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  #1  
Old 17-01-2019, 02:34 PM
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PRejto (Peter)
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Filter Offsets: Astronomik Deep Sky vs Type 2

Well, I have removed my original post! Thanks to Ray's comment I made an elementary mistake so there is no point in leaving this up at all. In my rush to avoid getting dust into my filter wheel I didn't check carefully enough what was put in which slot.



Sadly, I'm back to wondering if there is something not quite right about my TEC180 lens? I'm getting a shift of ca 200+ microns going from the Red focal point to the Blue focal point. It shouldn't be more than around 60 microns.


Thanks to everyone for helping me find this rookie mistake.


Peter

Last edited by PRejto; 20-01-2019 at 12:15 PM.
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  #2  
Old 17-01-2019, 09:53 PM
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Per Astrodon for filters to be parfocal they need to be very much the same thickness. Its dependent on the thickness of the filters.

I found Baaders not to be parfocal despite advertising they were. The only ones I have had that are parfocal are the Astrodons.

I routinely focus my luminance filter and its accurate for all of them including the narrowbands. Perhaps there is some slight deviation between them which I haven't bothered to chase down as they are workably close.

Greg.
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  #3  
Old 18-01-2019, 09:06 AM
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Peter Ward
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I suppose a sample of two is never going to be statistically significant, but my 65mm square Baaders are very much par-focal. As are my 50mm AstroDons.

That said, while the focal plane curves with refractors will vary across the spectrum, another aspect to consider is I/R or to a lesser extent UV leakage of the offending filter. BTW a micrometer will easily identify any thickness variations.
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  #4  
Old 18-01-2019, 10:36 AM
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I regularly get called upon to test the transmission of various filters with the spectroscope (R=10000) which has a resolution of close to 0.6A (at Ha)

The par focal requirements of narrowband filters is not related just to the thickness of the filter....
The formula for the shift along the optical axis when a Plane Parallel Plate (PPP) is used in air in a converging beam is well known and generally approximated to:
deltaZ = t -t/n2 for small angles where n2 the index of the plate.

For a constant filter thickness and base material, the only variable left is n2.
This unfortunately varies with wavelength.
Inherently then not all wavelengths can be par-focal unless ether the material of the filter changes or the thickness of the filter changes.
See p 40 of the attached:
https://wp.optics.arizona.edu/jcwyan...al_Testing.pdf

[Edit] Of course you're still left to handle any Chromatic aberrations in your system. I find using a Edmund Optics achromatic in the spectroscope the focus can vary up to 1.8mm between UV and Green and IR. In a 90/850 achromatic is can be 6mm difference!! This would never be able to be compensated by a narrow band filter.
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Last edited by Merlin66; 18-01-2019 at 03:04 PM.
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  #5  
Old 19-01-2019, 01:01 PM
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Thank you all for the thoughtful replies. I really appreciate them. Playing devil's advocate for the sake of discussion....



Consider the following. The Blue DS filter appears to be "out" by 1097 + 866 = 1963 steps = 216 microns. Worst case by design for my TEC180FL should be 60 microns, If this is correct (and it appears to be in spec using the R/B Type 2 filters) then I must account for an additional 155 micron shift between Red and Blue.


Consider thickness: Glass will shift the focus by ca 1/3 thickness. Astronomics are 1mm (1,000 microns) thick. To shift by 155 mm compared to the other 1 mm filters, the offending filter would need to vary in thickness by 465 microns! I cannot imagine that any reasonable manufacturing process designed to make a 1 mm thick filter could result in a filter thicker or thinner by nearly 50%.


As far as focal shift due to wavelength and index of refraction, I estimate that BK glass 1 mm thick will shift the R/B by 14 microns. I don't know what type of glass is used in these filters but it seems highly unlikely that refraction effects of the media would account for 155 microns.



So, perhaps the offending filters are leaking UV or IR. How might I confirm or refute this possibility?


Please let me know if I've stuffed up any of these calculations.


Thanks,


Peter
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Old 19-01-2019, 01:29 PM
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Peter,
I can't seem to find any Chromatic data for the TEC 180
You mention "Worst case by design for my TEC180FL should be 60 microns"
can you clarify?

Using a spectrograph on any/ all your filters will quickly confirm any leakage.
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  #7  
Old 19-01-2019, 05:15 PM
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Yuri was kind enough to give me his design specs for the TEC180FL. He wrote this paraphrased:


the 200+ micron shift from the F to the C line is not related to the design - which may vary only by 25-60 microns.



Peter
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Old 19-01-2019, 05:18 PM
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Ok, I accept that....
Have any chromatic curves been published for the TEC????
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Old 19-01-2019, 05:43 PM
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Going to stick my neck out here Peter - please forgive the impertinence .

The results would be consistent if the blue2 and green2 filters had been inadvertently switched - could this be possible?

If possible, you would have:

RedDS =866, Red2 = 745, Ha12 = 717, Ha6 = 664
GreenDS = 0, Green2 = -3
BlueDS = -1097, Blue2 = -1139

This could suggest that the filters could possibly be pretty close to parfocal - as you note, it is pretty hard to make them with offsets anything like you measured.

If the two filters were actually swapped, the offset due to OTA CA is maybe a bit more than expected, but not ridiculously so.

Cheers Ray
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Old 19-01-2019, 07:27 PM
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Hi Ray,


Well, I have to admit this is possible and I cannot know without checking. The OTA is remote so this is an ordeal to check first hand.


All I can say is that before I went remote I was using my Moravian G2-8300 with 31 mm Astronomik Type 2 filters. I found some old notes re filter offsets and the blue offset was totally normal. Memory says around 30-40 microns. When I first installed the DS set remotely and tried to use my auto focusing program FocusLock I had all sorts of issues. The offset was so great that the program could not deal with it at all. So, that is additional evidence that something may be up with the DS set.


However, I have not imaged with the Type 2 2" set and I simply opened the cases the way they were sent and installed. I don't recall verifying that the filters were in the correct cases. Stupid me. I admit that your numbers are pretty convincing!


Thanks for this!


Peter

Last edited by PRejto; 19-01-2019 at 08:13 PM.
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  #11  
Old 19-01-2019, 09:44 PM
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You might possibly be able to check by imaging something well known - a few RGB minutes on M42 or the running man should let you try different filter assignments to see if there is any obvious colour issue that could indicate that GB may have been swapped. Problem with the focuslock routine sounds a bit ominous though.

Cheers Ray
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  #12  
Old 20-01-2019, 04:15 AM
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Hi Ray,


My thoughts exactly. I will do something short with Running Man and hope I wont be the man running away in embarrassment!



Regards,
Peter
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