Capturing the Moon - 1st Stage of Astrophotography

[Dale Hawkes]

Hello,

I have a SkyMax 150 and ZWO 120 MC-S Colour Planetary Camera.

https://www.firstlightoptics.com/telescopes-in-stock/skywatcher-skymax-150-pro.html 
https://www.firstlightoptics.com/zwo-cameras/zwo-asi120mc-s-usb-3-colour-camera.html 

Setup 1
Took off the 90 diagonal and put the camera into a 1.25" to 2" adapter and put it on the tube on the back of the scope.  Pointed it to the moon, then used FireCapture, however I could only get a very white bright image. 

Setup 2
Put the 90 diagonal back on. Used the 1.25" to 2" to hold the camera into the 90 diagonal. Still no better image, however this time I got a white or black image. 

One of our members at Astro club asked me about Back-Focus. I have no idea on this, nor can I find a value for this. Any help would be appreciated, I am very new to this. Thank You 

[Earl]

You might need to add an extension tube to achive focus.

[ONIKKINEN]

If you see only white and no detail you need to lower your exposure time. The Moon is very bright so youll use something like 5ms for capture.

[Dale Hawkes]

I moved the exposure time right down to 1ms. 

Earl - When you "You might need to add an extension tube to achieve focus." - what do you mean? Can you show me a link please? 

[Cornelius Varley]

1 hour ago, Earl said:

You might need to add an extension tube to achive focus.

Probably not with a mak.

[Elp]

A few things here:

1. Your exp time is too high, dial it way back as mentioned.

2. You may not have the camera sensor at the right focal point, backspacing relates to getting the sensor at the point of focus and usually applies when using a corrector lens prior such as a flattener, reducer or coma corrector. As I assume you may not be using any, the sensor therefore is not at the point of focus. You can test this very roughly by getting an eyepiece in a diagonal into focus, then swapping to the camera and adjusting focus nearby, itll need to be adjusted because an eyepiece will contribute to the focus point whereas a camera is relying on the focus point of the telescope only. But, if point (1) is not right, you'll never see the focus point.

3. Your telescope is a long focal length, so with point (1) all you'll see is a bright light, even if pointed somewhere off target but near the moon. The brightening is worse the longer the exposure time and your scope can be further away from target but youll still see a brightening on the camera.

Potential solutions:

A. Dial back the exposure to less than a second, not necessarily fractional ms or uS (micro seconds) at this point, this is what I do. Make sure the target is a white disc. Get to near your focal point, when you look on camera the bright disc will get smaller as you're approaching focus and larger when away. Then move the scope off target a little and repeat. The disc should get smaller and smaller, if you're near focus and your scope it pointed at the edge of the moon you'll be able to focus to a near sharp circle edge, this will get you extremely close to the focus point.

B. Then dial back the exposure significantly to ms or uS exposure (most likely on the moon you'll use ms, uS is usually useful when doing solar). The bright disc will retreat to the moon colour, maybe a dim grey if the exposure it too short. The surface detail will show. Then very minutely adjust focus until areas of contrast are sharp as you can see, you'll have to work with the atmospheric seeing here and make a focus adjustment then assess when seeing "freezes" fractionally time to time. You'll have to to and fro a little here. The exposure should be such that your histogram peak isn't crushed to the left (you're "destroying" the darkest black/shadows areas merging them into one uniform blackest black, also known as clipping) and the whites/highlights also the same but on the RHS of the histogram.

C. Try the camera with diagonal and without (straight through). Note the latter you may need spacer/extension/backfocus rings because a diagonal takes up that distance from the back of the scope up to the eyepiece and you have to compensate for that sometimes, or rack the focus on the scope further than it would be with a diagonal+eyepiece. You might find without the extensions you cant get focus when imaging straight through.

D. Basic but be absolute sure your finder is aligned with the centre axis of your telescope, the finder will make it easier to ensure your camera is centred or slightly off target. With experience you can usually do without a finder and just use a deft touch with sweeping with your scope and using rough eye "finding". A larger camera sensor makes this bit easier, as well as not having to nudge the scope so much if you're not using a goto tracking mount to keep the target centered on camera.

 

[Dale Hawkes]

Thank you Elp. I will follow what you have suggested tonight if clear. 

Many thanks to everyone who has commented 

[bosun21]

With the 150 Maksutov you shouldn't need any extension pieces to achieve focus due to the large focus travel of the telescope. When setting up for the moon I set the exposure to 15-20ms which is still too much for the capture process itself but is fine for locating and centering the moon on the laptop screen. Once centered I reduce the exposure to 4-8ms depending on how bright and large the moon is. You can also change the gain setting along with the exposure so that you get a good histogram reading and a clear image on screen. This is a good time to focus on the surface details of the moon as good focus is crucial for a sharp image.Once this is achieved you can then start the capture process.