Connecting camera to filter wheel

[Rattler]

Hi guys. Any ideas what adapter I would need here to attach the camera to the filter wheel. Both are 42mm female. Cheers.

[RayD]

9 minutes ago, Rattler said:

Hi guys. Any ideas what adapter I would need here to attach the camera to the filter wheel. Both are 42mm female. Cheers.

FLO does a T adaptor (M42 M-M) which I would have thought would work, but it would provide a fixed connection with no way of rotating your camera independent of the filter wheel. You may want to check the depth of the thread on the FW side to make sure it won't hit the filters.

FLO T adaptor

[Adam J]

Yes a male to male t2 adapter.....shocked the camera did not come with one? Don't forget to factor it into your back focus for any Field flatner / reducer / coma corrector. 

[Rattler]

Cheers guys. Is there any adapter that would also allow to change the orientation of the camera?

[RayD]

25 minutes ago, Rattler said:

Cheers guys. Is there any adapter that would also allow to change the orientation of the camera?

Probably not with keeping the sensor close to the filters, which is best.

It shouldn't be a problem with a small filter wheel and is only relevant as it affects balance, so the T2 adaptor should work fine, but drop FLO a line and check the depth of the threads to make sure it isn't too long.

[Rattler]

Ok cool. It says on the page 14.5mm including 6mm central knurled grip so that must mean each thread is 4.25mm. Should be ok but I'll check tonight.

[Rattler]

I just need to work out the spacing required now.

[RayD]

4 minutes ago, Rattler said:

I just need to work out the spacing required now.

Let us have the details of the telescope, camera and any field flattener or focal reducer you are using and I'm sure we can help.  It is best to adjust the spacing between the FF and filter wheel rather than the FW and camera as this allows you to keep the camera sensor closer to the filters, reducing the likelihood of vignetting.

[Rattler]

Cheers bud.

Explore Scientific ED80 APO

Explore Scientific MPCC Field Flattner

Altair Astro 183m V2  

Starbright manual filter wheel 

And it looks like the adapter to connect the camera to wheel is 6mm

[RayD]

Well I believe the ES FF needs 55mm so:

Camera:  12.5mm

Adaptor:  4mm (this is assumed as it is what most FLO ones are, but adjust accordingly, so iff 6mm take 2mm off the spacer length)

Filter Wheel:  21mm

Spacer needed:  17.5mm

Not sure what filters you have, but you need to add 1/3 their thickness to the above, so if they are Baader, then add 0.7mm, meaning you need 18.2mm. 

 

[Adam J]

1 hour ago, RayD said:

Well I believe the ES FF needs 55mm so:

Camera:  12.5mm

Adaptor:  4mm (this is assumed as it is what most FLO ones are, but adjust accordingly, so iff 6mm take 2mm off the spacer length)

Filter Wheel:  21mm

Spacer needed:  17.5mm

Not sure what filters you have, but you need to add 1/3 their thickness to the above, so if they are Baader, then add 0.7mm, meaning you need 18.2mm. 

 

This is why I just bought a Baader Varilock. I could never tune my spacing to account for mixed manufacturer filter thicknesses without it.

Also, though I am not sure I want to get into this can of worms, but surly you need to reduce the spacing by 1/3 filter thickness as the filter is extending the optical path...making the spacing more would just further extend the optical path length would it not? From direct experience adding a 3mm thick astrdon filter has caused my coma to be over corrected not under corrected.

Adam

[RayD]

22 minutes ago, Adam J said:

This is why I just bought a Baader Varilock. I could never tune my spacing to account for mixed manufacturer filter thicknesses without it.

Also, though I am not sure I want to get into this can of worms, but surly you need to reduce the spacing by 1/3 filter thickness as the filter is extending the optical path...making the spacing more would just further extend the optical path length would it not? From direct experience adding a 3mm thick astrdon filter has caused my coma to be over corrected not under corrected.

Adam

 If it works for you by reducing it then by all means feel free to do it that way.  However, my understanding of how filters refract the light, and how I have spaced my sensors on all my set ups, is that it pushes the focal point out by 1/3 the thickness of the filter (assuming it is flat glass and not a lens of course).  Therefore if with no filter you need 55mm you would need 56mm (assuming 3mm Astrodon). 

I stand to be corrected of course, and it seems often am.

[Adam J]

1 minute ago, RayD said:

 If it works for you by reducing it then by all means feel free to do it that way.  However, my understanding of how filters refract the light, and how I have spaced my sensors on all my set ups, is that it pushes the focal point out by 1/3 the thickness of the filter (assuming it is flat glass and not a lens of course).  Therefore if with no filter you need 55mm you would need 56mm (assuming 3mm Astrodon). 

Yes your diagrams are correct but that is just the movement in the focal plain. The corrector needs a set optical path length. You compensate for the change in focal length by adjusting the focus, so forget about the change in focal position that is not what you are compensating for.

You are compensating for the change in optical path length not the change in focal length. 

See below, how many wavelengths are there between the corrector on the left vs the sensor on the left, there are more wavelengths between the two with the filter in place because the speed of light slowed down in the filter, so the optical path length was increased by the filter. So to compensated you need to reduce the physical separation between the corrector and the filter. 

Think about the extreme case where the filter takes up the entire distance between the corrector and the sensor....the distance required for the correction to take place and the focal length are equally reduced. So would you need to add an air gap following the corrector?

Hey I could be wrong....but my corrector is over correcting not under correcting. 

Adam

[RayD]

@Adam J as you said, a can of worms, and one which has been rumbling since long before you or I became members here, so not one I am prepared to open again on someone else's thread.

As we both know, there are tolerances which often need fettling with delrins or similar, so I'm sure the OP will get to the correct spacing.

[Rattler]

Thanks guys. I have Altair Astro 1.25" filters.

[Rattler]

Would the Baader Varilock be the best thing to use?

[RayD]

6 minutes ago, Rattler said:

Thanks guys. I have Altair Astro 1.25" filters.

Ok it doesn't say how thick they are, but I would assume 2mm, meaning 0.7mm spacing, and just work from there.  

There is an interesting thread on CN here which discusses the whole spacing issue, and the effect of reducing optical length, thus needing to add physical length to your imaging train.  In my opinion, and the final consensus of the thread, for our purposes i.e. how much physical spacer do I need to add, if you add a filter after a corrector, you do indeed shorten the optical length, but you need to get this back to where it needs to be for it to work, so you need to add physical spacers equalling 1/3 the length of the filter.

If you are in doubt, and it is very confusing, just start with a slightly shorter spacer and keep adding until you achieve the correct results.  This can be done with either an adjustable spacer such as the Baader one mentioned above or, as most do, with small shim like spacers such as delrin, which also help by preventing the spacers welding.

Hope you get it all sorted anyway.

[Rattler]

Appreciate your help guys. Cheers.

[Adam J]

3 hours ago, Rattler said:

Would the Baader Varilock be the best thing to use?

It's what I use and the least mucking about. There are three different versions though. You probably want the 20 to 29. It's not cheap however so with a little more work you can use spacers available for only 10 pounds. 

[Rattler]

Ok mate. How will the 20-29 be the one if I require 15.5mm?

[Adam J]

22 minutes ago, Rattler said:

Ok mate. How will the 20-29 be the one if I require 15.5mm?

Sorry I missed that bit was more focused on the filter compensation.

https://www.firstlightoptics.com/adapters/baader-t2-extension-12-16mm.html

It would be this with some spacers too...

So in actuality as you still need spacers you could also go with:

https://www.firstlightoptics.com/adapters/baader-t2-extension-tube.html

(the 15mm one)

these spacers:

https://www.firstlightoptics.com/adapters/baader-t2-delrin-spacer-ring-set.html

and this camera adaptor.

https://www.firstlightoptics.com/adapters/flo_t2_t2_ring.html

Up to you.

At that point you just have to hope that the thread on the camera adaptor does not go to far into the filter wheel and stop it rotating.....if it does as a spacer between the adaptor and the wheel.

No one said it was easy lol.

Adam

 

 

 

 

[Rattler]

Haha I'm fully aware of that bud. I've spent 8 hours out last winter and came home with only an hours worth of data. Perseverance is key.