Parfocal Filters - This must be a common problem!

[Tony Rodda]

I use Bessell (BVRI) 1.25" filters for photometry. With one bay in the filter wheel left empty.

As far as I know (that is, I can't find any) there are no 'Clear' filters with the same density as the Bessell 'colours'.

So whenever I need a 'Clear' photometry sequence (for a very faint object for example) I have to re-focus due to the changed optical path.

This means when I go back to the narrowband filters, not only have I got to re-focus, but I have to redo my flats.

Does anyone know of a 'clear' Bessell filter?  Or something around 3-4 mm depth that might be close enough to the density?

Regards

Tony

[davew]

I think I would email Optec on - sales@optecinc.com

If your filters are the very thick ones then maybe two clear filters in series somehow ?

Other then that, clueless,

Dave.

[opticalpath]

Not sure why density is important, Tony; I think only thickness matters to make it parfocal.

Astrodon clear filters are 3mm thick ...... but $150.

Adrian

[Physicist13]

you want to try and obtain a glass of the same refractive index and thickness. The manufacturer may be able to tell you what index the glass is and its effective thickness?

[Tony Rodda]

Thanks for the input guys.

I think this is coming 'round to a bespoke solution.  Can't see anything 'standard'.

Regards

Tony

[opticalpath]

Might be worth trying Knight Optical

http://www.knightoptical.com/php/showCatPage.php?cat=4

Or perhaps consider a simple stepper motor drive for your focuser that would allow accurate repeatable refocusing when required, which might make capture and calibration more convenient. I DIY'd two such, with Arduino/ PC control so I can refocus remotely from indoors.

Adrian

[Tony Rodda]

Adrian,

Sounds simpler (and cheaper) than buying a clear filter from Omega...

Are the details of your project available anywhere?

Regards

Tony

[ollypenrice]

Do you really need to redo flats? Is the difference in focus enough to be perceptible? I know that in theory there will be a change, but in practice?

Olly

[lensman57]

Do you really need to redo flats? Is the difference in focus enough to be perceptible? I know that in theory there will be a change, but in practice?

Olly

On the odd occassion that I do RGB with Asstronomik filters, only one set of flats seem to work even with the Baader Ha filter  ( usually used as L ) that is not parfocal with the Astronomiks.

Regards.

A.G

[Tony Rodda]

Olly,

Just following 'best practice' by re-doing flats after changing the optical train.  Incidentally, I also sometimes defocus for particularly bright objects (to spread the signal across more pixels).  As long as the AIP4WIN measuring radii encompasses the star's image it's still valid.  In such circumstances I redo flats.  I've been using that technique with the 'defocused' Clear filter.  It would be just nice to actually see what I'm capturing instead of simply counting photons across a number of pixels.

AG,

If your filters are almost parfocal you may not notice much difference.?

Tony

[lensman57]

Olly,

Just following 'best practice' by re-doing flats after changing the optical train.  Incidentally, I also sometimes defocus for particularly bright objects (to spread the signal across more pixels).  As long as the AIP4WIN measuring radii encompasses the star's image it's still valid.  In such circumstances I redo flats.  I've been using that technique with the 'defocused' Clear filter.  It would be just nice to actually see what I'm capturing instead of simply counting photons across a number of pixels.

AG,

If your filters are almost parfocal you may not notice much difference.?

Tony

Hi Tony,

The Astronomiks are parfocal to each other, no need to touch anything if I am using a Newt or the Apo, with the EDs the blue is the problem as ususal. 

Regards,

A.G

[ollypenrice]

Parfocality also depends on the F ratio and the colour correction of the optical train, but I'm sure you know that.

Olly

[ChrisLX200]

I find significant differences between each LRGB Baader filter, let alone filters of another make. I record the filter offsets needed for each but for that you need a stepper motor driven focuser (or something with an absolute readout - like a dial gauge).

ChrisH

[ollypenrice]

I find significant differences between each LRGB Baader filter, let alone filters of another make. I record the filter offsets needed for each but for that you need a stepper motor driven focuser (or something with an absolute readout - like a dial gauge).

ChrisH

Interesting. With the Baaders I find no perceptible difference in the FSQ 106 at F5 and found none in the FSQ85 even at F3.9. However, the pixel scale was pretty coarse (as it is with the 106/11 Meg. 3.5 arcsecs per pixel. Maybe this masks it a bit.)

Olly

[Physicist13]

there's no such thing as parfocal really, its a matter of where the (hiopefully slight) change in focal position affects you or not, which depends on the optical train being used, resolution, type of detector etc. You can only really tell by experimenting

P

[Tony Rodda]

All useful stuff.  Thanks for the input.

I bought Omega filters because they were 'good quality at an affordable price' (I must have got that from a marketing guy somewhere!).

I didn't think about 'Clear' with regard to photometry and that's what CBA often use for the dimmest, rapidly changing CVs.

I'll just use what I've got and attempt to get the focusing as precise as possible.

I'll do a few test runs to see if there's any difference (S/N ratios) without redoing flats and probably resort to Olly's suggestion.

It seems strange that this hasn't popped up before.

Tony

[opticalpath]

Adrian,

Sounds simpler (and cheaper) than buying a clear filter from Omega...

Are the details of your project available anywhere?

Regards

Tony

Tony - sorry for the delay responding. If you're still interested in the Arduino/ stepper motor focuser project, it is described on the sgl_observatory_automation Yahoo! group. (Look in the files section for the 'Software' folder that contains a good overview document.)

Adrian