Filters changing the magnification

[Matt S]

I’m sure this is exactly what I think it is, but I’ve not tried using any filters before and having now got my hands on some, whilst observing last night, noticed they had the effect of reducing the magnification of my eyepiece. 
 

Is that to be expected? 
 

Thinking about it, they obviously add ~5mm or so to the eyepiece. So if for instance I’m using my 7.5mm - could/should I consider getting something shorter for using filters with?

Unrelated last question to the thread topic, I was hoping a UHC filter would help give me some greater definition on the Ring Nebula. Last night I actually thought it made it harder to see - I think for some reason the seeing may not have been as good as my previous observations, but I still thought it might help define it a little more? 
I’ve got a relatively small scope (Skymax 127) so it’s not a great light bucket, but also thought a UHC would perform ok on something that size?! 
 

[vlaiv]

4 minutes ago, Matt S said:

Is that to be expected? 

No.

Filters are flat optical elements and they can't change magnification.

There are some special cases where they can alter magnification produced by other optical elements - because they add (minimal) optical path and very small physical path to light.

If you use them with barlow or reducer - then they would alter barlow / reducer element to eyepiece distance and change magnification / reduction factor of these elements - but you'd be hard pressed to tell the difference visually.

7 minutes ago, Matt S said:

Thinking about it, they obviously add ~5mm or so to the eyepiece.

You can get the same effect by slightly pulling eyepiece ~5 mm back in focuser and clamping it there - instead of pushing it all the way in. If you do that - it won't change magnification (again - with exception of above cases, barlow or focal reducer).

[Zeta Reticulan]

I don't know about reducing magnification. The problem with the 127mm Synta Mak' is with getting a wide enough exit pupil. As a consequence of this I only really use my 127mm SkyMax for lunar/planetary observing. One solution is to thread a  reducer into a 25mm eyepiece, effectively making it 50mm.

Either way, bear in mind the 127mm SkyMax is about f/12 (1540/127).

Edited August 20, 2022 by Zeta Reticulan

[vlaiv]

4 minutes ago, Zeta Reticulan said:

I don't know about reducing magnification. The problem with the 127mm Synta Mak' is with getting a wide enough exit pupil. As a consequence of this I only really use my 127mm SkyMax for lunar/planetary observing. One solution is to thread a reducer into a 25mm eyepiece, effectively making it 50mm.

That won't work very well.

25mm plossl has field stop of ~21mm. If you add x0.5 reducer - you'll want to provide 42mm of illumination to reducer element in order to fully illuminate field.

Neither 1.25" reducer can accept that large field having less than about 28mm of clear aperture - nor Mak can provide that much fully illuminated field given that rear port of 127mm mak is only about 25mm wide:

Best thing one can do if they want to maximize exit pupil is to take 1.25" eyepiece with long focal length and narrow field of view - like 40mm plossl in 1.25" variant (about 40 degrees of AFOV).

There will still be issues with such setup as very large eye relief - so it won't be very comfortable to use (might be for someone wearing glasses who can keep their head still on optical axis ).

[vlaiv]

I could not resist posting this:

https://www.skyatnightmagazine.com/advice/make-a-50mm-eyepiece/

That is actually amazing stuff. I have old Helios 44-2 58mm lens and 3d printer - might give it a go for my Mak102

 

[Zeta Reticulan]

31 minutes ago, vlaiv said:

That won't work very well.

25mm plossl has field stop of ~21mm. If you add x0.5 reducer - you'll want to provide 42mm of illumination to reducer element in order to fully illuminate field.

Neither 1.25" reducer can accept that large field having less than about 28mm of clear aperture - nor Mak can provide that much fully illuminated field given that rear port of 127mm mak is only about 25mm wide:

Best thing one can do if they want to maximize exit pupil is to take 1.25" eyepiece with long focal length and narrow field of view - like 40mm plossl in 1.25" variant (about 40 degrees of AFOV).

There will still be issues with such setup as very large eye relief - so it won't be very comfortable to use (might be for someone wearing glasses who can keep their head still on optical axis ).

It doesn't work too well with any kind of UHC filter admittedly. Although it's the only way to achieve a 4mm exit pupil with the 127 without using an actual 50mm eyepiece. Basically a 127mm Mak' isn't an instrument for low power observing. Celestron usually bundle a 40mm Plossl with most of their SCT's. Makes sense.

Edited August 20, 2022 by Zeta Reticulan

[Zeta Reticulan]

25 minutes ago, vlaiv said:

I could not resist posting this:

https://www.skyatnightmagazine.com/advice/make-a-50mm-eyepiece/

That is actually amazing stuff. I have old Helios 44-2 58mm lens and 3d printer - might give it a go for my Mak102

 

Let me know how you get on.

[Guest]

I have used broadband, narrowband and a OIII on the ring. To my eye the OIII brought it out the best 

[Zeta Reticulan]

2 minutes ago, Mike Q said:

I have used broadband, narrowband and a OIII on the ring. To my eye the OIII brought it out the best 

I have used broadband and narrowband OIII filters on M57. In my experience a broadband OIII can be used at a 1.5mm exit pupil. Although I've never tried this with a slow scope. It might work on a 127mm Mak'. It would take at least an 18mm eyepiece however. 

[Guest]

1 minute ago, Zeta Reticulan said:

I have used broadband and narrowband OIII filters on M57. In my experience a broadband OIII can be used at a 1.5mm exit pupil. Although I've never tried this with a slow scope. It might work on a 127mm Mak'. It would take at least an 18mm eyepiece however. 

My scope is a 10 inch F5 dob and i was using a 14mm eyepiece. 

[Zeta Reticulan]

2 minutes ago, Mike Q said:

My scope is a 10 inch F5 dob and i was using a 14mm eyepiece. 

That'll work.

[Matt S]

7 hours ago, vlaiv said:

u can get the same effect by slightly pulling eyepiece ~5 mm back in focuser and clamping it there - instead of pushing it all the way in.

I feel a little silly that I didn’t think of that - I’ll try this, I guess I assumed it was the length of the eyepiece that would affect it, rather than it’s depth in the focuser. 
I really noticed it using a 7.5 plossl whilst looking at Jupiter/Ring nebula. I was used to them being quite large in the view but once I added the filters the size seemed noticeable smaller.