New Chroma Filters and problems in SGP focusing

[Jkulin]

Hi All,

I wonder if I could have your thoughts?

I previously has Optolong Filters in NB and LRGB but was suffering from some reflections and halos, so I decided to slowly upgrade to Chroma, couldn't afford to do it in one go, so bought the LRGB in the first instance and wills ave up for the 3nm NB.

I'm using the 50mm unmounted in my Moravian 7 position FW and G2-8300MkII camera.

Now I have never had a problem in SGP focusing before, it has always worked faultlessly, but first time using the new filters, its focusing was all over the shop even settling at 10 on the HFR as the best it could do, it was a hit or miss affair, sometimes if I started with it near focus manually then told it to go off and get it better, it would and other times it would go off the scale.

I'm using on this OTA a Lakeside motor with a Pegasus Ultimate Power Box, I even tried their suggestions from here: - https://pegasusastro.com/wp-content/uploads/2018/07/autofocus_with_sgp.pdf

I tried varying the step size from 10 to 40 but it still got its knickers in a twist

20 would get it focused OK  as would 10, 30 or 40 but 20 was the most reliable, I tried the data points from 7 to 12.

When it would achieve focus, I could not get a V no matter how hard I tried: -

Apart from those two adjustments I have not played with anything else.

Does anyone have any thoughts, could it be a problem with the filters, they were ex demo and about 3 years old from MA, totally stumped on this one.

Appreciate any ones thoughts.

Thanks.

[RayD]

29 minutes ago, Jkulin said:

Hi All,

I wonder if I could have your thoughts?

I previously has Optolong Filters in NB and LRGB but was suffering from some reflections and halos, so I decided to slowly upgrade to Chroma, couldn't afford to do it in one go, so bought the LRGB in the first instance and wills ave up for the 3nm NB.

I'm using the 50mm unmounted in my Moravian 7 position FW and G2-8300MkII camera.

Now I have never had a problem in SGP focusing before, it has always worked faultlessly, but first time using the new filters, its focusing was all over the shop even settling at 10 on the HFR as the best it could do, it was a hit or miss affair, sometimes if I started with it near focus manually then told it to go off and get it better, it would and other times it would go off the scale.

I'm using on this OTA a Lakeside motor with a Pegasus Ultimate Power Box, I even tried their suggestions from here: - https://pegasusastro.com/wp-content/uploads/2018/07/autofocus_with_sgp.pdf

I tried varying the step size from 10 to 40 but it still got its knickers in a twist

 

20 would get it focused OK  as would 10, 30 or 40 but 20 was the most reliable, I tried the data points from 7 to 12.

When it would achieve focus, I could not get a V no matter how hard I tried: -

 

 

Apart from those two adjustments I have not played with anything else.

Does anyone have any thoughts, could it be a problem with the filters, they were ex demo and about 3 years old from MA, totally stumped on this one.

Appreciate any ones thoughts.

Thanks.

You need to redo your step sizes, John.  When you first set up your step sizes you base these on the in focus HFR value, and the 3-5 x out of focus HFR value.  This will now have changed so will need to be recalculated.

[Jkulin]

Thanks Ray, I never setup my stepsizes, just took the defaults until last night, how do I calculate the step sizes needed? What does 3-5 x out of focus mean?

Apologies if I don't understand the procedure for the calculations?

[RayD]

6 minutes ago, Jkulin said:

Thanks Ray, I never setup my stepsizes, just took the defaults until last night, how do I calculate the step sizes needed? What does 3-5 x out of focus mean?

Apologies if I don't understand the procedure for the calculations?

No problem, John.  For optimum auto focus you will need to set them for your focuser.  I did a video here explaining how to set them up which hopefully should help.

[Jkulin]

thanks Ray, I'll have a look

[sloz1664]

Looking at your set-up I see you are using 1 second exposures. I would try increasing this to 3 seconds and see if it makes a difference. If it is successful, also up the NB to 5-10 seconds, seeing as they are 3Nm.

Steve 

[Jkulin]

Thanks Steve, the one second was only while I got rough focus, I use 5 secs minimum for LRGB and 15 secs for NB

[RayD]

I think from memory that value is only applicable to a OSC, and isn't used for mono with filters.  You can set the separate filter values by clicking the button and entering them individually.  It will take that 1.0s value if you use a OSC camera.  I think the actual AF exposure value if using LUM to focus is on a different screen.

I could be taking nonsense as normal as I'm not in front of my imaging PC so can't remember for sure, but I seem to recall this.

Looking at that curve there is definitely a step size issue.  From an 'in focus' HFR value of 1.6 your first step should be around HFR 5.5 or so.