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IDAS LPS woes??

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#1 WhitenerJ

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Posted 29 November 2012 - 10:02 PM

Blah!! having issues with my new IDAS Light pollution filter. When it's in the imaging train it causes reflections of bright stars to the upper right of the star. Also did it with Jupiter. Also the colors are all split into 3 separate channels and there is a strong color gradient in the subs. Without the filter there are no reflections or gradients. Wondering if there is a better filter like the CLS clip in? Anyone else using a refractor and having issues like this with the IDAS 2" LPS-P2?

Jason

#2 WarrenS

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Posted 30 November 2012 - 12:17 AM

Sorry to hear about your reflection issues. I'm also using the 2" IDAS filter, screwed into my AT field flattener. Haven't noticed any unwanted reflections with either my 127mm or 80mm refractors. Where exactly in your imaging train is the filter? Some of the resident experts will have to weigh in on the gradient issue, I haven't noticed any additional or unusual gradients.

#3 mmalik

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Posted 30 November 2012 - 12:26 AM

I use it in this... format with a refractor and have never had any issues. Most images I have been posting lately were taken with this setup. I am quite sure issue is not the filter; keep looking, will get sorted out soon and with feedback from other folks.

#4 Silicon Owl

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Posted 30 November 2012 - 01:35 AM

No problem here, but at the front of my TV field flattener it is a long way from the focal plane and unlikely to cause visible internal reflections. We have low pressure sodium streetlights throughout the neighborhood and the filter is indispensible.

#5 WhitenerJ

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Posted 30 November 2012 - 07:10 AM

I have the Idas screwed into the AT field flattener at the end that goes into the focuser.

Jason

#6 mmalik

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Posted 30 November 2012 - 07:31 AM

Just as a 'test', take an image without FF (just the camera & filter) and see how it looks. Thx

#7 WarrenS

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Posted 30 November 2012 - 10:22 AM

I have the Idas screwed into the AT field flattener at the end that goes into the focuser.

Jason


Can you post a sample image of the reflections? Maybe try to exchange the filter for another one?

#8 WhitenerJ

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Posted 30 November 2012 - 11:10 AM

Here is a sample. Notice the reflection to the upper right of the star.

Posted Image

#9 WhitenerJ

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Posted 30 November 2012 - 11:41 AM

Oh and this is the second idas filter and both do the same.

Jason

#10 David Pavlich

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Posted 30 November 2012 - 11:50 AM

I have mine at the end of the flattener with no problem.

Edit: Forgot to mention that for a DSLR, I always used the clip in version with very good results.

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#11 Tonk

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Posted 30 November 2012 - 04:07 PM

Sounds very much like the planar surface of your filter is tilted a tiny amount to the axis of your optical train.

This gives both gradients and off axis reflections. Because these filters are a series of layers there are minor internal reflections between the layer interfaces - these only show up with very bright objects. If the filter is not tilted then these reflections travel back and forth in the same axis as the incoming rays and you should not see them - tilt the filter and the reflections cascade off to one side getting fainter as you move outwards

It could be the filter is misaligned (tilted) in its cell. The tilt is very slight.

#12 mmalik

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Posted 30 November 2012 - 04:41 PM

It could be the filter is misaligned (tilted) in its cell. The tilt is very slight.


I am quite skeptical of the tilt theory; two filters in a row will not behave the same or will have the same tilt. Could it be the flexure in the system causing it? One test would be to image something at zenith to get imaging train straight/vertical.

#13 WhitenerJ

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Posted 30 November 2012 - 04:44 PM

That's funny. The star was at zenith. So not that. I'm gonna get a clip on cls filter next and try that.



Jason

#14 lightyear44

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Posted 30 November 2012 - 04:55 PM

An IDAS filter inside my Canon camera.
Never any issues here.
Best of luck.
-David.

#15 WhitenerJ

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Posted 30 November 2012 - 07:42 PM

Well I tried again tonight without the Field Flattener and no reflections or color separation!! So with the FF the Idas filter is causing this?? How can I operate without my FF??

Jason

#16 mmalik

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Posted 30 November 2012 - 07:51 PM

Could try a different brand FF? Yes, pricey proposition.

#17 Tonk

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Posted 30 November 2012 - 08:47 PM

I am quite skeptical of the tilt theory


Please feel free!

I've just been looking into this on a crude optical bench with a Astronomik filter and an objective lens and a red laser (standing in for a bright star).

Slightly tilting the filter does indeed cause a readily observable but fainter offset point relection to apear where the camera sensor would be placed. Slightly varying the very small degree of tilt affects the fainter reflection displacement. The plane of the tilt affects the displacement direction.

The reflection light path is due to a small percentage of laser light reflected back off the flat surface of the filter back up the tube towards the objective (this ray is now at a angle to the principle axis). Some of this light is then reflected back off the back surface of the objective back to the filter. Net result is a faint offset reflection arriving where the sensor would be. Note that when all rays are traced this reflection may be distorted and slightly out of focus due to the objective lens rear surface being curved - my experiment isn't saying the reflected image of a star will slill be a point as I've only been looking at single rays with this set up.

My idea that it was due due to internal reflections within the filter was wrong - but refections off the flat filter surface back onto other optical elements could certainly cause the OPs observed effect - my experiment confirms it is possible via a small tilt - assuming the scope is a refractor of course!!!

Can't be discounted by sceptism now that the experiment has been done ;) ! However it may not be the actual cause - I suggest you play with a laser pointer - its a good diagnostic tool.

#18 fco_star

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Posted 30 November 2012 - 10:59 PM

some times the lock bolt at the focuser could cause a flexure, try not to use it, maybe it will work for you.






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