Repeatability of OK3Z focuser.

[mattssporre]

Dear all,

 

I am trying to find the filter offsets for my OK3Z focuser by using FocusMax and CCDAP5.

 

In CCDAP5 there is a wizard that automatically will loop through a given number of series (L - R- G- B - Ha - SII - OIII), focus each filter and noting down the focuser position. One should do at least 5 loops. The offset is then measured with the L-filter as the reference.

 

For a fast OTA (F3.6) the critical focus zone is only 9microns (CFZ = the zone within which it is not possible to measure a difference in focus). Thus I need to get the offset measurements for the different runs within 9microns. 

 

I now have 20 measurements for each filter.

 

The problem is that the different measurements differ too much (more than 9microns).

 

As an example here is the first 9 measurements for the R-filter: 156, 193, 181, 182, 174, 197, 185, 211,184. These are the value to add to the L-filter focus position in order to get the R-filter in focus.

 

The min value is 156 and the max value is 211. The difference between these are 55 microns which is much more than 9microns (CFZ).

 

Obviously if I add 156 or 211 to the L-filter position at least one of them will not give correct focus for the R-filter.

 

I thought the OK3Z focuser was meant to be reliable and able to repeatable. 

 

Anyone else having this issue? Or do you have a repeatable OK3Z focuser? Is there a way to optimize it (adjusting some screws or something)?

 

BR
Matts

[GerhardB]

Hi Matts,

 

Unfortunately the performance of FocusMax is seeing dependent, and the writers of CCDAP suggest to estimate the focus offsets on a night of good seeing.

Theses nights are extremely rare at my location, so I made a series like you did, calculated the mean values and used them as a starting point for finetuning by visual inspection.

The CFZ is not as critical as one would think because of above mentioned seeing effects and shifted with my average seeing of 2.5" - 3.5" to a  value of ~20microns.

 

As a side note, Phillip Keller's new ( and hopefully soon to come) version of Sequence should contain its own autofocus routine which compensates for the degrading effects of seeing.

 

Best regards,

Gerhard

[mattssporre]

Gerhard,

 

thanks for the input - I have been thinking along the same lines. How do you go about to do the fine tuning by visual inspection? Any tips?

 

BR
Matts

[GerhardB]

Hi Matts,

 

By measuring the FWHM of a few unsaturated stars in Maxim DL.

 

Best regards,

Gerhard

[mattssporre]

Ok,

 

so what you do then is

 

1. focus using FocusMax with the reference filter

2. use the median filter offset as found using CCDAP filter offset wizard to get e.g. the R-filter close to focus

3. take an image and check the FWHM on that image

4. move the focuser outwards and check the FWHM again

5. move to the other side of original focus and check the FWHM again

 

BR
Matts

[GerhardB]

Exactly, Matt.

 

Best regards,

Gerhard

[mattssporre]

Gerhard,

 

I tested it last night for the Ha-filter and the median value I had was correct. 

 

It is very easy to check the focus when you have the double spider vanes. If the OTA is not in focus an image of a bright star will produce double diffraction spikes (8 instead of 4).

 

Thanks again for answering.

 

BR
Matts