Unable to focus DSLR

[WiltsStarGazer]

Having previously got some reasonable images by mounting a smart phone to the eyepiece. I thought I would try for a more sophisticated set up.

So last night I tried to get some pictures of the moon using a Canon Eos D450 connected to a Skywatcher 130p Newtonian via a T2 connection on the eyepiece holder.

As the camera has automatic focusing built into the lens I thought I would have to adjust the focuser on the eyepiece holder to manually get a sharp image.

Basically the telescope acting as a manually focused lens for the camera.

But no joy, I just got a bright light which seemed to fill the camera view finder. I tried various settings on the camera, adjusting ISO and aperture etc,  I also had some extention rings for the camera lens so tired fitting those to extend the focal length but no better.

I sure there are many palms being slapped against foreheads reading this but as you can tell I have no idea, although I do have some of the gear.

Any pointers and/or advise would be gratefully received.

[fozzybear]

A photo of your setup would help as this would quickly solve your problem

[WiltsStarGazer]

[WiltsStarGazer]

[happy-kat]

Add far as I'm aware your telescope 130P will not reach infinity focus with a mirrored DSLR. You will likely need to use a barlow to increase the inward focus travel available.

The 130P-DS will as it has a shorter distance to the mirror and the heritage 130p will as the trusses can be lowered.

EDITL: the above is when not using an eyepiece, instead the telescope is a giant camera lens, prime focus.

Edited September 28, 2020 by happy-kat
Added 'infinity'

[AstroNebulee]

I have the same camera and can acheive focus on my skywatcher explorer 150p. I had to unscrew my ep holder and then screw the t ring onto that fitting to achieve focus also set the dslr to manual on the dial as autofocus won't work. I'm not sure what the fittings are called on my 150p but had same issues as you at first. 

Edited September 28, 2020 by LeeHore7

[WiltsStarGazer]

33 minutes ago, happy-kat said:

Add far as I'm aware your telescope 130P will not reach infinity focus with a mirrored DSLR. You will likely need to use a barlow to increase the inward focus travel available.

The 130P-DS will as it has a shorter distance to the mirror and the heritage 130p will as the trusses can be lowered.

Thanks for your reply, my 2x Barlow has a removable lens, I'm guessing I should remove that and just used the Barlow as an  extention tube.

[WiltsStarGazer]

28 minutes ago, LeeHore7 said:

I have the same camera and can acheive focus on my skywatcher explorer 150p. I had to unscrew my ep holder and then screw the t ring onto that fitting to achieve focus also set the dslr to manual on the dial as autoficus won't work. I'm not sure what the fittings are called on my 150p but had same issues as you at first. 

Thanks for your reply, am I right in assuming that when you click the lens to Manual Focus and that is displayed in the menu screen on the back of the camera the sensor is set on infinity and you use the telescope to focus the image.

[AstroNebulee]

2 minutes ago, WiltsStarGazer said:

Thanks for your reply, am I right in assuming that when you click the lens to Manual Focus and that is displayed in the menu screen on the back of the camera the sensor is set on infinity and you use the telescope to focus the image.

Hi 

That's what I do and use the focus wheels to pull into focus, I also use a bahtinov mask to help me get sharp focus. If I'm right it's called prime focus. 

Edited September 28, 2020 by LeeHore7

[fozzybear]

Switch your camera to manual mode and then set your ISO to its lowest possible setting. That’s likely to be 100 or 200.

Set your aperture to f/10.

Set your shutter speed to 60th second

and aim at the moon and see how you go from there

[WiltsStarGazer]

Thanks for the advise guys, I will try those on the next clear night, can't see a thing down here tonight.

[Art Gecko]

On my 130p I cut 15mm off the back of the tube and remounted the mirror to bring the camera to focus... a 2x Barlow will do the job but makes the image more unsteady and you need to increase open shutter time by something like 4x to gather the same amount of light 

[AstroNebulee]

1 hour ago, WiltsStarGazer said:

Thanks for the advise guys, I will try those on the next clear night, can't see a thing down here tonight.

You can try during the day to see if you can focus on anything, also can you unscrew the brass eyepiece holder from the draw tube and screw the t ring directly to the silver focus tube, may help but just guessing 

[bobro]

The 150P has a focuser with an eyepiece extension that can be unscrewed, allowing a DSLR to be closer to the OTA and achieve prime focus. A130P doesn't have a focuser that allows this, so the DSLR image plane will be too far away from the OTA and won't achieve prime focus. A Barlow can help, but has disadvantages as explained above. Shortening the OTA to move the primary mirror upwards, again as explained above, allows a DSLR to achieve focus. A well known issue with current 130P Newtonian scope designs when trying to get a mirrored DSLR to focus at infinity.

[WiltsStarGazer]

Thanks for everyone's advise, behold the beauty of my neighbours chimneys! 

Shame it's piddling down again tonight 🙄

[AstroNebulee]

Great work and lovely chimneys, so putting the 2x barlow in begore your dslr brings you into focus now? 

[WiltsStarGazer]

3 hours ago, LeeHore7 said:

Great work and lovely chimneys, so putting the 2x barlow in begore your dslr brings you into focus now? 

Yes, I have a Barlow that allows you to remove the lens and just use it as an extention tube but that didn't work, so I put the 2x back in and bingo.

[JamesF]

Adding the barlow moves the focal plane "further out" of the focuser, diverging the light cone from the primary mirror to the point where it will reach the image sensor in the camera, which is why it now works.  The problem without the barlow is that the focuser doesn't allow the camera to be moved inwards far enough to reach the un-barlowed focal plane.  That's why shortening the tube, thereby moving the primary mirror closer to the camera worked for Art Gecko -- effectively it moves the focal plane out of the focuser by the same amount as the mirror is moved upwards.  It's not a perfect solution however, because the secondary mirror may not be large enough to capture the entire image reflected from the primary, so you can end up "throwing away" part of the image.

Replacing the focuser with one that allows the camera to be moved in further would probably help, but quite possibly it would be cheaper to buy a 130P-DS than to replace the focuser.

If you're happy with what you're achieving with the barlow for the time being however, then just crack on and enjoy yourself

James