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Using Computar 75mm F/1.4 with front mounted 2" filters

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#1 John Vogt

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Posted 19 June 2021 - 02:14 PM

Has anyone tried using the 75mm Computar w/ front mounted 2" filters?

 

I realize there will be vignetting but not sure how much. I know a few others in the group have

resorted to using drop in 1.25" filters in the rear of the lens but figured I would give this a try.

 

 

John



#2 AllStarez

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Posted 20 June 2021 - 08:27 AM

I have that same lens but usually use 1.25 filters, but i would be interested too in using a 2’’ front mounted smile.gif



#3 longbond

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Posted 20 June 2021 - 10:32 AM

Hi John, you should be okay with a front-mount 2" filter. I have a pair of Pentax 75mm f/1.4 C-mount lenses. This is the same as a Cosmicar 75m f/1.4. It has a clear aperture of 53mm and uses 58mm x 0.75mm filters. If your Computar has the same front filter threading, you could use a 48-58mm step-down ring to front-mount your 2" filter. 2" filters usually have a clear aperture around 44mm, so you're not giving up too much to get the benefits of front-mounting.


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#4 AllStarez

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Posted 20 June 2021 - 10:50 AM

Thanks longbond, 48-58mm step-down ring should fits great. I have a 48-56mm and its a bit too short


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#5 John Vogt

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Posted 20 June 2021 - 11:48 AM

Thanks longbond!

 

I have a 58 - 48mm stepdown coming on Wednesday. I also just received a 2" Antlia 3nm Ha and

a Baader 2" 685nm long pass, looking forward to the views!


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#6 longbond

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Posted 20 June 2021 - 01:42 PM

Your welcome AllStarez. It helps that we both have the same lens! Now you'll be able to do front-mount and rear-mount comparisons with 2" filters.



#7 longbond

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Posted 20 June 2021 - 01:54 PM

John, you planned out that Antlia 3.0nm filter purchase pretty well! I have that same 2" filter and I've measured it to be good to about 14½ degrees TFOV. Beyond that, band shift shows-up and the outer field disappears. At 75mm (approx. 13 degree TFOV), you're just under the wire and good to go!



#8 AllStarez

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Posted 20 June 2021 - 02:17 PM

Your welcome AllStarez. It helps that we both have the same lens! Now you'll be able to do front-mount and rear-mount comparisons with 2" filters.

Yes thats’ exactly the idea :)

i just have one 2’ filter for now (7nm) that i’d like to compare to its 1,25 brother


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#9 John Vogt

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Posted 20 June 2021 - 03:47 PM

Thanks for the additional info longbond!

I debated spending that kind of money but am now glad I did based on your comments.waytogo.gif



#10 longbond

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Posted 20 June 2021 - 04:17 PM

John, I should clarify a little on the 2" 3.0nm Antlia. It will work fine front-mounted on your 75mm f/1.4, but this filter can be a harsh mistress. Some have reported band shift difficulties when rear-mounted (internally). I also have this 3.0nm filter in 1.25" and it's good to 17½ degrees front-mounted (versus 14½ degrees for the 2"). I don't know why...maybe a slight difference in bandwidth or center wavelength. That said, it's the only game in town for a 3.0nm at that price point and I'm satisfied with the purchase.



#11 John Vogt

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Posted 20 June 2021 - 07:43 PM

Hi longbond,

 

Thanks for the additional input. If I obtain either a 100mm or 135mm lens I might go f/2.8

and forgo f/1.8 so the filter could still be front mounted.

 

I'm waiting for a Televue 67/55mm combo to arrive. Obviously the 3nm will be mounted on the front

of that combo.


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