help with collimation and corrector spacing

[Atreta]

Hi everyone,

tonight, since the full moon is up, i took the opportunity to get my collimation right and to try the coma corrector with the 3d printed spacer.  The coma corrector is a gso, with all spacers and the adapter it's about 74.5 mm, it should be 75, but that's the closes i could get yet. would it be ok?  i took some pictures, but the cornes don't look good at all, and the center looks a bit blurry too. 

I also reviewed the collimation and did fine tune it with a star test, could somebody check it if it's  any good?

the telescope is a 130pds.

thanks a lot in advance.

here is a star field picture from tonight:, it's very noisy because it's really hot in here tonight ( middle of brazilian summer :D)  

 

[alacant]

Hi

Where's the focus position? Is the focuser tube protruding into the light path or is it a long way from the tube?

Anyway, assuming you nailed the collimation, it looks like the camera is tilted; too close at the bottom of the frame, OK centre, astigmatic/too far at the top.  It's a common problem with the sw focuser. Quite often, loosening the screws and re-seating the camera in the focuser is all that is required, albeit hit and miss. You can drill and tap m4 at 120º for a third screw to hold the camera more positively in the focuser. More definitively, the gso gives enough back focus to be able to use one of these, but then you're on your way financially to a proper replacement focuser. From bitter experience, these are best avoided;)

Also, unlike other ccs, the gso has a 10mm working range so maybe try a further 2 to 3mm away from the sensor too.

HTH and clear skies.

Edited January 11, 2020 by alacant
la gramática

[Atreta]

4 hours ago, alacant said:

Hi

Where's the focus position? Is the focuser tube protruding into the light path or is it a long way from the tube?

Anyway, assuming you nailed the collimation, it looks like the camera is tilted; too close at the bottom of the frame, OK centre, astigmatic/too far at the top.  It's a common problem with the sw focuser. Quite often, loosening the screws and re-seating the camera in the focuser is all that is required, albeit hit and miss. You can drill and tap m4 at 120º for a third screw to hold the camera more positively in the focuser. More definitively, the gso gives enough back focus to be able to use one of these, but then you're on your way to a proper replacement focuser. From bitter experience, these are best avoided;)

Also, unlike other ccs, the gso has a 10mm working range so maybe try a further 2 to 3mm away from the sensor too.

HTH and clear skies

Hi Alacant, 

The focuser is almost all the way out. I made a 30mm length ring to make some space between the camera and the top of the focuser to avoid it being inserted too far and having to pull by hand and misalign.

I'll print a longer m48 extension today to see if it gets better being more distant from the sensor.  

The baader click is interesting for a future upgrade,  I'll have it in mind. 

Thanks a lot for the help. 

[alacant]

34 minutes ago, Atreta said:

The focuser is almost all the way out.

Hi. yeah, that's what I found. The gso is perfect for reflectors with insufficient inward travel but works against those of us with telescopes designed for imaging.

Here on a 6" and with a good focuser, it was impossible to prevent tilt and very difficult to balance. The tube distorts at different angles with all the weight hanging off and this maybe leads to loss of collimation depending upon the angle. I think you may stand more chance with the 130 as the tube is not as wide. Bresser suggested I change the sensor to cc distance to make the focus point closer to the tube, but by then I had use of a gpu so didn't get that far. As I say, you have 10mm to play with so maybe worth a try? 

I'm sure it's doable, so here's hoping that someone with the same combination can point you in the right direction.

Cheers and HTH

   

[Atreta]

I'll try again tonight if it's clear and report back. 

I can make a tad smaller ring so the corrector + camera can be inserted deeper into the focuser tube and have it  focus more inward without protruding into the light path. 

[Atreta]

I think i made some progress tonight, I switched the 12mm spacer for a 20mm one and the total backfocus for the gso coma corrector+ camera became about 76mm.

Then i also reviewed the collimation and took some time doing it with a star test and when i thought it was ok, i went for the test shots.

the corners look a lot better, yet not perfect, one other thing i noticed is that it was a bit harder to get the best focus ( maybe i'm getting old, lol) 

anyway, here is a picture to showing the result of tonight`s efforts.

ps: a noob question: is a coma corrector also a field flattener?

Edited January 12, 2020 by Atreta