Filters - worth going from 1.25" to 31 mm?

[MartinFransson]

I´ve decided to get a ZWO ASI 1600mm with a 8-slot filter wheel. There is a choice between 1.25" and 31 mm filters to be done. ZWO claim that the 1.25" filter are good for f/5 scopes and slower, whereas the 31 mm filter will work down to f/2.8. Since I will be imaging with both f/4 and possibly f/2.8 Canon lenses I guess I should get the 31 mm? What could be the effect of 1.25" and a fast lens? Vignetting?

[Peter Drew]

The short answer is no, 1.25" and 31mm are virtually the same.  .  Did you mean 50mm?

[kirkster501]

I asked Astrodon himself this question.    He suggests that anything faster that F3.8 you get the 31mm.  Good flats will help if you do use 1.25 but the 31mm are better.  And that is from the man himself.

[MartinFransson]

No, 31 mm The 31 mm ones are unmounted, if that makes a difference.

This is the info on their webpage:

" LIMITING FOCAL RATIO

Please note that the sensor diagonal size is very close to the internal clear aperture of a 1.25" filter, therefore with very fast telescopes or if you will use the camera with a fast photographic lens, telephoto lens etc. there might be vignetting. (lens with small f-ratio: photographers call it a fast lens because it takes less time to achieve the same brightness of the image)

The limiting focal ratio with the ZWO manual filter wheel depends on the thickness of the filter glass and its clear aperture. The latter is usually 24 or 26mm for 1.25" filters. The cheaper ZWO LRGB filters that were developed for planetary imaging have 24mm clear aperture, but fortunately, the new LRGB filter that were developed for the ASI1600MM and ASI1600MM-COOLED cameras have 26mm clear aperture...

Earlier calculations were approximate figures as those were based on assumptions rather that precise measurements as at that time we did not yet have the new filters and electronic filter wheels in stock.

We have now took some measurements and recalculated the limiting f-ratio and to our surprise the results are much better, there will be much less vignetting...

limiting focal ratio= (filter to sensor distance + thickness of filter) / (filter clear aperture - sensor diagonal size)

Calculation for a 1.25" filter with 2mm glass (EFWMini or EFW 8*1.25"/31mm)

(8.4mm + 6.5mm + 2mm + maybe 2mm more due to the cell) / (26mm - 21.9mm) = f/4.6

(f/4.85 with 3mm glass and f/5.1 with 4mm glass)

good for a fast newtonian at f/5 or SCT at f/6.3

Calculation for a 31mm filter with 2mm glass (EFWMini or EFW 8*1.25"/31mm):

(8.4mm + 6.5mm + 2mm) / (28mm - 21.9mm) = f/2.8

(f/2.94 with 3mm glass and f/3.1 with 4mm glass)

this would be good for a fast Newtonian or APO at f/4 or even faster...or to be used with a telephoto lens at f/2.8 or slower "

 

[Knight of Clear Skies]

I went for the unmounted 31mm filters in the end. It probably doesn't make much difference but I wasn't sure if the mounted ones would cut into the light path and affect the size of the airy disks when using fast lenses, I want my stars to be as small as possible. I might go for a 1.25" mounted for Ha, where the stars are small anyway - to make it easier to swap in an OIII later (my filter wheel has 5 positions).

@Gina gets very good results with the 1.25" filters and fast optics, but I think she mostly shoots narrowband? She says the vignetting is easy to deal with, I'm just not sure whether star size would be affected significantly.

[MartinFransson]

I'm considering a package deal with filters included. There is about a 80 GBP price difference between 1.25" and 31 mm when buying all 7 filters (LRGB + narrowband). Not that much but I'm on a very tight budget.

[kirkster501]

Kevin Nelson who was the founder of QSI (he's now at Google) and Don at Astrodon both have said to me that 31mm are better at that fast ratio - i.e. faster than F3.8. You can use 1.25 but there will be more vignetting.  Yes flats will help with that but if you can afford to go with 31mm then you should do so.  And that is from people who really know.   I know that Sara Wager uses 1.25 at fast ratios just fine as just Steve Richards.  And if they use them then rest assured it will work.  But if buying new then why not got for 31mm and reduce that vignetting?  The cost delta is very slight.

[whizzbang]

I originally started with 1.25 on an AA 102 at F5.7 with the Atik 383 and filter wheel, with no major problems, however when i changed scopes to a Borg 89 at f4.3 i had significant vignetting, the 31mm unmounted filters cured this, while also allowing me to use a 7 position carousel, allowing me to mount LRGB and Ha, OII and SIII.

i would say go for the 31mm, as  other than a little extra outlay, there are no downsides 

[MartinFransson]

On 2017-07-22 at 12:06, whizzbang said:

I originally started with 1.25 on an AA 102 at F5.7 with the Atik 383 and filter wheel, with no major problems, however when i changed scopes to a Borg 89 at f4.3 i had significant vignetting, the 31mm unmounted filters cured this, while also allowing me to use a 7 position carousel, allowing me to mount LRGB and Ha, OII and SIII.

i would say go for the 31mm, as  other than a little extra outlay, there are no downsides 

Thanks!  I will go for the 31mm version. 

[TareqPhoto]

So it is about 1.25" and 31mm, i was in same dilemma, but i decided or thought i just cut it off and go with 36mm, so before i do that in the future, is it worthy really?

I have Canon lenses ranging from f/1.4 wide angle/standard lenses up to f/2.8 zoom or primes, as scopes it is most likely i will be in f4-F7 range, maybe if i use a reducer on a fast scope it may go down to f/3.5 or closer, so with those, are there many advantages to use 36mm over 31mm? [i will neglect 1.25"].

[STEVE333]

On 19/07/2017 at 09:18, MartinFransson said:

No, 31 mm The 31 mm ones are unmounted, if that makes a difference.

This is the info on their webpage:

" LIMITING FOCAL RATIO

Please note that the sensor diagonal size is very close to the internal clear aperture of a 1.25" filter, therefore with very fast telescopes or if you will use the camera with a fast photographic lens, telephoto lens etc. there might be vignetting. (lens with small f-ratio: photographers call it a fast lens because it takes less time to achieve the same brightness of the image)

The limiting focal ratio with the ZWO manual filter wheel depends on the thickness of the filter glass and its clear aperture. The latter is usually 24 or 26mm for 1.25" filters. The cheaper ZWO LRGB filters that were developed for planetary imaging have 24mm clear aperture, but fortunately, the new LRGB filter that were developed for the ASI1600MM and ASI1600MM-COOLED cameras have 26mm clear aperture...

Earlier calculations were approximate figures as those were based on assumptions rather that precise measurements as at that time we did not yet have the new filters and electronic filter wheels in stock.

We have now took some measurements and recalculated the limiting f-ratio and to our surprise the results are much better, there will be much less vignetting...

limiting focal ratio= (filter to sensor distance + thickness of filter) / (filter clear aperture - sensor diagonal size)

Calculation for a 1.25" filter with 2mm glass (EFWMini or EFW 8*1.25"/31mm)

(8.4mm + 6.5mm + 2mm + maybe 2mm more due to the cell) / (26mm - 21.9mm) = f/4.6

(f/4.85 with 3mm glass and f/5.1 with 4mm glass)

good for a fast newtonian at f/5 or SCT at f/6.3

Calculation for a 31mm filter with 2mm glass (EFWMini or EFW 8*1.25"/31mm):

(8.4mm + 6.5mm + 2mm) / (28mm - 21.9mm) = f/2.8

(f/2.94 with 3mm glass and f/3.1 with 4mm glass)

this would be good for a fast Newtonian or APO at f/4 or even faster...or to be used with a telephoto lens at f/2.8 or slower "

 

Very nice analysis Martin. Very helpful too. Much appreciated.

 

Steve

[dannybgoode]

Have you considered 36mm unmounted one? Bit more flexibility when it comes to sensor size etc. 
 

Makes more sense than going from 1.25” mounted to 31mm unmounted to me   

[Adam J]

On 19/07/2017 at 16:44, MartinFransson said:

I´ve decided to get a ZWO ASI 1600mm with a 8-slot filter wheel. There is a choice between 1.25" and 31 mm filters to be done. ZWO claim that the 1.25" filter are good for f/5 scopes and slower, whereas the 31 mm filter will work down to f/2.8. Since I will be imaging with both f/4 and possibly f/2.8 Canon lenses I guess I should get the 31 mm? What could be the effect of 1.25" and a fast lens? Vignetting?

In your situation I would get 31mm. You could correct it using flats yes, but you would lose significant SNR in the corners of the image. 

[Davey-T]

On 19/07/2017 at 17:09, Peter Drew said:

The short answer is no, 1.25" and 31mm are virtually the same.  .  Did you mean 50mm?

1.25 and 31mm may be practically the same on a tape measure but the Astrodon 1.25 mounted filters have significantly smaller area than the 31mm unmounted versions due to the overly large metal frames on the 1.25.

In imaging terms I guess the steeper the light curve the more this impacts on the image.

Dave

[Gina]

When I went over to the ASI 1600MM-Cool cameras after smaller sensor Atik CCD cameras, I already had 3nm Astrodon NB filters in 1.25" size.  I am also imaging in widefield with high quality vintage SLR lenses used at full aperture.  I have successfully been imaging with lenses of f2.5 in focal lengths from 28mm to 135mm.  I use the ZWO filter wheels to get the filters as close to the camera as possible and do get a bit of vignetting but this is completely corrected with flats.  31mm unmounted filters should be a bit better but I haven't had the chance to test this.  "Bottom line" :- with flats, the slight vignetting seems not to be any problem.

[Adreneline]

I am also happy with the results I get from 1.25" Baader filters mounted in an eight position ZWO-EFW and used with an ASI1600 with Samyang 135mm and Canon 200mm lenses.

This is a screen shot of my flats - Samyang 135mm at f2 Ha on the left - Canon 200mm at f2.8 Ha on the right. Flats taken using SGPro Wizard with an iPad Lightbox app as the light source and tee-shirt cloth stretched over the lens. (Exposure times are all down to the level of brightness I set on the iPad app).

I nearly always end up cropping off the outer 10% anyway due to suspect stars in the corners.

HTH with your deliberations.

Adrian

 

[AbsolutelyN]

7 minutes ago, Adreneline said:

Flats taken using SGPro Wizard with an iPad Lightbox app as the light source and tee-shirt cloth stretched over the lens. 

Hi Adrian - can I ask what ipad app you use? Sounds very handy. Thanks

[Adreneline]

1 hour ago, AbsolutelyN said:

can I ask what ipad app you use?

Hi Tristan,

The app was called Light Pad and it is indeed very handy but unfortunately no longer available; I use it on my old iPad 2.

There are similar apps available (like Trace Table and LightBox Extra) but nothing quite like Light Pad.

Adrian