Which filters are parfocal?

[earth titan]

As I plan my move (and save the cash) to mono CCD away from DSLR, I'm trying to cross the whatnot's and dot the thingummies.

I've decided on a 314 as the camera on my 80mm scope (initially) which is reasonably well corrected (FPL53 objective) and long term move toward using it on my C9.25.

Filter wheel seems simple enough, as shall be primarily doing LRGB, but I'm fuzzled by the filters themselves.

FLO state these are very nearly parfocal:

http://www.firstlightoptics.com/rgb-filters-filter-sets/baader-lrgbc-ccd-filter-set.html

How parfocal is 'very nearly'? Will I need to adjust focus?

Are there any alternatives?

I've read of people having to readjust every time they shift filters, which will take up valuable imaging time.

[Velvet]

Most LRGB filters say they are parfocal but i would take that with a pinch of salt. Always check focus if you

change filter. Dont know if anyone else has any filters you do not need to check....but I would think it is

always wise to check focus when you change

Velvet

[johnrt]

It is your scope that is more likely to be not parfocal rather than the filters themselves. Focus whenever you change filter, that's a fact of imaging I'm afraid.

[JamesF]

It can also vary depending on how fast the scope is I believe. With a fast scope you may need to check all the time regardless of which filters you have, whereas with a slow scope you may get away without checking at all.

James

[kirkster501]

I am doing preliminary reading about the whole CCD thing as well; i am good with my modded DLSR for a while yet. However, I thought the idea of the programmable filter wheel was that the imaging PC could change the filter unattended? Focusing after every colour change would be a right royal PITA........???

[dephelis]

I am doing preliminary reading about the whole CCD thing as well; i am good with my modded DLSR for a while yet. However, I thought the idea of the programmable filter wheel was that the imaging PC could change the filter unattended? Focusing after every colour change would be a right royal PITA........???

I'm looking into this also, I believe that this is where computer controlled auto-focusers come into the picture!

[Sp@ce_d]

I use the Baaders. On my 314L+/ZS66 setup they are parfocal now I have the WOflat/red II on it. Before without a reducer the G & OIII focused at the same point with the LRBHaSII focussing at another. So as above, more down to the scope.

[ollypenrice]

The Baaders have been parfocal for me down to F3.9.* Below that I cannot say. But when I say parfocal I mean it. I fuss like hell over focus, even in RGB when, arguably, it is less critical. I've tried Astrodons, briefly, and saw no life changing experience arising from them, but close aquaintance might have changed that. Astronomiks I have also tried, including the 'post-halo' ones, and I don't use mine. I'm happy with Baader.

You can't check focus too often but my feeling is that Baader have done a mighty fine job.

Olly

http://ollypenrice.s...39556&k=FGgG233

*That is with Takahashi colour correction, be it said.

[Cath]

It is your scope that is more likely to be not parfocal rather than the filters themselves.

I understand why lenses aren't normally parfocal, but I haven't yet worked out why a reflector system could not be parfocal because as far as I know a mirror will always reflect/focus at the same angle no matter what the frequency/wavelength is (whether it's at RF frequencies or visible light frequencies).

[johnrt]

I understand why lenses aren't normally parfocal, but I haven't yet worked out why a reflector system could not be parfocal because as far as I know a mirror will always reflect/focus at the same angle no matter what the frequency/wavelength is (whether it's at RF frequencies or visible light frequencies).

You're quite right, I was referring to refractors not reflectors.

[Cath]

You're quite right, I was referring to refractors not reflectors.

oh sorry John, didn't realise.

[ollypenrice]

Most reflectors need glass for field correction, though. Not an RC if you can get someone to make you a curved CCD chip, maybe...??? A few million slipped Sony's way...

Olly

[Cath]

Why don't they shape the secondary mirrors to get the secondary to do the field correction and so removing the need for having to insert glass to it ?

[JamesF]

Why don't they shape the secondary mirrors to get the secondary to do the field correction and so removing the need for having to insert glass to it ?

I imagine because in the main it's not considered economically worthwhile, perhaps they think only a very small percentage of people buying the telescopes would care.

James

[laser_jock99]

Why don't they shape the secondary mirrors to get the secondary to do the field correction and so removing the need for having to insert glass to it ?

Might be possible on an RC or SCT but on a Newt secondary you would need to generate non-rotationaly symetric curves which is very specialst optical work indeed. I have seen it done with three axis diamond turning for CO2 laser mirrors

http://www.iiviinfra...lty-optics.html

but whether this will work at visual wavelengths I'm not sure? You can certain make some intersting shapes on these lathes.

These are lathe turned parts!

[Chris Rowland]

The RC scope does change the shape of the mirrors to correct for most aberrations but you are still left with field curvature. These non spherical mirrors are expensive to make and it's a lot cheaper to do it with correctors such as with the SCT. The EdgeHD scopes use lenses to remove the field curvature as well. I think that in theory you could do it with mirrors but would need more than two. Smoke would help of course.

And I find the Baader filters to be exactly parfocal, that's through the UV/IR block, RGB, Ha, O3 and S2. It makes life so much easier.

Chris

[laser_jock99]

There is a good reason to try and make all the corrections with reflecting surfaces and that is chromatic abberation. Since all lenses (even the best corrected triplets) introduce some form of CA. You might not see it visually but if you're at all intersted in IR or UV imaging then it's even more apparent.

Going back the original topic- to be parfocal all the filters need to be exactly the same thickness, the same substrate glass type, parallel faced and similarly mounted (for position). So you almost certainly need buy a set from one manufacturer.

[Martin-Devon]

I do a lot of imaging at a fast F ratio of 3.6 with my Tak, and at times I have to refocus when changing filters since a few microns either way makes a big difference on optimal or suboptimal focus here. Independent of the switch of the filters, as the temperature drops during the imaging run, the focus is also prone to change so you have a double whammy - you probably will need to change filters + the temperature probably will drop over a typical imaging run of several hours - both of these factors are likely to challenge the accuracy of your initial focusing later in the imaging session.

Having set up my imaging run early in the evening, I now follow it remotely using LogMeIn on the desktop in my study. As the subs roll in, I open then up in Maxim and check for any field rotation (if there I try switching the choice of PHD guide star) and for the FWMH of a few selected stars across the subs. If there's a significant change in FWMH I will suspend the imaging run, and refocus again to get the FWMH value back down to its sharpest figure again - this is also done remotely since I have a Lakeside focuser fitted that I can control with software on my desktop screen. When I switch filters, I always recheck the FWMH focus value before initiating the new run, and then again during the run check how the FWMH figures vary over time and whether I need to intervene again or not.

In my early days of imaging, admittedly using a less-fast scope (F 5.6) I used to just set the optimal focus up, and then leave it all night un-interrupted to run and change filters with the EFW2. However, knowing now that the focus will likely drift over time, I intervene much more than before. The EFW2 is still useful, since this is also controlled remotely, so I don't have to keep going in/outside just to change a filter.

As pointed out in earlier posts there are software packages now that claim to auto-focus for you during an imaging run; I think that Sequence Generator Pro will do this but I've not tried it yet. If these software packages are reliable and reasonably priced, this may be the way to go.

[Gina]

I have the Baader LRGB filter set and on checking I found that after careful focussing in L the R, G & B were spot on. I also checked the Astrodon 5nm Ha filter and was surprised to find that the focus was still at it's best. This is with the Evostar ED80 without FR ie. f7.5. So I can do HaLRGB without refocussing. I use the Atik EFW2 wheel and 314L+ mono CCD camera.

[perfrej]

And I refocus at every filter change and at least every hour throughout the night. All automated with CCD Autopilot. I need sleep and sharp subs

/per

[kirkster501]

Discussing focus. With a CCD does one check for focus in the same way as a canon DSLR - I.e with a baht mask and some sort of live view on the CCD?

Rgds, Steve

Sent from my iPad using Tapatalk

[Sp@ce_d]

Discussing focus. With a CCD does one check for focus in the same way as a canon DSLR - I.e with a baht mask and some sort of live view on the CCD?

Rgds, Steve

Sent from my iPad using Tapatalk

Just about.. I set it up looping short exposures either with Artemus capture or Nebulosity & a bahtinov mask.. same way I do with the DSLR & Neb.

EDit: there is no live view as such on the actual CCD though... just to be clear

[perfrej]

I would suggest using Focusmax instead of manually focusing. There is no way that anyone can come close to perfect focus without something like that. I have tested the difference between using a Bahtinov mask masnually, with Bahtinov grabber and then Focusmax, and it is substantial.

Focusmax focuses out of focus and is thus less affected by the seeing conditions. Sounds strange, but that is the way it works Mine takes one to two minutes including slew to a suitable focus star and back to the target with plate solve and adjustment. Result is extremely sharp.

/per

[pyrasanth]

I would suggest using Focusmax instead of manually focusing. There is no way that anyone can come close to perfect focus without something like that. I have tested the difference between using a Bahtinov mask masnually, with Bahtinov grabber and then Focusmax, and it is substantial.

Focusmax focuses out of focus and is thus less affected by the seeing conditions. Sounds strange, but that is the way it works Mine takes one to two minutes including slew to a suitable focus star and back to the target with plate solve and adjustment. Result is extremely sharp.

/per

I echo your sentiments. I use a Celestron RASA & focus at F2.2 is critical. I don't think I have ever got a satisfactory sharp focus manually. FocusMax does the job quickly & accurately.