Filter Focus

[joecoyle]

Hi

Attached are 2 images (2 minute subs) from a couple of nights ago of M1. One is a luminance image taken with an IDAS P2 Light Pollution filter and the other is a Red image (Baader). Is the red one out of focus? Or is it just that red data is less defined? The blue image is of a similar focus to the LP. Green is a little bit worse off.

I have a vague recollection of needing to adjust focus for each filter. Would this be true in my case? I do not have a motorised focuser (however, I may buy one to save effort!)  Would i need to Bahtinov mask each filter? Which would mean removing my dew shield, taking focusing images, and re-attaching dew shield. Which is going to add a large amount of time to each imaging run. Normally i just set a sequence off in SGPro and go inside to let it work its magic.

I notice there is a setting in ASCOM for filter offset... is that to do with focusing?

Would a motorised focuser auto focus and move to a specific stored focus for each filter? Or does it need to measure / change focus each session?

Thanks

Joe

Light Pollution Filter:

 

Red:

 

[Catanonia]

The bottom one is out of focus. Always best to buy par focal filters of the same brand that meet focus at the same point. Otherwise you will need to manually change focus during each filter change that can be a pain.

Less defined will not give out of focus stars, look at the 2nd image bottom left quadrant and you can see the out of focus stars. Not by much but enough to see it.

Your guiding or collimation is slightly off too by the looks of it.

Not much experience of motorised filters, but with the right software, you can change the focus per filter, but again it can be a pain. Best to have par focal filters.

[ollypenrice]

It isn't just the filters which have to be parfocal. The optics have to be as well - which is to say they have to be fully apochromatic or colours will focus at slightly different distances even with perfect filters. Focus also drifts with temperature so needs regular checking. I think the lower image is out of focus and I'm not sure collimation is right, as mentioned above.

Olly

[joecoyle]

My LRGB and narrowband filters are all Baader. I'm hoping that means they're parfocal. I was advised to get the IDAS LP filter as the best out there (according to FLO).

 

Any focus issues / odd stars are always in the same quadrant of any of my images.

 

These images were my first attempt at drift aligning with PHD so potentially things were out. 

The collimation hasn't changed from new in 3 years - I checked it last night as it happens. I don't think I'd be able to centre the mirror much more. I'll have to take a photo to show you. Perhaps all it needs its the smallest of movements  

 

Then there's always the issue of imaging with an f/10 SCT reduced to f/6.3. I don't make things easy for myself!

Joe

 

 

[Catanonia]

What scope / focuser / camera are you using ?

If your stars are behaving badly in the same quadrant it could be down to the dreaded camera tilt, depending on the F ratio you are running at.

I had massive issues on my set up with a fast F2.8 Newtonian and heavy camera set-up. It was causing the focuser to bend and tilt the image giving massive issues. The faster your scope in F stop, the worse it gets.

Bad collimation will also make it more evident.

 

[joecoyle]

I'm using a Celestron Nexstar 6SE, mounted on an Advanced VX.

 

Got a focal reducer / corrector to bring it from f 10 to f 6.3 so the focal length goes from 1500mm down to 1000mm.

 

No focuser. Camera is Atik 414EX. 

 

Guider is Orion magnificent mini (star shoot auto guider plus 50mm guide scope)

 

Joe

[theastrodragon]

Might be an idea to set up the mask and go through your filter set, seeing which ones need a focus change for good results. Once you know that, you can make more informed decisions.

A motorised focus setup can help if it allows you to measure the focus (ie a computer operated one rather than a hand controller), as you can change filters and just apply the relevant correction.

[DaveS]

Are you sure you have to remove the dew shield? I never have with my Meg 90 but perhaps a 'frac is more forgiving there.

I tend to take only one or perhaps two (If I can get a long enough run) different filter exposures on any one night.

[joecoyle]

Yeah that was my thought as well - currently I'm trying to grab all 4 filters in one fell swoop. Leaving about 20 minutes for each colour. I may just have to resign myself to 2 filters per night to acquire a lot more data anyway. Only ever manage an hour of luminance for example. 

I'm just too impatient and want to get processing my images as soon as  

I just figured the bahtinov mask would have to go on the actual scope. Not really sure why or if it would make a difference. I can only test and find out. 

Joe

[Catanonia]

1 hour ago, joecoyle said:

Yeah that was my thought as well - currently I'm trying to grab all 4 filters in one fell swoop. Leaving about 20 minutes for each colour. I may just have to resign myself to 2 filters per night to acquire a lot more data anyway. Only ever manage an hour of luminance for example. 

I'm just too impatient and want to get processing my images as soon as  

I just figured the bahtinov mask would have to go on the actual scope. Not really sure why or if it would make a difference. I can only test and find out. 

Joe

I think you may be right. Your set up  as far as focusing is concerned, is not the same for each filter. You have 2 options

1. Manually change the focus per filter shot or concentrate on one filter run at a time

2. Buy a computer controlled focuser, set and calibrate it to do the manual shifts when the filter changes.

This will not stop temperature differences, unless the software you use in option 2 has a measuring method to automatically pick the correct focus. But that said, I have never run into temperature issues even at F2.8 once everything is cooled down.

The BMask will work for the current filter in place which in your set up is not the same for all filters hence the issue. When I bought my QHY 2inch filters, I made sure that they were par focal (same focus point) to make sure this didn't happen to me. But I have the same issue with my Ha filter which is not par focal to my LRGB ones and hence I concentrate on either Ha or LRGB for a session or re-focus when I switch 1/2 way through the night. At F6 your are on the edge where small amounts are noticeable in images.

 

Also, you may not have focuser flex, but your CCD may not be totally square with the imaging plane = tilt and hence the star shapes differ in the image. Focusing will "hide" this some what, but I would check your CCD is square. I use digital callipers to check and adjust the camera housing screws to try and dial it out.