Newbie Focusing Problem

YvesLeb

Hello,

I bought the ASI290 camera to use with my Celestron NexStar 5SE telescope and can't get the camera to focus on objects (Moon, planets, local objects).

I follow the instructions provided where I first focus on an object with the eyepiece, then switch out the eyepiece for the camera (using the 1.25" adapter provided). I then try to refocus the image without success. I've used both ASICap and SharpCap 3.2, and get the same results. I tried the camera with the CCTV lens provided (not connected to the telescope) to check if the camera was faulty and always obtained a crisp image in focus. I tried focusing on objects during the day (with the same results) and lately with the full Moon on October 31 under clear conditions. I was able to focus the Moon with a 25mm eyepiece and had a very clear image in focus. When I switched out the eyepiece for the camera the image was much larger and not in focus.

Do I need a lens along with the 1.25" adapter to bring the images into focus?

I've searched the web and could not find any help or suggestions for my configuration/setup. Any suggestions? Your help would be appreciated.

Thanks,
Yves

w7ay

YvesLeb I was able to focus the Moon with a 25mm eyepiece and had a very clear image in focus.

Are you certain that it is a focus issue with the Moon, and not an exposure issue?

Take a look at the Max ADU number just to the right of the guide graph in the main window, Reduce the exposure time and the gain of the camera so that the Max ADU is under 65000.

If that still produces a bloated moon, then you need to adjust focus. While you are attempting focusing, monitor the Max ADU to make sure you are not overexposed.

It is even easier if you have a clear, moonless night. Point the OTA at a bright star. If you can see a dim but very large disk (even the size of your sensor), you are halfway there. Adjust the focus in one direction and then the other direction, and observe the Max ADU again. As you approach focus, Max ADU will increase (the total flux from the star lands on fewer pixels of the camera sensor). At the same time, keep re-adjusting the exposure and gain to stay below 65000. As long as the defocused star's flux is above the camera noise, this method will work even if you cannot visually see a large disk at the beginning. Just rack the focuser in and out to see which direction gives you a larger Max ADU.

By adjusting for largest Max ADU, it should get you a compact star that is close enough to use a Bahtinov Mask to finely focus the telescope.

If you hit the focuser limit, you will need to shorten or lengthen the back-focus distance.

If you are not using any focal reducer with your 5SE, the back focus should be close to 127mm. This is the distance from the metal back of the SCT to the sensor in your camera. 127mm (5 inches) may seem quite long, but your telescope is meant for visual viewing usng a star diagonal; the star diagonal needs a large back-focus distance.

If you have the large hockey puck shaped ASI290, its flange to sensor distance is 12.5mm, if you are using the cooled ASI290, the flange to sensor distance is 17.5mm. Subtract these numbers from the 127mm back-focus value to get the distance that the flange of your ASI290 needs to be placed behind the metal back of your telescope.

If you are using a Celestron 0.63x flattener/reducer, the back-focus from the metal back of the reducer to your camera sensor should be 105mm.

Chen

YvesLeb

Hello Chen,

Thanks for the quick response and the suggestions. My camera is the non-cooled version (ASI290MC).

Regarding overexposure, during my tests I tried a number of settings to avoid this and was successful. I was able to see the images so I don't think that this is the problem.

I checked the measurements for back-focus as you suggested and confirmed that the camera sensor falls within the range of the back-focus for my telescope. This morning I tested my camera by trying to focus on a distant hydroelectric tower. I was able to focus correctly with my 25mm lens but with the camera, I couldn't get the image in focus. I even tried inserting the 1.25" adapter into the telescope at different positions (moving it further out from the flange) and tried to refocus, always with the same results. Note that the image with the camera is about 2 to 3 times larger than what I see with the 25mm lens. Even if the camera sensor falls within the back-focus, don't I need a lens to better focus the image? I've seen articles on corrector lenses so I'm wondering if I need to add a lens of this type in my setup. FYI I have attached one of the pictures taken of the hydroelectric tower to give you an idea of what I mean by out of focus.

Yves

w7ay

YvesLeb Hi Yves, that does not look to me like an out of focus image, but looks more like weird de-Bayering happening (you said the camera is an OSC, right? but the image you posted looks monochromatic) and very blown up. You can almost see individual pixels. It may even be caused by a poorly collimated SCT.

If you are using an iPad, try to pinch the image to reduce the image scale so that the sensor fits the size of the ASIAIR Preview window. If you are using Android, I don't know if Android has also copied the pinching action in iOS, if not, check to find out how to reduce image scale on Android..

Next, check to make sure you are using Bin 1. This button is on the right side of the Preview window, just above the large round button. Larger bin factors will definitely give blurry images when it is scale up.

Further, go into the Advanced Settings of the Main Camera Settings and turn off Mono Bin if it is set. Turn on Auto White Balance in Advanced Settings to get a more natural color image.

To confirm that it is not a faulty camera, you can try using a program like ASIcap or SharpCap and view the image on your computer to see if it too looks weird.

The weirdness may be cause by a horribly decollimated SCT. When you look at a bright star that is defocused, do you see a dim donut shaped disk where the dark hole is perfectly centered in the disk? If not, you need to spend some time carefully collimating the SCT. The disk becomes dimmer and larger the more you defocus the SCT, but the dark hole (which also becomes larger) should remain perfectly centered.

Chen

YvesLeb

Hello Chen,

Thanks again for the quick reply and suggestions. FYI I'm viewing the images on my MacBook Pro using SharpCap 3.2 (Windows session with Parallel Virtual Machine). I understand what you mean about pinching the image. Is there a setting in SharpCap to scale the image down? It looks as if the image is scaled up and when I try to zoom smaller (ie. 25%) it doesn't seem to improve the image quality or the focus. I'm also using Bin 1 and also tried Mono Bin, as well as settings for auto white balance. I'll check the images again tomorrow during the day with ASICap and FireCapture to see if I can scale the images down and improve the quality. Regarding decollimated SCT, I'll try this when the conditions allow (it's currently cloudy).

Once again thanks for your time.

Yves

w7ay

YvesLeb If you are using SharpCap, you may have to specifically tell it to do the proper de-Bayering. I haven't used SharpCap ever since the ASIAIR release that supported Polar Alignment; I have not renewed my subscriotion (also on Mac OS X) I hate yhe subscription model and try never to support programs that use it (for the same reason, I have left PhotoShop for Affinity Photo).

If you use ASIcap, I thnk it should understand their own camera and use proper de-Bayering for their one shot camera.

Good Luck,
Chen

stuart8415

Hi have you tried using a Bahtinov mask? It is a diffraction grating which goes over the end of your telescope and splits the light up into beams which you then bring together by turning the focusing knob.I use one on my scope whilst using my ZWO camera.At first I couldnt see anything but if you turn up the exposure and gain on an object it will appear with its light split up into beams then it is easy to bring them together,There are plenty of videos on the net about how to go about getting focus with a mask.The masks cost about £20 or so they are just a piece of plastic with slots in - just tape it to the front of your scope with any dew shield removed at first.Once you have brought the beams together you can lower the gain and exposure again until the object you are viewing appears.Hope this helps 😃