Lunar image sharpness

[jACK101]

I took this shot last night, conditions were excellent. In the area of the terminator, where I was focussing, the picture is sharp, but sharpness falls off to the RHS and lower part of the picture.

 

I don't believe that this can be due to a depth of field issue. At the lunar distances involved I would expect everything to be in focus i.e. infinity.

 

Equally, I don't think it can be due to camera shake because parts of it are sharp.

 

The only reason I can think of is the T Mount I am using, which is a cheapo from China. There is a certain amount of "play" when the tube is inserted into the eyepiece holder. This is perhaps exacerbated by the grub screws which clamp from the side. The combined effect of these may well be to skew the camera so that the sensor is no longer at right angles to the light coming into the camera. This, I think, could explain the symptoms.

 

My questions are ,  Is this a reasonable assumption? and  can I improve this by buying a more expensive adapter?

 

If so what would you recommend.

 

Thank you

 

 

Jack

[ecuador]

I can't really tell from a single lunar shot whether you have an issue or not. Especially when the part you refer to is the side of the moon that has no shadows.

In any case it is much easier to tell if you have an issue with tilt if you take a starfield image. But I haven't heard of tilt introduced by a bad t-ring - at most they allow a little rotational play. It is usually the way the focuser holds the t-adapter, especially if the t-adapter barrell has grooves.

[Knight of Clear Skies]

What scope and camera are you using please? You may need a flattener to get a sharp image to the edge of frame. I had a similar problem when I forgot to fit one when shooting through an ED120.

[jACK101]

38 minutes ago, Knight of Clear Skies said:

What scope and camera are you using please? You may need a flattener to get a sharp image to the edge of frame. I had a similar problem when I forgot to fit one when shooting through an ED120.

I am using a Celestron CPC 925 AND A Nikon D3200

 

Jack

[SilverAstro]

Nice image , always something to delight along the terminator, endless photo ops

You are right about DofF not being a prob. The lack of contrast in the fully illuminated bits is always a prob.

I have done bit of crop&paste&200%  to do a side-by-side comp. Top, middle and bottom-right of the edge. Looking similar ?

 

[cjdawson]

looking at those blow ups, it does look to be that the focus is ever so slightly off.  It's close, very close.  I suspect that the cause might be atmospheric,  that is turbulance caused by the seeing.

With a single image, this is always a possibility.  This is why it's worth while taking a series of images - say 2000, then stacking the best 100 of them, throwing the rest away.

 

Also, Depth of field is very much an issue on the moon.  Whilst it's 250,000 miles away, it's spherical (ish) an so the limb is further away than the terminator, The difference is 1000 miles. Again, this won't shift the focal point by much, but it will shift it a little. That little may be enough to register on a camera, so I'd not discount it completely.

[SilverAstro]

11 hours ago, cjdawson said:

Also, Depth of field is very much an issue on the moon.  Whilst it's 250,000 miles away, it's spherical (ish) an so the limb is further away than the terminator, The difference is 1000 miles. Again, this won't shift the focal point by much, not kidding!

 with this equipment and this image,  insignificant , but this is not the place to discuss sensor ability to distinguish image circles of confusion, hyperfocal distances etc&etal

but I think JACK101 may have gone to bed   

[SilverAstro]

On 06/02/2017 at 15:11, cjdawson said:

looking at those blow ups, it does look to be that the focus is ever so slightly off. can you point to those examples that make you think so (annotate the pic ? )

 

Yes ! So, as you say, lots of  lucky exposures thro' the seeing, but let's not conflate that / too many issues into one topic

 

[jACK101]

8 hours ago, SilverAstro said:

Yes ! So, as you say, lotsof  lucky exsposures thro' the seeing, but let's not conflate that / too many issues into one topic

 

i'm not sure what points you are making. I have been an amateur photographer for about 60 years and I understand hyperfocal distance etc from a terrestrial photography standpoint.  I am however very new to astronomy. I don't know if these relationships hold when the lens has a focal length of 2700mm+.

I still think that any play in the tube holding the camera is important. The slightest movement here is likely to move the sensor sufficiently out of truth for focus problems to occur.

 

I'm awake now.

 

Jack

 

Jack

[Davey-T]

Not sure that it's confusing the issue suggesting stacking multiple images, it's the best way to capture as much detail as possible on solar system objects which are greatly affected by the atmospheric seeing.

Focussing is critical with SCTs, I usually focus on a reasonably bright star to start with as it's easier to tell if it's focussed, a decent replacement after market focuser also helps a lot.

Dave

[SilverAstro]

5 hours ago, jACK101 said:

i'm not sure what points you are making. 

the hyperfocal /DofF was a point to cjd, who was under the impression that we all thought the moon was flat ! Dont worry about it

The point I was originally  making  trying to make was that I did not see, to my eyes !,  any significant focus variation across your image in the wee crops I selected and that it may be the lack of contrast in the high illumination parts that was making it look out of focus.  So I put them up to see if you or others would agree.

[jACK101]

2 hours ago, SilverAstro said:

the hyperfocal /DofF was a point to cjd, who was under the impression that we all thought the moon was flat ! Dont worry about it

The point I was originally  making  trying to make was that I did not see, to my eyes !,  any significant focus variation across your image in the wee crops I selected and that it may be the lack of contrast in the high illumination parts that was making it look out of focus.  So I put them up to see if you or others would agree.

Yes, I agree with you. This is only my second attempt at astro photography, the first attempt was very similar. Perhaps I need to avoid taking shots of the moon when there are such large expanses of highly illuminated area.

 

Thanks for your comments.

 

Jack

[SilverAstro]

14 minutes ago, jACK101 said:

 This is only my second attempt at astro photography,

such large expanses of highly illuminated area.

And an excellent 2nd it is too  

No dont avoid the moon, just avoid the highly illuminated bits the terminator is where all the action is !

(  Sorry I cant help with your C925 mechanical questions, someone else may be along in a min. Nice scope though, I have been thinking of the similar 9.25 Evolution version, not sure if I (my back) could handle it and not got a showroom near to see one   )

 

 

[jACK101]

2 hours ago, SilverAstro said:

And an excellent 2nd it is too  

No dont avoid the moon, just avoid the highly illuminated bits the terminator is where all the action is !

(  Sorry I cant help with your C925 mechanical questions, someone else may be along in a min. Nice scope though, I have been thinking of the similar 9.25 Evolution version, not sure if I (my back) could handle it and not got a showroom near to see one   )

 

 

Thanks for your comment. Although my telescope experience is very limited I have no regrets about buying the CPC925. It is heavy and I can't manage to handle it on my own. I have however overcome that by building a trolley which allows me to move it within the house and I have rigged up a block and tackle which allows me to easily get it onto the tripod outside. I am however planning to build a small observatory with a pier and this will avoid all manual handling.