Does Using a Filter Between Sensor and MPCC Affect Spacing?

[richyrich_one]

Hi

i am trying to understand the effect of a filter in the light path.

I use a Baader protective T-ring for its zero tolerance features but without a filter fitted. Baader's documentation on specific use isn't clear. It doesn't say you have to use a filter but neither does it say you can use it without one. It can be puchased without a filter though.

My question is, if the t-ring provides the correct spacing for the MPCC with a 3mm filter fitted does this affect the spacing as the filter is between the sensor and the MPCC? Thereby causing spacing issues without a filter.

I suspect my field is being over corrected and this could be the problem.

Many thanks

 

[alacant]

Hi. I can't answer your specific question about the Baader fitting, but I do have a filter in front of the cc on the telescope side. It makes no difference to the correction. FWIW, the only way I could get the correct spacing for my 700d was: camera - 11mm m48 t-ring- 4mm spacer - mpcc. HTH.

[richyrich_one]

23 minutes ago, alacant said:

Hi. I can't answer your specific question about the Baader fitting, but I do have a filter in front of the cc on the telescope side. It makes no difference to the correction. FWIW, the only way I could get the correct spacing for my 700d was: camera - 11mm m48 t-ring- 4mm spacer - mpcc. HTH.

So much for  57.5 +/- 1mm

But as you say placing it on the scope side would not affect spacing.

[ronin]

Owing to the refractive index of the glass I would have expected a change of about 1/3 the thickness of the filter glass. Slight problem is I cannot recall 100% which way but I think it pulls the image plane backwards. Olly is the best to ask. So I think that if it is 57.5 +/- 1mm without filter then it would be 56.5 +/- 1mm with filter.

Not a lot but you would need to refocus for best results.

[richyrich_one]

Thanks, actually just reading this thread...(Mental note, use the search!)

Olly has an input on it there, I'll see what he had to say.

It's specifically the MPCC spacing I am concerned with.

 

[richyrich_one]

Ok, so having just read all 3 pages I think I am even more confused than ever.

My take on it is that introducing a 3mm filter would require an additional spacing of 1mm between the sensor and MPCC. Now if Baader have already allowed for this in the mechanical construction of their t-ring then using it without a filter would make it over-spaced by 1mm.

I think

[sloz1664]

I totally agree that adding filters does cause confusion on whether you add or subtract the amount of spacing between your MPCC and the sensor. I have had this same issue with one of my set ups as I couldn't a achieve a flat field. Until I realised I also  had an IDAS LP filter in place. To achieve my flat field I had to add 1/3rd the thickness of my filter to the stated spacing.

Steve

[richyrich_one]

Thanks for the confirmation.

So if I want to keep using the baader t-ring I need to use a filter to get the correct spacing.

[alacant]

11 hours ago, richyrich_one said:

So much for  57.5 +/- 1mm

I don't think there's a correct answer. Perhaps the best -only?- way to find what works is by trial and error. I think the only way you can do that is camera - m48 t adaptor - spacer - mpcc. 59mm works for me: a 208mm f3.9. Other stuff I needed to fix: Is your camera sensor square? Are the primary springs up to the task? Does the camera sit square in the focuser. Maybe a message to Baader? HTH.

**edit. One last thing: loosen the lens retainers one at a time on a horizontal surface until when you shake it, you can hear the glass-metal rattle. Now slowly tighten until -still shaking and tapping- the rattle just disappears.

[ollypenrice]

We've been round the houses on this before and please don't regard me as an authority, but IIRC Steve Richards agreed with me and that makes me feel a lot better! It's the verbal descriptions which introduce the confusion. (I rather suspect QSI of being behind this but that might be unfair!) Try this diagram, and the arrival of the term 'Snell's Law' makes me feel cosily scientific!!

https://www.google.fr/search?q=refractive+effect+of+filter+in+light+cone&espv=2&biw=1920&bih=990&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjXvtXTybPSAhXDHxoKHQ1BCCgQ_AUIBigB#imgrc=4JjpfEeue-I0qM:

The yellow diagrams strike me as being in perfect accord with common sense. 

I'm stuffed with the flu and in no fit condition for coherent thought (am I ever?) but I hope the link is of some help.

Olly

[richyrich_one]

1 hour ago, ollypenrice said:

We've been round the houses on this before and please don't regard me as an authority, but IIRC Steve Richards agreed with me and that makes me feel a lot better! It's the verbal descriptions which introduce the confusion. (I rather suspect QSI of being behind this but that might be unfair!) Try this diagram, and the arrival of the term 'Snell's Law' makes me feel cosily scientific!!

https://www.google.fr/search?q=refractive+effect+of+filter+in+light+cone&espv=2&biw=1920&bih=990&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjXvtXTybPSAhXDHxoKHQ1BCCgQ_AUIBigB#imgrc=4JjpfEeue-I0qM:

The yellow diagrams strike me as being in perfect accord with common sense. 

I'm stuffed with the flu and in no fit condition for coherent thought (am I ever?) but I hope the link is of some help.

Olly

Sorry Olly, I can't see the yellow diagrams you are referring to in that link.

[richyrich_one]

Is this what you were meaning? It didn't come up in the original image search, added snells law to it and there it was.

[richyrich_one]

Agreed, that makes perfect sense.

[Merlin66]

Agreed, also.......

But what impact (if any) does the  lengthening of the optical spacing have on the  imaging properties of the corrector. 

[steppenwolf]

How strange, I have already answered a similar post on this subject tonight! Olly and I have pondered this before from different viewpoints and it seems that Olly does agree with my analysis but the debate continues because there are some convincing arguments the other way too!

[Merlin66]

Steve,

I don't think there is much to debate on the fact that a plain filter will move the focus point further outwards by approx 1/3 the thickness.

The only "shadow" in the back of my mind is what effect does the change in the optical path length have on the design characteristics/ performance of critical components like correctors.

I'm sure it would/ could be an easy question for the manufacturer to answer..........

[richyrich_one]

Found this related to baader's t-ring:

The glass thickness (3mm) of the built-in 50.4 x 3mm filter is taken into account in the
mechanical construction. The resulting effective overall optical length for each CANON EOS
camera thus amounts to 55mm - precisely the overall optical length of standard T-rings.
If, instead of a 3mm thick filter, a normal Baader 2" Filter with a low profile filter cell is used, the
effective overall height of 55mm deviates from the standard by just +0.3mm. This deviation is
negligible.

Clear as mud to me!

Discuss

[Merlin66]

Rich,

I don't know the animal being discussed but two things...

1. the "" built-in 50.4 x 3mm filter "" - the increase in optical spacing (+1mm) seems to be (by design) compensated for in the width of the T ring body, thus maintaining the "correct" optical length required.

2. If a "" normal Baader 2" Filter with a low profile filter cell " is used ( in lieu of the built-in filter) then the combination of the T ring body and the low profile cell are not perfectly compensating, leaving a difference of 0.3mm.

Simples! ;-)

 

 

[richyrich_one]

So if you buy one without a filter as I did, the last thing I wanted was more glass, you are already 1mm overspaced.

As I suspected then.

Thanks for interpreting the "baader speak".

Now what to do about it...

[iapa]

This is the sort of confusion that males consider taking up dentistry as a hobby instead

I think I have it now tho', or do I?

If you just add a filter to the light path, then the focal plane moves further away from the source of the light. To keep your focus, you move the sensor further away. Easy enough.

However,

If you must have a fixed distance between source and sensor, i.e. also have a corrector of some sort, then you need to remove some of the metal work in between to move the focal plane towards the light source.

In the first case you are trying to achieve focus - so can move the sensor to the focal plane, add a 1mm spacer.

In the latter, where the distance must remain fixed, you need to move the focal plane so it falls on the sensor. Reduce the distance by 1/3rd of the filter's thickness.

The reduction in distance needs to the between light source, and filter.

For a clip in filter, that could be a thinner t-ring for example, 9mm instead of the more normal 10mm (based on those I have).

I have a thin t-ring e.g. which I would need to add spacers to to bring it back to 9mm. 

[richyrich_one]

1 minute ago, iapa said:

This is the sort of confusion that males consider taking up dentistry as a hobby instead

I think I have it now tho', or do I?

If you just add a filter to the light path, then the focal plane moves further away from the source of the light. To keep your focus, you move the sensor further away. Easy enough.

However,

If you must have a fixed distance between source and sensor, i.e. also have a corrector of some sort, then you need to remove some of the metal work in between to move the focal plane towards the light source.

In the first case you are trying to achieve focus - so can move the sensor to the focal plane, add a 1mm spacer.

In the latter, where the distance must remain fixed, you need to move the focal plane so it falls on the sensor. Reduce the distance by 1/3rd of the filter's thickness.

The reduction in distance needs to the between light source, and filter.

For a clip in filter, that could be a thinner t-ring for example, 9mm instead of the more normal 10mm (based on those I have).

I have a thin t-ring e.g. which I would need to add spacers to to bring it back to 9mm. 

Well I'm going to throw a spanner in here...

Having measured the baader t-ring it's thicker than a standard one.

[iapa]

take a grinder to it LOL

From my understanding that's shifting the focal plane after the filter to get focus.

Need to think a bit more

 

[richyrich_one]

I think that is my only option if I want to retain the zero tolerance benefits.

Baader...Design a CC with a barrel that tilts when you tighten on it. Design a t-ring fits nice and snug but has to be used with a filter otherwise it's too thick. Arrrrgh!

 

[richyrich_one]

One thing has become clear...This IS rocket science!

[iapa]

1 hour ago, richyrich_one said:

One thing has become clear...This IS rocket science!

rocket science is easy - apply a force to make it make it go up, then it falls.

Working with pieces designed to do what you want, but not all you want, is the hard part.

We know we can put a filter in, we know we cause focal reducers/correctors; lots of companies make each.

But not to work together:( Thats science.