Flattener/filter conflict

[Mickoid]

Hi guys,
I really need some advice here. I have a problem with my field flattener when I introduce a filter into the optical chain. The flattener works great when used on it's own but when I place a filter between the flattener and my T ring adapter the results look as if the flattener wasn't even there. It is frustrating me because I want to use my LP filter with the flattener to get those pin point stars to the edge of the field. I'm using a modded 550d on a SW f 5.5 Esprit 100. Any suggestions to solve this problem would be greatly appreciated.

[Camelopardalis]

Michael, putting the filter in the optical path changes the space necessary between the flattener and the sensor. If you're getting what I suspect - stars stretched out towards the edges and corners - then try putting a short spacer between the flattener and scope.

[Mickoid]

Thanks for your suggestion Dunk, can these spacers be purchased or are they a DIY job you keep having to trial until you get the correct spacing?

[luka]

Are you using any spacers between the flattener and the t-ring adapter?
edit: I meant before you used filter when everything was working.

In my case I had to put the filters before the flattener. The flattener required the distance to the sensor to be exactly the thickness of the t-ring + DSLR distance between the flange and sensor. In other words it is not possible to insert anything else between the flattener and the DSLR.

[Camelopardalis]

How bad are the stars? Can you post an edge or corner?

[Mickoid]

Here is a crop of the worst corner of one of my recent shots. They vary slightly but all exhibit the same distortion.

[cfranks]

Rule-of-Thumb says the distance between the flattener and camera chip has to be increased by 1/3 the thickness of the inserted filter.

[Camelopardalis]

That's the top right corner, yeah?

If there's sufficient thread on the flattener, you could put a shim in there to extend it a little. Try for 1mm first and see if that makes any difference. Ideally, you want to be able to vary the distance for the purpose of that experiment.

[Mickoid]

Thanks for all your help and advice guys. Seems like there's a little experimentation to try until I find the correct spacing. Each scope, flattener and camera configuration is different and thus there's no generic magic spacer distance.

[Merlin66]

Michael,
Charles has provided a very good explanation of what is happening.
If you need to maintain an absolute spacing , then nominally 1mm has to be removed from other spacers between the corrector and the image plane.

[casstony]

I think you mean add 1mm to the spacing Ken? The filter screws inside the existing flattened housing.

With my esprit 80 adding an IDAS LPS filter doesn't seem to make any difference.

[Merlin66]

Tony,
That's why I said "absolute spacing".......
If the corrector requires say a 55mm spacing then the physical spacing with a 3mm filter in the path would require reducing the other spacers by 1mm.
Do you agree??

[casstony]

Ken, I don't agree but it may just be that I'm confused by the word absolute ☺

If you add a filter inside a fixed length reducer housing and between the camera and reducer, then you usually need to add a spacer between the camera and reducer - is that right?

[Merlin66]

Tony,
Doing that would increase the optical length say from 55 to 56mm.
If you reduce the spacers then you could maintain the optical spacing at the design 55mm.
Make sense??

[cfranks]

I must say I always get confused with the 'add' or subtract' which is why I posted that diagram. It shows the focal point pushed out by the filter so surely (?) you would have to add an L/3 spacer to move the chip to the new focal point. The flattener setting will be the same as before.

[Merlin66]

Tony, et al,

Reviewing available info on the web, I can't honestly say that any manufacture of correctors/ reducers etc. categorically say that an "absolute" spacing distance (as distinct from an effective optical distance) is required by these products.
I therefore must conclude that the general solution applies - add approx 1/3 the filter thickness to maintain focus.

(One reason for the "absolute" comment was the subtle change in plate scale brought about by the increased optical distance....)
Sorry for any confusion! :rolleyes:

[garymck]

I bought these to assist with fine spacing adjustment:

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

Freight will be a killer unless you combine with other items here (many things way cheaper than here - prices reduce by UK VAT in shopping cart)

Perhaps they may post them rather than courier if you email them.

cheers
Gary

[Merlin66]

I can vouch for these Baader delrin spacers - they have helped with with spacing issue on filter wheels, spectroscopes etc.
(I did have great difficulty getting them to fit over some of my T threads...Thomas Baader when asked said the spacers were OK and my threads were not!! I ended up cutting a slot through a couple to get them into position.)

[cfranks]

I tend to agree with Baader's comment. I have made a lot of adaptors and find that I cannot make a 'generic' cupboard full because of the variation in tolerances among various manufacturers threads. The accessory attachment on my DSI RC10C, for example, is threaded 2.7" 24 tpi according to the manual but it is actually 2.74", a whole millimetre out. The 'standard' 42 mm 0.75 pitch varies a fair bit too.

[Mickoid]

I am now totally confused as to remedy this problem, it looks as though there are mixed ideas, methods and procedures from you guys to finalise a solution. Perhaps if I'd bothered to read the manual I could have come to the conclusion that introducing a filter into the optical chain at this point was going to create problems. See the attached shot of this explanation from the manual that warns of stretched stars at the edges. At least I now know why I got the result I did when using the filter. Luka nailed it with the comments in his post. I just have to find a way of placing the filter before the flattener.

[Merlin66]

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. :sadeyes:

[luka]

Michael, what size is the filter and how is it mounted?

[Mickoid]

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?

[casstony]

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.

[Mickoid]

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.

[casstony]

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.

My T ring has a 48mm thread (ie. wide T ring).

[Mickoid]

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.

[luka]

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.

[luka]

Actually I just though of another option, telescope express sells filter drawers for DSLRs (http://www.teleskop-express.de/shop/product_info.php/language/en/info/p6128_TS-Optics-Optics-M48-Adapter-fuer-Canon-EOS-Kameras-mit-eingebautem-Filterwechsel.html). 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.)

[Mickoid]

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.

[luka]

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.