Michael,
I honestly don't think there is any general confusion...
Adding, say a 3mm filter between a corrector/reducer etc. will add approx 1mm to the required spacing.
The issue, which I think is confusing, is the IMPACT of the change to the performance of the corrector/reducer etc.
Determining the position of the "new" focus is easy, but I can't find any definitive data on the impact of this on the performance of the correctors/reducers etc.
Michael, what size is the filter and how is it mounted?
A standard 2 inch filter with a 48mm thread. I was told to screw the filter onto the camera T- ring adapter and then screw the camera/filter combination onto the telescope/flattener optical chain. The filter is basically connecting/holding the camera to the OTA, which as the manual states, is not the place to put it but how do I fit the filter in front of the flattener?
. The filter is basically connecting/holding the camera to the OTA, which as the manual states, is not the place to put it but how do I fit the filter in front of the flattener?
The filter should not be holding any weight at all or connecting anything together - I think you've got it in the wrong position. The filter screws into the telescope side of the T-ring adapter, then the adapter is screwed onto the flattener, then T-ring then camera. So the filter is between the parts where the red arrow is pointing.
That's how it is on my Esprit 80 anyway, I assume the 100 has a similar setup.
The filter should not be holding any weight at all or connecting anything together - I think you've got it in the wrong position. The filter screws into the telescope side of the T-ring adapter, then the adapter is screwed onto the flattener, then T-ring then camera. So the filter is between the parts where the red arrow is pointing.
That's how it is on my Esprit 80 anyway, I assume the 100 has a similar setup.
OK, that's similar to my set up. The difference ( and I may have got the terminology wrong here) is you have a T ring adapter and I don't. My T ring has a 48mm thread so the filter screws straight on. You obviously have a 42 mm T ring and need the adapter to step it up to 48mm so you can fit it to the flattener. You still have the filter between the camera and the flattener which according to the manual will increase the distance between the camera sensor and the flattener to more than the critical 63mm and so cause the stars to be blurry at the edges of FOV.
Did you try unscrewing the knurled section from the rear of the reducer to see if there's a filter thread inside that you can use? The filter glass thickness alone will only cause very minor elongation of stars at the edge of field which isn't noticeable at normal image viewing scales. You can always add a 1mm spacer if it bugs you.
There is an inside thread but it's way bigger than 48mm. So this is where my confusion lies, or maybe I'm just not getting my head around the physics, why would you introduce a spacer which will increase the flattener/sensor distance even further than the filter already has? According to the manual it must be 63mm.
Michael, let me try to explain why I think there is confusion. There are two things that affect the distance:
1. filter is glass and has a different refractive index from air. This will shift the light beam roughly to about 1/3 of the glass thickness. So, for a 3mm thick filter glass you will need 64mm distance instead of 63mm and this can be achieved with a 1mm spacer.
This is what everybody is talking about.
2. Physical thickness of the filter mounting. I think this is the point that most of people did not think of. Usually filters are inserted into filter holders/wheels and do not add to the distance between the flattener/sensor. However, in my case (and probably yours as well) the filter screws in between the flattener and camera and this will extend your existing distance. If the filter mounting is 10mm thick you suddenly have 63+10mm distance and you need to shorten it by 10mm to achieve the optimum 63mm.
(if you include the effect of the glass as in point 1 above you will need to shorten the distance by 10mm - 1mm = 9mm. Of course the numbers are examples and you have to measure your gear).
So, you have two choices:
1. Mount filter inside something so it does not extend the distance.
2. Mount the filter before the flattener. My flattener has thread at the scope side which matches 2" filter threads and this is where I placed the filter.
Actually I just though of another option, telescope express sells filter drawers for DSLRs. It is like the t-ring for the camera where you can slide in the (unmounted) filters. It has the same thickness as the normal t-ring so it does not affect the flattener-sensor distance.
(If you are interested I think I have one which I am not using any more and which I was planing selling.)
Thanks for all your help Luka, I get what you mean now, I was thinking more about the thickness of the filter mount and not that the light refracting through the glass would focus further from the sensor and thus would require a 1mm spacer to correct the difference. I'll be thinking of ways to put the filter before the flattener but in the meantime I'll just concentrate on shooting smaller objects when using the filter the way it is for now. I can then just crop the bad star shapes out of the picture and no one will ever know! I have a modded camera anyway, so any shots of larger objects, or if I want nice stars to the edges, I just won't use a filter. There is so much to learn with this hobby.
Well, let me tell you about my temporary solution... sticky tape... I taped the edges of the filter to the front of the flattener. It worked well but eventually I managed to crack the filter. Not sure how.
So sticky tape is not a good idea
Does your flattener have a thread on the telescope side? If yes you could get an adapter to screw the filter into the flattener.