calculating Chip setback

[harry page]

Hi All

I am often asked about working out focal set backs for reducers / flatteners when imaging etc

I have meant to put up on me old website ,  how to do this for ages -I have finaly got round to it

http://www.harrysastroshed.com/backfocus.html

some might find it useful

Cheers

Harry Page

[sharkmelley]

That's a clever solution and a good write-up!

Mark

[david_taurus83]

That's a great test and worth the effort! I measured my 6D sensor to be 41.5mm deep using a digital caliper. Its modified and been reshimmed so to be expected. I am also of the opinion that optical distance is important and that mechanical distance should be REDUCED if a filter is added.

[harry page]

1 hour ago, david_taurus83 said:

That's a great test and worth the effort! I measured my 6D sensor to be 41.5mm deep using a digital caliper. Its modified and been reshimmed so to be expected. I am also of the opinion that optical distance is important and that mechanical distance should be REDUCED if a filter is added.

Hi

I am afraid I disagree with you here

If any glass / filter is introduced into the light path, the focal point will be shifted out by approx 1/3 the thickness off the glass - so the mechanical lenth must be increased to compensate

I admit it all gets very confusing sometimes

 

Regards

 

Harry

 

[david_taurus83]

1 hour ago, harry page said:

Hi

I am afraid I disagree with you here

If any glass / filter is introduced into the light path, the focal point will be shifted out by approx 1/3 the thickness off the glass - so the mechanical lenth must be increased to compensate

I admit it all gets very confusing sometimes

 

Regards

 

Harry

 

Yes I agree but is the optimal distance for a flattener not optical distance? So adding a 3mm filter would make the optical distance 1mm bigger? So mechanical distance would be the usual 55mm but optical distance from the lens of the flattener to the sensor would be 56mm? Therefore the need to reduce? See my findings below.

 

https://stargazerslounge.com/topic/340444-flattener-spacing-does-it-work/?tab=comments#comment-3702062

 

I'm not convinced that adding spacers is correct though its always a case of trial and error. Many people follow the rule of thumb and add spacers and achieve the results they are after. I can only comment on my own personal experience and I seem to get better results by subtracting 1/3rd filter/glass thickness.

Ultimately I suppose it's a question the manufacturers should be stating clearly. Are the quoted spacings for a given scope and matching flattener mechanical or optical?

 

Though this is slightly off topic so I'll leave it at that. Your demonstration is excellent and it's something I hope to try with my 6D on one of these nights suffering from a severe dose of cloud fever!

 

David

[harry page]

Hi

Nothing beats a good discussion

IMO  the optical distance never changes I.E a reducer with a FL of 55mm will always be 55mm regardless of what glass you put in line ( flat glass only ) hence why the pinhole setup works

its just that the phsical length will change with the introduction off filters etc , I have measured this with various set ups .

So I always add to the physical length to adapt to various inline filters .

Very often there are so many other posible errors ( i.e camera chip set back ) that practical trial and error at the scope is sometimes easier

Regards

Harry

Edited January 2, 2020 by harry page

[newbie alert]

Thanks Harry..I'm going to have to try this..

There must be a mathematical calculation and a test to get optimal sensor spacing also...55mm seems to be too much of a roundabout figure to me

Edited January 2, 2020 by newbie alert
Added info

[IanL]

16 hours ago, david_taurus83 said:

Yes I agree but is the optimal distance for a flattener not optical distance? So adding a 3mm filter would make the optical distance 1mm bigger? So mechanical distance would be the usual 55mm but optical distance from the lens of the flattener to the sensor would be 56mm? Therefore the need to reduce? See my findings below.

 

https://stargazerslounge.com/topic/340444-flattener-spacing-does-it-work/?tab=comments#comment-3702062

 

I'm not convinced that adding spacers is correct though its always a case of trial and error. Many people follow the rule of thumb and add spacers and achieve the results they are after. I can only comment on my own personal experience and I seem to get better results by subtracting 1/3rd filter/glass thickness.

Ultimately I suppose it's a question the manufacturers should be stating clearly. Are the quoted spacings for a given scope and matching flattener mechanical or optical?

 

Though this is slightly off topic so I'll leave it at that. Your demonstration is excellent and it's something I hope to try with my 6D on one of these nights suffering from a severe dose of cloud fever!

 

David

David, basic physics doesn't agree with your findings. The diagram in your linked post is absolutely correct - refraction through the filter glass moves the point at which the rays converge to focus physically further away from the reducer, so you have to add more spacing to compensate. Where you're getting confused is that 'optical distance' assumes that the light travels in a straight path from the back of the reducer to the focus point - once you add a filter or any other optical element after the reducer that assumption is broken as the rays converge less during the path through the filter.

It may be that the backfocus specifications for your setup aren't correct/clear in the manufacturer's advice. I'm struggling with the same issue right now with my new WO setup, see post below. Basically WO are quoting two different figures for spacing for the same reducer/scope combination, only one (or neither) of them can be right. I'm pretty sure I need to reduce the spacing but need some clear skies to test it out!