Clarification please….🤔🤔

[ollypenrice]

Here's chapter and verse from OPT. Scroll down for the section on filters and backfocus. The filter increases the backfocus by a third of its thickness so you need to add a spacer to adjust for the filter.

https://optcorp.com/blogs/deep-sky-imaging/how-to-set-the-correct-back-focus

Olly

[vlaiv]

There you go - answer provided by Olly.

Here is handy diagram - in converging beam - filter pushes focus point further out so yes - for example it will be 56mm instead of 55mm so adding spacer is required.

[bosun21]

Thanks for this, I was obviously told the contrary by a less learned individual. Spacers already fitted.👍

[vlaiv]

18 minutes ago, bosun21 said:

Thanks for this, I was obviously told the contrary by a less learned individual. Spacers already fitted.👍

It is rather confusing thing. I always have to stop and think deeply about it and even then I'm not certain that I've got it right.

It depends on refraction indices of glass and air. There is law of refaction that says

n1 * sin a1 = n2 * sin a2

When we rearrange this we get n1 / n2 = sin a2 / sin a1.

So relation of angles is inverse to relation of refraction indices.

Air has n1 = ~1 and glass is "thicker" and has higher refraction index - so angle in glass must be closer to normal (smaller angle).

It shows in above diagram - angle in air is larger than angle in glass - and that is similar to what barlow does and it pushes focal plane further away (unlike reducer that brings it inward because it does the opposite).

[ollypenrice]

2 hours ago, bosun21 said:

Thanks for this, I was obviously told the contrary by a less learned individual. Spacers already fitted.👍

 

3 hours ago, vlaiv said:

There you go - answer provided by Olly.

Here is handy diagram - in converging beam - filter pushes focus point further out so yes - for example it will be 56mm instead of 55mm so adding spacer is required.

I think that all this confusion is linguistic in origin.  Vlaiv posted a visual representation of the lightpath and, years ago, when this issue came up, I drew myself a ray diagram and came to the same conclusion. It's in the verbal descriptions that the ambiguities arise and, from memory, (and this may be unjust) it was a piece of text on the QSI website which was ambiguous.

Trust the drawing.

Olly

[JeremyS]

Now I remember why I like visual observing 😊

[gorann]

It is fun to see that this question comes up regularly on SGL. I agree with @vlaiv that it is easy to get confused and I have to stop and think every time. I probably should put up a note about it in the obsy saying:

"ADD 1/3 OF THE FILTER THICKNESS TO THE IMAGE TRAIN"

since there usually are enough other things to think about there and then.

But as someone said here, never trust what scope or flattener/reducer manufacturers claim should be the correct backspace - check corner stars and be prepared to adjust.