Bloated stars with Baader Oiii CCD 8.5nm 1.25" filter

[ChrisEll]

Hi,

I've read there's been some problems with this filter.... my Ha and Sii stars look fine and are about the same size but the Oiii stars are noticeably larger.

Can anyone recommend a replacement which is parfocal with the other Baader filters? 

Thanks,

Chris

[Hallingskies]

I assume you are refocusing after filter changes.  My old Astronomik Olll was utterly hopeless and gave horrendous haloes on bright stars, but I’ve found the Baader Olll to be pretty good.  Maybe you have just been unlucky, might be worth talking to the supplier?

[ChrisEll]

No, I'm not refocusing... I'm assuming these filters are parfocal. However next time when I have the bahtinov mask on I will check each filter to see if this is the case.

[ChrisEll]

>might be worth talking to the supplier?

I bought it 2nd hand so I'm not sure I have any warranty.

[vlaiv]

Try refocusing first.

There are few other reasons for this:

- might be due to seeing. Shorter wavelengths are more affected by it.

- If you are using refractor - it can be due to optical performance of it. This one is also linked to refocusing. Filters might be parfocal (to an extent), but if you are using refractor, OIII part of spectrum can have slightly different focal position. With refractors you might also have lower performance in blue part of spectrum - most are corrected in green, some are corrected towards red (ones aimed at AP) - so blue will have the lowest Strehl of all.

[ChrisEll]

Yes using a refractor. Fair point... I appreciate light of differing wavelengths will focus in different positions, and Ha and Sii are almost next door to each other in the EM spectrum.

[steppenwolf]

I never rely on filters (even my Astrodons) being parfocal when part of an imaging system so your problem is very likely to be one of focus. Even in my excellent Esprit 150ED, there is a 50.22 micron difference in the focus position between my 3nm Ha and 3nm OIII filters but I don't know how much of that difference is purely between the filters.

[Hallingskies]

2 hours ago, ChrisEll said:

However next time when I have the bahtinov mask on I will check each filter to see if this is the case.

Let us all know how you get on, but if you take the time to carefully refocus after filter changes then I suspect the problem will probably be a lot less noticeable.  It’s a pain because you’ll need a new set of flats if you change focus, of course...

[carastro]

I find the Oiii stars are always a bit bigger than the Ha and Sii - and it also lets more light through when I come to do my flats, so I think part of it is to do with the nature of the Oiii wavelength.  

I also have Baader HOS filters, I find they are mostly parfocal but if any of them need a bit of a tweak it is Oiii.  I don;t understand why sometimes they are parfocal and others times not quite.  

Carole 

[Stu]

Didn't Baader replace a batch of filters which were causing haloes? I can't recall the detail but there was definitely an issue which they resolved by swapping them out for new ones.

[fireballxl5]

36 minutes ago, Stu said:

Didn't Baader replace a batch of filters which were causing haloes? I can't recall the detail but there was definitely an issue which they resolved by swapping them out for new ones.

Yes, I bought one that came with free halos back in March 2013, replaced foc later that year.

Also, CCD's are usually more sensitive in the green and so this will make the OIII data seem brighter than you may expect.

[Stu]

This was the problem, does that look like it? Dated back to 2013 to 15 I think.

 

[ChrisEll]

Thanks all for your input.

My Oiii stars don't have halos so I don't know if my issue is related to that.

However this thread got me thinking about electronic focusing and I can buy an auto focuser for my ED80 for £45 and an ASCOM compliant controller is available. Then I can set up filter offsets and (hopefully) automate the whole process

[Adam J]

Well the Ha and SII will be the same size they are both near IR / deep red.

The OIII is blue / Green and so Blue stars will be larger in OIII.

You can get reflections from a OIII filter, its harder to prevent than with the Ha and SII but its not clear if you have reflections or bloating or just poor focus without some example images.

[Shibby]

I found that, with the Baader filter, the raw star sizes were pretty similar. However, as Oiii is generally fainter we tend to push the data harder, increasing the chance of star bloat. I now have a 3nm Oiii which seems to match up pretty well with the 7nm Ha stars after processing.