but it looks small.

 

Here is a link where you can see two different scopes, one with a fair amount of chromatic aberration (requiring refocus with filters) and the other quite good.

 

  https://www.cloudyni...dpost&p=7628600

 

And, some other scopes:

 

  https://www.cloudyni...k/#entry9481054



#8 cengell

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Posted 30 January 2021 - 03:21 PM

Why don't you just use autofocusing software, then you won't have to worry about it. Just set it to autofocus every filter change.

Oh yes I understand, but what my point was the company's of these LRGB filters all say that they are Par-focal filters, and by my testing the LRGB set is not and yes AF would correct that but that's not what I wanted to better understand.

 

fetoma are you talking about NINA and SGP software programs, I plan to get on NINA and that's my plan down the road, but wanted to know if using a Bahtinov Mask could be used for this testing and looks like so..

 

Thanks

Christopher



#9 cengell

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Posted 30 January 2021 - 03:22 PM

Well, that image of the Bahtinov mask is a little off focus, but it doesn't look bad in terms of the chromatic aberration (as far as I can tell give the pixelization in the image you uploaded). I think I can see a little shift in the blue (closest to the center), but it looks small.

 

Here is a link where you can see two different scopes, one with a fair amount of chromatic aberration (requiring refocus with filters) and the other quite good.

 

  https://www.cloudyni...dpost&p=7628600

 

And, some other scopes:

 

  https://www.cloudyni...k/#entry9481054

I agree james7ca, it's very close but a tad off, I may try my ZWO 0294 OSC camera to see in color and higher mp. Thanks I will check out these links you sent thanks again!

 

Christopher



#10 cengell

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Posted 30 January 2021 - 03:42 PM

I wonder if back focus is a factor as I am not yet using a reducer or flattener but could the back focus from the filters to the ZWO 0178mm make any effect as I was thinking it would not? I understand that whatever that back focus is but stays the same would only effect the original focus?

 

From each filter to the other filters when focused with the Bahtinov Mask on say the L filter?

 

Anybody have any advice?

 

Thanks again

Christopher 



#11 fetoma

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Posted 30 January 2021 - 04:43 PM

Oh yes I understand, but what my point was the company's of these LRGB filters all say that they are Par-focal filters, and by my testing the LRGB set is not and yes AF would correct that but that's not what I wanted to better understand.

 

fetoma are you talking about NINA and SGP software programs, I plan to get on NINA and that's my plan down the road, but wanted to know if using a Bahtinov Mask could be used for this testing and looks like so..

 

Thanks

Christopher

Nothing is ever parfocal in my book, and yes, I'm talking about those programs. Backfocus is critical only when using reducers/flatteners. As long as you can come to focus without any additional glass you should be OK.



#12 cengell

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Posted 31 January 2021 - 05:05 PM

Nothing is ever parfocal in my book, and yes, I'm talking about those programs. Backfocus is critical only when using reducers/flatteners. As long as you can come to focus without any additional glass you should be OK.

Thanks fetoma, I was thinking that! Other than light fall off in the corners if the back focus is too far then causing that issue. So when using a reducer and or flattener I also ask here in CN why won't using a Bahtinov Mask to confirm center & corners that would get the correct back focus?

 

So far have not got much feedback on using a Bahtinov Mask to set & find out the true back focus, as posted back focus numbers are close but not the real back focus as it does change based on the FL of the scope, ect? So I know that if your back focus is wrong you will see in the corners the stars pointing in or if too far or too close pointing in, so why not use a Bahtinov Mask inside with a artificial star to set the correct back focus?

 

Thanks

Christopher



#13 james7ca

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Posted 31 January 2021 - 09:49 PM

Using an artificial star with anything other than a small refractor could be problematic since most optical systems behave differently when imaging at infinity versus a closer distance. This is one of the reasons why camera lenses are more complex than a typical telescope, because camera lenses have to do well at short distances whereas a scope is typically limited to large distances or infinity. However, if you have a modest focal length refractor and a working distance between the scope and artificial star of more than 40 feet then you should be okay doing some testing with a Bahtinov mask (one rule of thumb is for the distance to be no less than 25 times the focal length of the refractor). Fast and large Newtonian reflectors are particularly sensitive to the distance between the scope and artificial star, requiring hundreds of yards between the two. Plus, it's possible that when using a reducer on a refractor that these working distances become even more critical (i.e. the artificial star may have to be even further away).

 

When all is said and done, it's probably best to use real stars for this kind of testing and adjustment. However, there is nothing that would prevent you from doing the spacing tests with an artificial star, but I'd make certain to verify the results with a real star test, going between your test spacing to that recommended by the manufacturer (so, if the manufacturer recommended 55mm and your tests indicated 52mm I try both of those spacing with a real star).

 

Here is a reference on working distances and other factors when doing artificial star testing:

 

  http://www.telescope...g_telescope.htm



#14 Pete_xl

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Posted 01 February 2021 - 02:19 AM

Hi Chistopher,

 

I use filters from 3 different suppliers (Baader, Astronomik, Chrome) in a 7 x Filterwheel with 2 refraktors of different focal length and each with and without a 0.79x reducer in NINA So nothing is parfocal here. I use NINA to handle the offsets automatically and for the evaluation of the correct sensor distance for all combinations

 

 

fetoma are you talking about NINA and SGP software programs, I plan to get on NINA and that's my plan down the road, but wanted to know if using a Bahtinov Mask could be used for this testing and looks like so..

 

For each combination I have measured the filter offsets relative to the L-filter with a Bahtinov mask in the form of motor steps and stored the values within the related NINA profiles. This works great and saves me a lot of autofocusing with filter changes.

 

Thanks fetoma, I was thinking that! Other than light fall off in the corners if the back focus is too far then causing that issue. So when using a reducer and or flattener I also ask here in CN why won't using a Bahtinov Mask to confirm center & corners that would get the correct back focus?

 

So far have not got much feedback on using a Bahtinov Mask to set & find out the true back focus, as posted back focus numbers are close but not the real back focus as it does change based on the FL of the scope, ect? So I know that if your back focus is wrong you will see in the corners the stars pointing in or if too far or too close pointing in, so why not use a Bahtinov Mask inside with a artificial star to set the correct back focus?

 

Thanks

Christopher

To find out the correct sensor distance I would advise to use the implemented NINA function for the corner stars on a natural star field. The tool can be found in the top of the screen. Its icon shows 9 little squares.




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