Different brand CCD filter focus point

[Spoon]

Hi,

Just a quick question, will filters from different brands eg. Astronomik + Baader have different focus points?

Cheers

Cam

[ronin]

There will be small differences.

The biggest change is caused by the thickness of the glass being used.

Then comes (very small) the refractive index of the glass, which is related the the wavelength going through.

[johnrt]

Never mind different brands, even matching sets from the same manufacturer claiming they're parfocal have different focus points!!!

[swag72]

Never rely on the parfocality claims of one filter manufacturer - Let alone thinking that different manufacturers will have the same focal point with their filters!

I always refocus between every filter change .............. well, my auto focuser does!!!

[nightster]

Will the quality of the scope optics affect this also?

[Spoon]

Never rely on the parfocality claims of one filter manufacturer - Let alone thinking that different manufacturers will have the same focal point with their filters!

I always refocus between every filter change .............. well, my auto focuser does!!!

Is the difference massive or not that big? I have a Baader 7nm Ha and Astronomik 12nm O3 (03 on the way), and I'm putting them in a FW

[swag72]

In my experience, focus makes or breaks an image..... Assume parfocality at your peril and expense of your data ..... Just my opinion

I would think the difference in focus between two different brands would be quite significant.

[swag72]

Will the quality of the scope optics affect this also?

If it does..... I need a better quality scope

[Physopto]

Is the difference massive or not that big? I have a Baader 7nm Ha and Astronomik 12nm O3 (03 on the way), and I'm putting them in a FW

I think Astrodon are 3mm thick, Baader are 2mm thick and Astronomik are 1mm thick nominal.

Some time ago I asked Astronomik ( Eric -Sven Vesting who was very helpful ) for their exact filter thickness and Index of refraction. The answer was 1.1mm thick and the refractive index was 1.469 at 656nm and 1.48 at 435nm. The thickness and refractive index are used to calculate the optical reduction in the light path.

Derek

[ollypenrice]

Don't try to mix brands and expect to be parfocal.

Whether a brand seems to be parfocal to you or not depends on two things above all. 1) Is the scope highly colour corrected (highly apochromatic?) 2) What is your pixel scale? Sara does not find her Baby Q parfocal but, when it was mine, I did. Explanation? I was using it with larger pixels which are more tolerant. I strongly suspect that we are both fusspots on focus!

If not using autofocus why not run an electric wheel set to RGB, RGB, RGB etc on one run. If there is mild focus drift in RGB it won't much matter. Then refocus for a dedicated luminance run. In lum it really does matter.

Sara is mainly a NB imager and, for her, focus is more important in every channel than it is in LRGB where only the L is hypercritical. If you are dong NB follow sara, no question. Refocus.

Olly

[Physopto]

In my experience, focus makes or breaks an image..... Assume parfocality at your peril and expense of your data ..... Just my opinion

I would think the difference in focus between two different brands would be quite significant.

Hi Sara,

Just out of interest what is the F number you are working at.

Derek

[swag72]

I have found the same non parfocality of my filters across the following.

F10 SCT

F8 RC

F3.9 refractor

I can honestly say not one system has given me acceptable focus across the filter range.

[Physopto]

I have found the same non parfocality of my filters across the following.

F10 SCT

F8 RC

F3.9 refractor

I can honestly say not one system has given me acceptable focus across the filter range.

Now that is very interesting to me. I don't have your experience with imaging, but so far I have not noticed any problems. Once focused with the Bahtinov mask I seem to be OK in all subs. I am only at f 7 on my WO 132mm. My filters are also very close to the CCD .That is why I was asking.

Maybe in time I will see some differences.

Derek

[swag72]

I use a QSI and so my filters are equally as close as yours to the sensor - The difference could be as Olly suggests that I am using a chip with smaller pixels (3.4um)

[Physopto]

I use a QSI and so my filters are equally as close as yours to the sensor - The difference could be as Olly suggests that I am using a chip with smaller pixels (3.4um)

Ah! did not think of that I keep forgetting that there are other CCD imaging chips in use. I get tunnel vision at times. I forgot you have the Sony chip.

Thanks Sara

Derek

[Physopto]

Sara,

Have a look at this.

http://blog.astrofotky.cz/pavelpech/?p=864

I'm wondering if it is not just the size of the pixels but an effect of the microlenses. The smaller sizes may affect the focus because of the incidence angles.

Derek

[swag72]

Sara,

Have a look at this.

http://blog.astrofotky.cz/pavelpech/?p=864

I'm wondering if it is not just the size of the pixels but an effect of the microlenses. The smaller sizes may affect the focus because of the incidence angles.

Derek

Interesting ................ but with autofocus every filter can be as unparfocal as it likes....... frankly, I don't give a damn

[Spoon]

I use a QSI and so my filters are equally as close as yours to the sensor - The difference could be as Olly suggests that I am using a chip with smaller pixels (3.4um)

The pixels on my camera are 6.45um I think. What effect does this have?

Cam

[ollypenrice]

I use a QSI and so my filters are equally as close as yours to the sensor - The difference could be as Olly suggests that I am using a chip with smaller pixels (3.4um)

I wonder if the close proximity also plays a part? I'd never  thought of this. Just a thought. Did you find the same lack of parfocality in the Atik?

Olly

PS, Mind you, your pixels are a quarter (by area) of the ones I use. I'd bet on that being the explanation.

[swag72]

I don't know about that Olly.....as I had a manual wheel in the atik and manual focus at the time I always refocused regardless.

[Physopto]

It would be interesting to see the differences between the same  CCD chip with and without the  microlenses.

Also at 3.69 micron it is getting closer to the wavelength of the light so any orifice edge in front of the CCD will have an effect causing some  diffraction. The microlens may exacerbate the problem if not perfectly focussed. The diagrams are for the coloured version of the Sony CCD so the mono version will I presume not have that Bayer matrix at least not coloured. ( I strongly suspect not there at all.)

http://blog.astrofotky.cz/pavelpech/files/2013/03/SonyExviewHAD.jpg

Derek