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Is there any such thing as parfocal filters?

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#1 astrovienna

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Posted 20 June 2012 - 04:02 PM

My Astronomiks certainly aren't. L,R, G and Ha are all close enough that I don't feel the need to refocus, but B and OIII are both quite off. Are there any sets that really don't require refocusing? I use an 11" SCT with the Celestron f6.3 focal reducer, giving me about f/6.

Kevin

#2 SGT500

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Posted 20 June 2012 - 04:05 PM

I don't have to refocus my astrodon gen 2 LRGB and Ha at all. I focus in the luminance and then that's good for all 5 filters.

#3 Vladimir Elvov

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Posted 20 June 2012 - 04:09 PM

Same for me with Astrodon. Baader are also great in this respect without reducer

#4 vpcirc

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Posted 20 June 2012 - 04:38 PM

The only way to tell is to run a focus in focusmax and compare the position. That's how normally focus offsets are determined. Of course if you're focusing by hand it really probably doesn't matter since you can't get as precise anyway as an autofocuser. You could also try measuring your stars FWHM between filters. It would at least tell you how much of a difference there is. I highly doubt that Astronomiks are even close to parfocal. That would be reflected in the cost.

#5 korborh

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Posted 20 June 2012 - 04:46 PM

I find my Astrodon LRGB to be quite parfocal. I use focusmax to focus on L for all LRGB. However, for Ha, I focus using Ha since there no filter changes to worry about.

#6 astrovienna

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Posted 20 June 2012 - 04:48 PM

I highly doubt that Astronomiks are even close to parfocal. That would be reflected in the cost.


Well, they *claim* to be parfocal. ;) I can tell that they're not just by running through the filters with my Bahtinov mask on. The OIII and especially the B are quite obviously off. And the images tell the tale.

Kevin

#7 dawziecat

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Posted 20 June 2012 - 04:55 PM

I use Astrodon filters . . . LRGB Series E and 3nm Ha/OIII.
This focusing issue is not so simple IMHO and experience.
I use a Bahtinov with rather fast (F2.8 and F4.0) camera lenses.
It is easy enough to get the Bahtinov whiskers "centered," more or less. But "perfectly" centered? Judging "perfect" center is not so easy.
I center the whiskers as best I can and then make micro adjustments assessing the images of faint field stars.

And I do see small differences in focus between the LRGB and the NB filters. I haven't looked hard enough to see differences between the individual LRGB filters.

I have not really noticed the focus shifting with time though . . . not even from night to night when I leave the gear set up and do not touch the focus.

These are impressions . . . not rigorously tested. I definitely check focus if going from NB to LRGB and vice- versa though.

#8 Konihlav

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Posted 21 June 2012 - 02:00 AM

Dear Kevin,

your initial question is wrong. Let me to put it correctly:
is there anything such as a parfocal imaging setup?

the answer is (in 99%) no.

It doesn't matter if how high quality your filters are (how perfectly parfocal - same thickness) when you use any kind of imaging setup (telescope + reducers) that screw things up.

Even Astrodons (I own both I-series and E-series for two of my main imaging CCDs) are NOT (far from) parfocal on ANY kind of refracting optics.

On the other hand, on SCT or RCs with as good corrected "lens" they are almost nearly parfocal, especially at F/20 or F/30.

At F/4 it's much more obvious that filters are not parfocal (not filters, your setup! namely focal reducers are the source of messing things up as they are made from poor glass) than at F/10 (well, even my EdgeHD11" at F/10 - I see the RGB are not parfocal with each other and I am using Astrodons). I am using manual focus with Bahtinov mask and take my time to precisely focus (find the spot to focus on, make sure your focusing-subs are not too short not too long, judge on better seeing etc.).

this is how I see the things.

ad Blue - it's always problematic e.g. due to possible light leak in UV which is Astronomik filters known for (from my point of view Astronomik filters are the worst ever ;D no matter what they sell TODAY).

#9 Konihlav

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Posted 21 June 2012 - 02:05 AM

here's a sample to show how ED optics (only doublet, not "APO" triplet) at fast speed looks like when you focus one filter and check the spider of other filters using Bahtinov mask:

Posted Image

ad why I do not like Astronomik filters - I do not like if someone "lies" to me or is not fair. I like to trust specifications of products but I also like to check myself if what's being advertised is really true (you know my huge concern on noise in cameras as that's another marketing trick that is not fair if not met). So I tested few filters on spectrograph and these ones are the poorest where on the website the chart was "cut" to show the good and did not show the "bad" part of the transmittion curve...

my 0.00002 cents ;D

#10 freestar8n

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Posted 21 June 2012 - 03:53 AM

Hi-

The fact that you see a shift in OIII and B indicates a change in the short wavelength region of the spectrum and not something random. This is consistent with spherochromatism in the SCT/Reducer combination. The filters may be adequately parfocal, but the optimal focus of different wavelengths is slightly different for your OTA. This doesn't confirm the shift is due to the optics, but it's all consistent.

I use Astronomik IIc filters with a C11/Reducer at about f/6 and I see about the same shift. The good news is that - if the shift is real - you are noticing it and should be able to compensate for it while imaging. That's better than not noticing it and ending up with soft blue frames that leave a halo that has to be dealt with in processing. It is also something that can't be dealt with using an OSC camera.

If your focus is tight then you will probably need to refocus regularly with an SCT anyway - so I tend to image with one filter at a time and refocus between filters as-needed. This may mean more planning than cycling through filters, and you may miss a filter if clouds come in - but you'd probably do adding more data another night anyway.

If this is due to the ota, then spending more for improved parfocal filters would have no benefit. Pure reflecting optics like a Newtonian or R/C, both without any correcting lenses, should be parfocal if the filters are parfocal - and that is where there would be more of a clear win.

If you use focusmax and carefully find the exact focus offsets for your filters and optics, then none of it will matter as long as you apply the shift between filters. I sequence the filters in order of focus shift so that I can step through without any focus backlash - until the end of the cycle when I need to go backwards to the first one. This lets you move through filters quickly and without refocusing - until you need to due to temperature or other changes.

Frank

#11 astrovienna

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Posted 21 June 2012 - 07:34 AM

Thanks for the info, everyone. I learned a lot on this in the last 12 hours! From what I've read here and elsewhere, I doubt the issue is the filters - as Frank suggested, it's the optical system. When I tested last night with a Televue 2x barlow instead of the reducer, the Bahtinov indicated all six filters were parfocal. But I've also read that an f/24 system like that will have a critical focus zone 16x as wide as the f/6 I tested the other night. So it may be more forgiving to begin with.

So, also as Frank suggested, it looks like it's time to get up to speed on automated refocusing. Another learning curve to climb . . .

Kevin

#12 Peter in Reno

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Posted 21 June 2012 - 12:03 PM

If you are going to have different focus for different filters, don't forget to take flats for current filter before refocusing for next filter.

Peter

#13 astrovienna

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Posted 22 June 2012 - 03:51 PM

Here's some more info, and a question. On the Solar System imaging board, I learned that the CFZ is usually determined by the *native* f-ratio of the scope, not the f-ratio as modified by reducer, barlow, etc. So the CFZ was not actually wider in the barlow, it was just that the Celestron reducer induced a color-related focus shift, while the Televue barlow did not. So shortwave light is out of focus.

So - does anyone know of a reducer that won't induce a focus shift?

Kevin

#14 JoseBorrero

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Posted 22 June 2012 - 04:14 PM

Thanks for ask because I found same problem, I got three scopes: Orion ED80, AT6RC and a Meade 8". The only problem become evident on SCT's scopes. No issues on refractor or ritchey chretien. The only filter that is way off is the OIII, I can't tell about newtonians.


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