14 replies to this topic

[skywatcher01]

Hi

 

I have Canon 600d (entry level DSLR) + Dobson 10" and I was trying to follow below instruction video : how to capture DSO with DSLR + Dobson   

https://youtu.be/b1E3K9rU9A4?t=160

.... however I am unable to focus any star. I was trying to focus Vega, but in my Liveview was only dark.

With capturing planets: Jupiter, Saturn: no issues, I use TV mode to modify shutter speed ( with planets I used also Barlow, but here nothing helped)

Looks I have same settings ( iso 6400..) but in Liveview only dark... probably I have something missing in camera settings

Edited by skywatcher01, 06 August 2020 - 05:36 PM.

[kathyastro]

It is not normal to be able to focus a DSLR when mounted on a Dob.  Normally, you would need a barlow to be able to focus the camera.

[skywatcher01]

Ok, I tried also 2xbarlow but still only dark view

[kathyastro]

Have you tried a rough focus in daylight on a distant object so you know roughly where the focuser should be positioned?  Is your finder properly aligned with the scope?  Have you cranked up the ISO on the camera for live view? 

[skywatcher01]

Another issue: today I checked my new 3x barlow to focus on Juiter, but looks like I am unable to focus clearly, with  telescope focuser I am in lowest position so unable to focus more...

Do I need some another accessories to fix this ?  With 2x barlow: no issues, no need to focuser to get too low position.

[doolsduck]

Are you able to confirm that the images you got of the planets were in focus? Just let’s say for argument sake that you weren’t able to focus due to the common back focus problem with Newtonians, the planets are so bright and wide that they would still appear on your screen, but Vega may dissolve into A diffuse ring that you can’t discern on the screen. If you’re sure of focus then up your ISO until you can see it to confirm, but you probably need to then optimise the ISO for imaging with that camera.

[doolsduck]

Sorry I posted my reply as you were doing yours. So, you have a back focus problem. It is not uncommon, I have the same too. Do you by chance have a collapsible dob? Google back focus telescope

[skywatcher01]

Have you tried a rough focus in daylight on a distant object so you know roughly where the focuser should be positioned?  Is your finder properly aligned with the scope?  Have you cranked up the ISO on the camera for live view? 

Thanks for hint, Finally I focused the star ! Firstly I aligned my focuser with Jupiter and then I did focus also Vega

[skywatcher01]

Sorry I posted my reply as you were doing yours. So, you have a back focus problem. It is not uncommon, I have the same too. Do you by chance have a collapsible dob? Google back focus telescope

No I don't have collapsible dob, it's one solid tube.

And reg. back focus telescope, how can I fix it please ? is there some accessories ? 

As I understood I need longer focuser tube to be able to get more deep with focuser. Is there some focuser reduction or something ?

[doolsduck]

No I don't have collapsible dob, it's one solid tube.

And reg. back focus telescope, how can I fix it please ? is there some accessories ? 

As I understood I need longer focuser tube to be able to get more deep with focuser. Is there some focuser reduction or something ?

But is it all academic now?  You can get focus with the 2x and 3x Barlow?  

[kathyastro]

No I don't have collapsible dob, it's one solid tube.

And reg. back focus telescope, how can I fix it please ? is there some accessories ? 

As I understood I need longer focuser tube to be able to get more deep with focuser. Is there some focuser reduction or something ?

To get it to focus without a barlow, you need a shorter focuser.  The camera needs to move in closer to the tube than an eyepiece does because its sensor is way at the back.  The other option, which requires some tube modification, is to move the primary mirror up in the tube.

[skywatcher01]

But is it all academic now?  You can get focus with the 2x and 3x Barlow?  

Currently I can focus with 2x Barlow, issue is with 3x and 5x Barlow.

Also without barlow > no issues to focus.

Edited by skywatcher01, 07 August 2020 - 01:47 AM.

[doolsduck]

Currently I can focus with 2x Barlow, issue is with 3x and 5x Barlow.

Also without barlow > no issues to focus.

So if you can focus without a Barlow it means there is enough inward travel of the focusser so the camera sensor can reach the focal plane, ie the the telescope has enough 'back focus'- this sounds confusing given we're talking about inward travel of the focusser, but think of it as the scope throws the focal plane far enough back or out to meet the camera sesor without the need for a Barlow .  If you can focus with a 2x Barlow who's divergent lens throws the focal plane further out, it means there is enough outward travel of the focusser so the camera sensor can still reach the focal plane.  The 3x Barlow throws the focal plane even further back and if you cannot focus the dslr it means now the focusser cannot travel out far enough to reach the focal plane and you need to add a spacer.

[skywatcher01]

So if you can focus without a Barlow it means there is enough inward travel of the focusser so the camera sensor can reach the focal plane, ie the the telescope has enough 'back focus'- this sounds confusing given we're talking about inward travel of the focusser, but think of it as the scope throws the focal plane far enough back or out to meet the camera sesor without the need for a Barlow .  If you can focus with a 2x Barlow who's divergent lens throws the focal plane further out, it means there is enough outward travel of the focusser so the camera sensor can still reach the focal plane.  The 3x Barlow throws the focal plane even further back and if you cannot focus the dslr it means now the focusser cannot travel out far enough to reach the focal plane and you need to add a spacer.

Indeed, sorry for confusing,  yes camera can focus, but telescope  manual focuser needs travel little bit more down and I have no space, so I need probably some accessories correction btw T-ring and camera, respectively  between T-ring and barlow....

[doolsduck]

Indeed, sorry for confusing,  yes camera can focus, but telescope  manual focuser needs travel little bit more down and I have no space, so I need probably some accessories correction btw T-ring and camera, respectively  between T-ring and barlow....

Take a step back, because that statement is confusing.  I assume you're doing prime focus imaging, so the telescope replaces the camera lens.  It makes no sense then to say that 'yes the camera can focus' AND the telescope focusser needs to travel down a bit more.  If the telescope focusser needs to move down more then the camera cannot focus.  So the question is; when does the focusser need to move down a bit more?  Without Barlow, with 2x Barlow or just with 3x Barlow?  If it's just the 3x Barlow then as I explained in previous reply you need a spacer to move the camera further out.  Or have I got completely confused?