Back focus - no image??????

[Len281249]

This is probably a stupid newbie question but it's really got me head-scratching!!

I am just moving to a ASI533MC camera from modified Nikon D5300 DSLR, I have a TS65 Quad scope F6.5 and have been getting images from the DSLR with the DSLR, a 2 inch adapter straight into the scope drawtube and focused with the scope focuser and Bahtinov mask and all good. I installed the new camera and all drivers etc are functioning and originally I forgot to add any extension tubes to account for the 55mm back focus required by this ASI533MC camera. I took a star image and long or short exposure times and low and high gain combinations, the stars were the smallest of smallest pinheads where previously with the DSLR they were normal. I thought the new camera was faulty!!!. Then I saw and remembered and fitted 55mm length of extension tubes and tried again. Exactly the same outcome, impossibly small star pinheads at any setting. In the daytime I can focus on far away objects using the focuser and acquire a correct sized image on the new camera.

I have of course now realised that my scope which includes a built in flattener, requires 110mm of back focus and I am now waiting for additional extension tubes to arrive to get to 110mm from the rear of my scope to the ASI533MC sensor.

My question is would such a way off back focus produce this symptom of crazily small pinheads stars that even disappear from the image when I increase the gain or exposure times and only reappear in their incredible small size at minimum gain and exposure time and of course are useless for AP.

I am trying to convince myself that my scope is still OK and all will be well when I get the correct scope back focus of 110mm

Any explanations that this incorrect back focus is causing this effect would be very reassuring

Thanks so much 

[andrew s]

Are you sure they are stars and not hot pixels? Try imaging the moon to see if the focus is good. Regards Andrew 

Edited October 6, 2020 by andrew s

[Tomatobro]

I think you are just struggling with the layers of technology and certainly have a grasp of the basics.

If you can focus on a distant object in daylight and get a good image with the new camera then focusing on the stars is only a little way further out. I would repeat this and measure the focus distance and make a note of it.

A combination of a little way out of focus short exposure times will produce a blank screen. So next clear night set the focus distance to what you had in daylight and a longer exposure time and then move the focuser out a fraction and wait for the next image to download. You don't say what software you are using but if its Sharpcap then click on the auto stretch symbol after each download.

In the excitement of it all its easy to leave the dust cap on the end of the scope.  Everyone has done this at some time.

To check if they are hot pixels just keep zooming in on the monitor till its squares. If its a hot pixel they will be single illuminated squares.

 

[Len281249]

43 minutes ago, andrew s said:

Are you sure they are stars and not hot pixels? Try imaging the moon to see if the focus is good. Regards Andrew 

Thanks Andrew I'll try the Moon when I get the extension tubes

[Len281249]

15 minutes ago, Tomatobro said:

I think you are just struggling with the layers of technology and certainly have a grasp of the basics.

If you can focus on a distant object in daylight and get a good image with the new camera then focusing on the stars is only a little way further out. I would repeat this and measure the focus distance and make a note of it.

A combination of a little way out of focus short exposure times will produce a blank screen. So next clear night set the focus distance to what you had in daylight and a longer exposure time and then move the focuser out a fraction and wait for the next image to download. You don't say what software you are using but if its Sharpcap then click on the auto stretch symbol after each download.

In the excitement of it all its easy to leave the dust cap on the end of the scope.  Everyone has done this at some time.

To check if they are hot pixels just keep zooming in on the monitor till its squares. If its a hot pixel they will be single illuminated squares.

 

 

[Len281249]

Thanks Tomatobro, Like Andrew said, they may well be hot pixels and hence I think would cause the strange phenomenon of disappearing with gain and exposure changes. I am comforted by your comment re "a little out of focus and short exposure times producing a blank screen"....that really settles my mind a bit.

No definitely remembered the dust cap

I'll do as you suggested if we ever get a clear sky, I'm in St Asaph so quite near to you in Shropshire and we both have horrible skies at the moment.

I am using the capture in APT though do have ASICap from ZWO and Sharpcap, not really familiar with either as used to APT with my DSLR and stupid me thought it would be a simple switch to CCD (CMOS)...unfortunately not.

I will wait for extension tubes to arrive and set exactly 110mm from rear of scope to sensor and try again.

Do you mind if I contact you again please to try to move along this learning curve?, are you on Messenger? 

[Adam1234]

I second using the moon to check the focus, I've spent a couple nights spending ages struggling to focus after getting new equipment, to then realise I can use the moon to get close to focus