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Old 27-02-2017, 05:22 PM
Mickoid (Michael)
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Flattener/filter conflict

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.
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Old 27-02-2017, 05:28 PM
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Camelopardalis (Dunk)
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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.
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Old 27-02-2017, 06:04 PM
Mickoid (Michael)
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Quote:
Originally Posted by Camelopardalis View Post
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.
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?
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Old 27-02-2017, 08:02 PM
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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.

Last edited by luka; 27-02-2017 at 08:40 PM.
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Old 27-02-2017, 08:32 PM
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Camelopardalis (Dunk)
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Quote:
Originally Posted by Mickoid View Post
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?
How bad are the stars? Can you post an edge or corner?
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Old 27-02-2017, 11:54 PM
Mickoid (Michael)
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Here is a crop of the worst corner of one of my recent shots. They vary slightly but all exhibit the same distortion.
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Old 28-02-2017, 08:29 AM
cfranks (Charles)
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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.
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Old 28-02-2017, 09:34 AM
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Quote:
Originally Posted by Mickoid View Post
Here is a crop of the worst corner of one of my recent shots. They vary slightly but all exhibit the same distortion.
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.
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Old 28-02-2017, 02:38 PM
Mickoid (Michael)
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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.
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Old 28-02-2017, 02:58 PM
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Merlin66 (Ken)
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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.
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Old 28-02-2017, 04:35 PM
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Quote:
Originally Posted by Merlin66 View Post
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.
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.
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Old 28-02-2017, 04:41 PM
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Merlin66 (Ken)
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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??
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Old 28-02-2017, 04:57 PM
casstony
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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?
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Old 28-02-2017, 05:23 PM
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Merlin66 (Ken)
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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??
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Old 01-03-2017, 08:30 AM
cfranks (Charles)
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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.
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Old 01-03-2017, 08:31 AM
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Merlin66 (Ken)
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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!
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Old 01-03-2017, 09:06 AM
garymck (Gary)
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I bought these to assist with fine spacing adjustment:

https://www.firstlightoptics.com/ada...-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
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Old 01-03-2017, 09:12 AM
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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.)
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Old 02-03-2017, 07:44 AM
cfranks (Charles)
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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.
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Old 02-03-2017, 07:18 PM
Mickoid (Michael)
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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.
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