Flattener Spacing - Add or take away?

[david_taurus83]

Hoping theres a general consensus on this as I've been wrestling for months now with flattener spacing. Let's take a hypothetical situation. You need 55mm from flattener flange to sensor. With your camera and filterwheel you already have 45mm. So you add a 10mm spacer to get to 55mm. However, in the light path after the flattener, you have a 2mm LPS filter, a 2mm filter in your filterwheel and 2mm glass screen above the sensor. So theres 6mm of glass to account for as well. Using the rule of thumb consensus for allowing a third the thickness of glass, you need to also allow for 2mm of glass.

 

Do you add 2mm to your extra 10mm spacer making 12mm extra in total or do you take it away and use 8mm extra instead?

 

Previously I had interpreted (rightly or wrongly) that I had to take away. I got ok results but not perfect. A rethink lead me to believe that I should add instead. Again, ok results but not perfect.

 

What do you kind folks do? Add or take away? Or do you ignore the extra glass altogether?

 

[symmetal]

You do need to add the 2mm to your spacing making 12mm extra needed. The sensor glass as you say has an effect as well as the filters, though the filter thickness is the only one commonly mentioned as needing to be taken into account, with the sensor glass being overlooked.. 

Alan

[mos]

I add with my WO GT81 and Flat6Aiii and I get very good results. If you calculate that you need 55mm of back-focus you will need 57mm physical distance measured to achieve the 55mm.

I think the confusing part is that glass etc shortens the back-focus distance from an optical light path point of view, so while you will have say 55mm of physical back-focus, the light path travel distance is actually shorter at about 53mm meaning you need a few more mm added to your physical requirements. 

So because you subtract, you need to add.

But then, I also found that a mm had very little noticeable effect on my stars when tweaking the spacing so who knows really! 🤯

 

[Jkulin]

The general rule I have always used is the you add a 1/3rd of the thickness of the filter, so a 2mm filter requires you to 0.6mm and a 3mm filter requires 1mm, if you are using an LP filter than treat that as another filter and measure the thickness.

[symmetal]

9 hours ago, Jkulin said:

The general rule I have always used is the you add a 1/3rd of the thickness of the filter, so a 2mm filter requires you to 0.6mm and a 3mm filter requires 1mm, if you are using an LP filter than treat that as another filter and measure the thickness.

Don't forget to add 1/3 the thickness of the sensor glass too.  For the ASI1600 it's 2mm thick the same as the ZWO filters.

Alan

[Jkulin]

22 minutes ago, symmetal said:

Don't forget to add 1/3 the thickness of the sensor glass too.  For the ASI1600 it's 2mm thick the same as the ZWO filters.

Alan

Hi Alan, I have never seen any reference to the sensor glass as I thought that was too near to the CCD to have any effect, any pointers as to where that is recommended?

Thanks

Edited July 2, 2019 by Jkulin

[symmetal]

35 minutes ago, Jkulin said:

Hi Alan, I have never seen any reference to the sensor glass as I thought that was too near to the CCD to have any effect, any pointers as to where that is recommended?

Thanks

As you say it never seems to be mentioned, but in the diagram David posted at the top, the piece of flat glass will move the point of focus back by a set distance irrespective of the actual location of the glass as the incident angle of the light cone reaching the glass doesn't change, just the diameter of the cone. That's my interpretation but am willing to be proved wrong. 🤔

Alan

[AngryDonkey]

Best thing is to go 1 or 2mm short and then use thin spacer rings to get best distance by trial and error.

Also it probably depends on whether the camera manufacturer includes these things in the quoted distances i.e.  optical vs. physical path from flange to sensor.

Edited July 2, 2019 by AngryDonkey