Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

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

  • Please log in to reply
9 replies to this topic

#1 cengell

cengell

    Vostok 1

  • ****-
  • topic starter
  • Posts: 111
  • Joined: 08 Nov 2007

Posted 23 January 2021 - 07:05 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 Der_Pit

Der_Pit

    Vanguard

  • *****
  • Posts: 2,044
  • Joined: 07 Jul 2018
  • Loc: La Palma

Posted 23 January 2021 - 07:19 PM

Yes, you can do that test.  However, it's not only the filter that affects the focus position, it can also be the telescope.  As soon as there are some refractive (i.e., glass) elements in the OTA, the focus will also depend on wavelength.  So you can measure the difference for two filters.  But if you have, e.g., a refractor, and see that the focus isn't the same, you can't really tell if the filters are not parfocal, or if it is residual color error of the main optics...



#3 cengell

cengell

    Vostok 1

  • ****-
  • topic starter
  • Posts: 111
  • Joined: 08 Nov 2007

Posted 24 January 2021 - 01:30 AM

Thank you Der_Pit 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 - 01:38 AM.


#4 TxStars

TxStars

    Vanguard

  • *****
  • Posts: 2,455
  • Joined: 01 Oct 2005
  • Loc: Lost In Space

Posted 24 January 2021 - 01:50 AM

If you can place all the parts in the same place every time this would show if the filters focus in the same place or not.

Would be much easier with a filter wheel or filter slider but it can be done.



#5 Der_Pit

Der_Pit

    Vanguard

  • *****
  • Posts: 2,044
  • Joined: 07 Jul 2018
  • Loc: La Palma

Posted 24 January 2021 - 09:22 AM

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?

Yes, I think your experiment is valid. That only the blue filter deviates is a consequence of the 'other factors' I mentioned.  If you look at a typical focal curve of a triplet lens, e.g., here, then you notice that it runs off really fast at the blue end close to 400nm.  If your blue filter has a lower cutoff limit at (or even below) 400nm such a shift is normal.  Even for excellent APOs like your TAK.  Yes, you can directly convert that to 'real world' and conclude that you should slightly refocus when taking blue images.  Another option would be to get a blue filter with a higher cutoff, like Astronomik, Antlia, CHROMA, and a matching L.
 

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'm quite sure that the filter is fine, and even highest quality filters would show that slight offset (unless they cut off at 420nm+).  Most acquisition software can take care of adjusting the focus automatically on filter change (if you have a motorized focus, that is wink.gif)
 

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?

If you set up such a filter offset, yes, you'll have to do the measurement on real stars, focused at infinity.  Also make sure to look at backlash in the focus drive.



#6 cengell

cengell

    Vostok 1

  • ****-
  • topic starter
  • Posts: 111
  • Joined: 08 Nov 2007

Posted 24 January 2021 - 04:52 PM

I will be testing my setup with a ZWO 7 filter wheel and see...

 

Update soon.

Christopher



#7 cengell

cengell

    Vostok 1

  • ****-
  • topic starter
  • Posts: 111
  • Joined: 08 Nov 2007

Posted 24 January 2021 - 05:40 PM

(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?)

 

Yes, I think your experiment is valid. That only the blue filter deviates is a consequence of the 'other factors' I mentioned.  If you look at a typical focal curve of a triplet lens, e.g., here, then you notice that it runs off really fast at the blue end close to 400nm.  If your blue filter has a lower cutoff limit at (or even below) 400nm such a shift is normal.  Even for excellent APOs like your TAK.  Yes, you can directly convert that to 'real world' and conclude that you should slightly refocus when taking blue images.  Another option would be to get a blue filter with a higher cutoff, like Astronomik, Antlia, CHROMA, and a matching L.

 

 

Thanks that make perfect sense, what's funny NOT, is I asked the company if their LRGB filters were Par Focal and they said yes. I know these are the lowest cost LEGB at $250 for the set but I can't see paying $500 or $1000 for a set of LRGB but maybe what you are saying is the reason those cost 2x to 4x more?

 

I attached is both spectral curve and it shows that the B starts at 375 nm but at 90% starts at 400 to 500, which I think is very close to those "Astronomik, Antlia, CHROMA" filters? I attached the Antlia LRGB that cast $350 and the curve is more straight up and down, so that's what you are saying and that's why the focus is not the same point as the LRG filters?

 

(I'm quite sure that the filter is fine, and even highest quality filters would show that slight offset (unless they cut off at 420nm+).  Most acquisition software can take care of adjusting the focus automatically on filter change (if you have a motorized focus, that is wink.gif)

 

Which software are you talking about that would take care of the adjusting the focus on a filter change, you mean Auto Focus like NINA or SGP?

 

 

If you set up such a filter offset, yes, you'll have to do the measurement on real stars, focused at infinity.  Also make sure to look at backlash in the focus drive.  

 

Yes I understand that and that makes perfect sense as real world conditions, now the backlash is normally not an issues with softwares like NINA when you input an offset? Well that's from Cuiv with NINA as he wrote that code. 

 

Thank you for your help and I hope this also helps others!

 

Christopher

Attached Thumbnails

  • S-Vbony Spectrial Curves LRGB Filters Small.png
  • Antlia lrgb-v_spectrum_curve_1.jpg


#8 Der_Pit

Der_Pit

    Vanguard

  • *****
  • Posts: 2,044
  • Joined: 07 Jul 2018
  • Loc: La Palma

Posted 25 January 2021 - 08:19 AM

Thanks that make perfect sense, what's funny NOT, is I asked the company if their LRGB filters were Par Focal and they said yes. I know these are the lowest cost LEGB at $250 for the set but I can't see paying $500 or $1000 for a set of LRGB but maybe what you are saying is the reason those cost 2x to 4x more?

No, this is just a design choice, and those companies choose to cut higher.  Not (directly) related to their price.  
 

I attached is both spectral curve and it shows that the B starts at 375 nm but at 90% starts at 400 to 500, which I think is very close to those "Astronomik, Antlia, CHROMA" filters? I attached the Antlia LRGB that cast $350 and the curve is more straight up and down, so that's what you are saying and that's why the focus is not the same point as the LRG filters?

The blue curve of the Antlia filter starts going up only at 420nm.  Those 'missing' 45nm is the reason their filter would show less focus shift on your telescope.  The focus shift is from your telescope, not from the filter!  And the Antlia filter just blocks that light that is at a slightly different focus.  Yes, you get less light, but also crisper star images (this focus variation is often the reason of so-called 'blue bloat').
 

Which software are you talking about that would take care of the adjusting the focus on a filter change, you mean Auto Focus like NINA or SGP?

Yes, for example.  Autofocus itself isn't crucial, but the ability to change the focus motorized.  But if you're not running automated on first hand you can of course just change it manually...

 



#9 cengell

cengell

    Vostok 1

  • ****-
  • topic starter
  • Posts: 111
  • Joined: 08 Nov 2007

Posted 25 January 2021 - 08:30 PM

Thanks Der-Pit! I wanted to post a few pics that I took from a screen grab.

 

Pic 1 Red filter focused here first. Dead on focus.

Pic 2 is the same brand but Blue. Off enough to effect focus?? I think it's a bit right?

Pic 3 is 1000 Oaks OIII filter. Not perfect but quite close and would this effect focus maybe not?

 

Also I did need to adjust the exposure, but on a LRGB filters set that's normal that the RGB filters does require different exposures? 

 

Any feedback?

 

Thank you to all that replied!

 

Christopher

Attached Thumbnails

  • 1-SV Red Filter 12.20.00 AM Small.png
  • 2-SV Blue  12.21.12 AM Small.png
  • 6-O3 1000 Oaks 12.26.45 AM Small.png

Edited by cengell, 25 January 2021 - 08:32 PM.


#10 cengell

cengell

    Vostok 1

  • ****-
  • topic starter
  • Posts: 111
  • Joined: 08 Nov 2007

Posted 25 January 2021 - 08:44 PM

No, this is just a design choice, and those companies choose to cut higher.  Not (directly) related to their price.  
 

The blue curve of the Antlia filter starts going up only at 420nm.  Those 'missing' 45nm is the reason their filter would show less focus shift on your telescope.  The focus shift is from your telescope, not from the filter!  And the Antlia filter just blocks that light that is at a slightly different focus.  Yes, you get less light, but also crisper star images (this focus variation is often the reason of so-called 'blue bloat').

 

ME: So if I understand what you are saying the Antila filter and higher end filters have less spread the 45nm as the SVBony is wider spread and there is the cost difference? So If I used a difference scope my filters would the Blue be closer or farther in focus as you say that my telescope has a great effect, so a wider FL scope would maybe have less of an image focus shift and a longer scope would have more of a focus shift or change?  
 

Yes, for example.  Autofocus itself isn't crucial, but the ability to change the focus motorized.  But if you're not running automated on first hand you can of course just change it manually...

 

 

Thanks again Der-Pit

Christopher

I understand!

 

Also to help me and others, I found the curve from Chroma on there LRGB as these are 1 of the most expensive LRGB set. Der-Pit can you compaire these 3 curves to each other so i and others can learn more on what to look at please.

 

Thank you again! 

 

Attached Thumbnails

  • Screen Shot 2021-01-25 at 8.15.16 PM.png

Edited by cengell, 25 January 2021 - 09:16 PM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics