Focusing the Samyang 135 with NB filters

[Richie092]

I finally managed to get my spacing sorted using the Samyang 135, Atik filter wheel and 383l+. The LRGB focus point with my Baader filters is exactly where it should be and the Baader Ha is only 35 steps away. All good so far. 
 

The problem I have is the Astrodon Oiii and Sii filters won’t focus. By quite a bit. 
 

my first question is - is it possible I have installed them in my filter wheel backwards and this is making a difference?

If it is just a case of them being so different to the Baader and I have to change them should I stick with the Baader regular NB filters and hope they are parfocal or go for the F2 filters? 
 

To be honest I’m pretty surprised how far out the Astrodon filters are. Just when I thought I had everything nailed...
 

 

[Hallingskies]

I think the threads on the filters (if threaded) should point towards the lens but in practice I’ve found it doesn’t make much difference.  But yes, there seems to be a big difference in focus position between makes of filter.  I had the same problem with Baader LRGB and an Astronomik 12nm Ha - the narrowband was off the mark.  When I swopped it out for a Baader, all was good.  On the f2’s, yes they do seem to give brighter images when used at f2, but I see no difference to the standard 7nm filter at f2.8, which is where I tend to use the Samyang anyway.  

 

Edited April 14, 2021 by Hallingskies

[Adreneline]

I use a mix of Baader LRGB and Astronomik LHOS in my 8-position filter wheel and the focus position for the Baader compared with the Astronomik is significant. I have opted to have the Astronomik focus within the 'L' with the Baader focussing someway before the 'L'.

I understand the Baader filters are ~3mm thick and the Astronomik are ~2mm thick; this difference in thickness will have an effect on lens focus position. Recognising the introduction or removal of 0.15mm spacer can make a significant difference in the focus position with the Samyang it is not surprising that a change of filter thickness will also change things. The lens to sensor spacing (nominally 44mm) will change by approximately 1/3 of the thickness of the filter.

Other than the fact that a filter may have some sort of anti-reflective coating on one side I can't see it makes any difference which whay round they go.

I have found the Astronomik filters to be more 'parafocal' than the Baader when using them with the Samyang 135mm. Interestingly when using the Astronomik with my RedCat they are exactly parafocal - the LHOS all focus in exactly the same position.

HTH

Adrian

Edited April 14, 2021 by Adreneline
Typo

[Richie092]

Thanks both for your replies. Much appreciated.

I did wonder if the filter itself would be exactly in the middle of the threaded ring so turning it around "might" change the distance.

Like you I found even .25 mm spacer made a big difference when setting the rig up so I'm now thinking about laser cutting a 1.25 spacer to go on the filter wheel. I need to reduce the back focus of the filter so this might do the trick. I'm just trying to find something thin/strong enough I can cut safely.

[Richie092]

or maybe just unscrewing it half a turn?!

[Adreneline]

1 minute ago, Richie092 said:

or maybe just unscrewing it half a turn?!

I've used that approach when trying to fine tune to establish the spacer thickness I need BUT the image train will droop unless you are really careful and then you'll have a whole new problem to solve.

Mixing filter brands inevitably means compromise somewhere. Because I tend toward NB I decided that I'd compromise with the LRGB; I just wasn't prepared to buy new LRGB filters.

Rotating the filter in the holder won't change anything - the refractive index of the filter is a function of the glass used and not it's rotational position. Introducing a medium in the light path will push the focal point away from the lens hence the need to move the sensor away by ~1/3 the thickness of the filter.

It took me ages and loads of trial and error (and mainly error) to get the best position with the Samyang. It pays off in the end though.

Good luck.

Adrian

[Richie092]

Hi Adrian. I meant unscrewing the filter on the wheel, no way am I messing with the spacing of the lens now lol.

[Adreneline]

4 hours ago, Richie092 said:

I meant unscrewing the filter on the wheel

Sorry - I misunderstood. I can't see that would make any difference. Once the filter glass is in the light path then a few millimetres backwards or forwards has no effect - it's all down to the thickness of the glass.

Hope you manage to get it sorted.

Adrian

[Richie092]

I see your point. Still, nothing to lose so have given it a go but I am probably looking at 2 new Baader NB filters and a looong wait. ouch.

As a .p.s. I said the OIII and SII were Astrodon, they are in fact Astronomik.

[Greg Shaw]

I'm currently imaging with the Astronomik 6mm S,H,O filters with my Samyang 135mm and the ASI1600MM-Cool

I have in the past just used a single focus point for all the filers, this was when using them on my 150PDS I compared them by using the pegasus focus cube and the new focusing system in APT. Three was a slight difference but it was like 100 steps which is nothing for the Focus Cube.

With the Samyang I am using manual focus with a small focusing jig that fits onto the lens. I was really surprised to see quite a large difference between the filters with OIII being the worst. I have the lens set to f2.8

[Adreneline]

6 minutes ago, Greg Shaw said:

With the Samyang I am using manual focus with a small focusing jig that fits onto the lens. I was really surprised to see quite a large difference between the filters with OIII being the worst. I have the lens set to f2.8

I’ve had exactly the same experience with the Samyang. It must be related to the lens because when I use the Astronomik L3HOS or Baader LRGB with my RedCat they are truly parafocal.

Adrian

[Greg Shaw]

2 hours ago, Adreneline said:

I’ve had exactly the same experience with the Samyang. It must be related to the lens because when I use the Astronomik L3HOS or Baader LRGB with my RedCat they are truly parafocal.

Adrian

I wonder if its because we are using this lens with it still quite wide open and fast? I think I've read that these filters are suited up to f4. I thought that was more for vignetting issue but maybe it also causes focusing difference between the filters?

[Richie092]

Can I ask why you aren’t using it wide open?

[Greg Shaw]

2 hours ago, Richie092 said:

Can I ask why you aren’t using it wide open?

I have read many blogs and posts that say its best to stop down slightly to help increase star sharpness 

 

[Richie092]

OK, sounds reasonable. Once I get a couple of clear nights I will test it and see if I can tell the difference. I'm not a pixel peeper so not after perfection and would prefer to maximize my clear nights/imaging time but not at the expense of a rubbish image.

 

[Greg Shaw]

I was out imaging again last night with the 135mm Samyang and thought I would look at the differences between the Astronomik filters with the Bahtinov mask.

I focused using the 6nm H alpha and then swapped to the other filters to see what the focus would be like. This was all done at f2.8. I'm going to redo this target using f4 to see if it helps with the poor coma around the edges (reminds me of my newtonian without a coma corrector!) 

This was the focus using H alpha

 

This was OIII

 

This was SII

 

Will be interesting to see if I get the same result when using f4.