Backfocus and OAG help!

[nakis87]

Hi guys and Happy New Year to all!

Got a new dedicated camera recently and looks like I am having some newbie issues… so I could use your lights.

Equipment:

12” Orion Optics Newtonian @ f/4 with Televue Parracor type 1 comma corrector (yes the old one with the parrot) – is that supposed to be a 55mm backfocus?

Main camera: QHY268C

Guide camera: ZWO ASI224mc paired with the ZWO OAG

Problem is that cannot seem to get into focus on both camera and the guidecam at the same time no matter what (I tried adjusting the prism up and down with the small allen screws but stars were never clear points and had the classic elliptic moon shape ))))

Looks like since the light path is out of the field of view of the main sensor and my backfocus might have an issue this is something that cannot be fixed?

 

Anyone here maybe have a similar set up that can provide some tips/pictures?

 

On a separate note, since I was really fed up with trying to focus the OAG and didn’t want to miss another clear night, I reverted back to my 50mm Orion mini guide scope for guiding and I did some test shots with the new camera.

Used Sharp cap pro live stacking function and NINA to plate solve which worked well –  started with the recommended settings of 27 gain and 30 offset at 60 and 120 sec exposures – Image was appearing really dark even after some stretching so I decided to up the gain a bit to be able to see what’s going on at the screen.

No darks no flats at this stage, but I have to say that I am having a real nightmare using FITS files. Even after I convert them to TIFFs and change the mode to RGB instead of Greyscale the images appear nothing like the ones on the live stack screen.

Also all my images seem to have some strange artifacts like cosmic noise??? Does anybody knows if that’s normal and will go away with darks/flats or something is going on with the sensor/optics?

Picture below for reference:

  

It’s my first dedicated astronomy camera as I am coming from a DSLR background so please be kind with your replies 😊

 

Thank you for your time,

Ananias

[almcl]

Can't help with your focus issues beyond saying that I use a click lock holder that allows the guide camera to slide in and out without moving the stalk (see below), but your 'cosmic ray effects' are walking noise, hot pixels.  You are practically guaranteed to get these with a guidescope and a Newtonian and are a very good reason to persevere with the OAG.  Dithering between the shots (I think you stacked 15, from counting the hot pixels) can help together with a Sigma clip stacking algorithm, but you need at least 20 lights and more would be better. 

Edited January 7, 2021 by almcl

[newbie alert]

Regarding your back focus you should setup your imaging camera with the correct back focus( ie 55mm) if you then measure the distance from your prism to the imaging camera sensor, and then setup the same distance to the guide camera sensor from the prism you should then be good to go..note your back focus distance from the imaging and guide cam will be different so you would need to take that into consideration.. a friend has a good idea and point at a globular cluster , so something with lots of stars to do fine adjustment

[michael8554]

On 07/01/2021 at 12:29, nakis87 said:

Problem is that cannot seem to get into focus on both camera and the guidecam at the same time no matter what (I tried adjusting the prism up and down with the small allen screws

Moving the prism up and down adjusts how far into the main imaging light path the prism protrudes, it doesn't affect the guidecam focus..

You adjust that to just avoid a shadow on Flats.

Once that is adjusted and locked-off, focus the guidecam by moving it's position up or down the periscope, after loosening the other screws.

You can do this during the day on distant landmarks.

Star shape may not be perfect, but PHD2 will handle misshapen stars.

As newbie says, get the imaging focus correct before adjusting the guidecam focus.

Then in future only adjust the imaging camera focus.

Once that is correct, the guidecam will be in focus too.

Michael

Edited January 10, 2021 by michael8554

[nakis87]

Thank you all for the responses.

 

So the walking noise issue looks like it’s quite common, but using a guidescope is so much easier on finding guidestars vs the OAG. Will try dithering and calibration frames properly on the first clear night.

 

I hadn’t had the chance yet to play around with the focus so I just assume at this stage that 55mm is the correct BF for my set up ( I do not have a clip focuser on the OAG side so it will be just a case of moving up and down the camera in the nosepiece hoping that will reach focus) Once again I was able to focus ok when using my DSLR and off axis set up.

Agree on trying making the adjustments in daytime to avoid frustrations and potential damages to the surrounding area – I was that close to start throwing things from the rooftop the other night!🦹‍♂️

Will keep you posted!

 

Thanks,

Ananias