Objects not in focus on the rim of the field of view and very shaky

Draco (Niv) · #1 03-07-2014, 09:10 AM

Hi All

I was out last night, and my oh my, it was such a beautiful night to be out and gazing at the heavens. Apart from the cold, everything else was amazing.

I have a Meade 10" ACF in Alt/Az. I was viewing Saturn last night, which had just passed Zenith and was moving towards the West. I noticed that when I had Saturn in the center of FOV, it was crisp and in focus however when I used the slew arrows to move to the moons, and incidently moved Saturn to the edges, Saturn became quite out of focus.. so much so that I was seeing doubles of it, though they were very close. I found this to be same for Mars as well.

Is this normal behaviour with Meade ACF? I was under the impression that ACF (Advanced Coma Free) would ensure the above didn't happen?

Also, while I was using the telescope, if I tried to focus the object, I found that the image would shake quite abit. It will settle a few seconds after but it was quite annoying since I didnt know which way to turn the focus knob since the shakes blurred the object. This was more evident in the 10mm eyepiece than the 30mm. Is this normal as well or is there something wrong with my telescope?

Thank you for your help and look forward to views from all.

Have a great day.

Regards
Niv

Now I have a question

astro744 · #2 03-07-2014, 09:55 AM

The eyepiece/telescope combination determine the flatness of the field and I would say your eyepiece is contributing some significant field curvature.

As far as vibration goes; yes you will notice it more at high power but I would have thought and expected your telescope to settle within 2-3 seconds preferably less than 2 and any longer is annoying. You can get anti-vibration pads for the tripod feet but having never used them I'm not sure of their effectiveness. Note any strong breeze and your telescope will oscillate.

mental4astro (Alexander) · #3 03-07-2014, 10:00 AM

Expanding on astro744...

What eyepiece were you using?

One thing that is next to never mentioned about eyepieces is that they DO NOT PERFORM THE SAME IN ALL TELESCOPES!

Telescopes produce a focused image that is not flat. The shape of this 'plane' depends entirely on the optical system. Newtonians produce a concave shaped focal plane. Maks & SCTs a convex shape. Refractors also convex. Then, add to the mix the focal ratio & the radius of the focal length of the scope (yes !), and the ultimate focal plane is one heck of a fruit salad!

Eyepieces too respond best to certain focal planes than others. Manufacturers won't mention this as it could mean to some people that their products are "faulty" when this is not the case.

If the scope & eyepiece are not an optical match, you can get a terrible image, or change the ease of use (less eye relief or more critical eye placement), or see other aberrations such as chromatic aberration or astigmatism, even field flatness.

Even within a model range of eyepieces performance can vary greatly between scope designs.

Big brands are also no garrantee of optical match.

Best thing is to give some examples: Baader Hyperion eyepieces are excellent in SCTs, but very poor in Newtonians. Nothing wrong with Hyperions - I love them in my C8, but no good in my Newts. Only the 5mm Hyperion is good in Newts & SCTs, which is why the 5mm is the only Hyperion in my kit. If you only have an SCT, all the Hyperions will be great.

Pentax XW eyepieces. Many people believe these are the best eyepieces in the world. They are very bloody good. BUT, depends on the scope you use them in. The 14mm is excellent in Maks, but requires a coma corrector to improve them significantly in Newts. The 10mm is exquisite in Newts, but in an SCT, eye placement is really, really fine, which is ok if you are just observing, but for me with my sketching & constant back and forth to the eyepiece, it is a significant drawback.

AFC does not change the shape of the focal plane. The image you see is still due to the optical mismatch between the scope and eyepiece you are using.

Mental.

Eden (Brett) · #4 03-07-2014, 10:22 AM

Hi Niv,

Vibrations or movement at the scope or around the pier will definitely be more noticeable at longer focal lengths. Were you using a diagonal or was the eyepiece connected directly to the scope?

Draco (Niv) · #5 03-07-2014, 10:26 AM

Thanks Astro744 and Alexander

I am using the eyepiece that came with the telescope, it says Meade Super Plossl on the eyepiece however from the bintel website it says it is a series 4000 (cant see this written anywhere in the eyepiece).
I also used a 10mm eyepiece. This has no name on it and was gifted to me by a generous and warm hearted IIS member

Astro744, the shakes do last for around 4seconds or less. The annoying part is that it just takes too long to focus since I cant fine focus while the shakes are happening

wish I could focus via the keypad as well. Also, if the handset cable hits the scope, it vibrates/shakes it as well

Draco (Niv) · #6 03-07-2014, 10:28 AM

Hi Eden

I am using a diagonal.

Quote:

Originally Posted by Eden

Hi Niv,

Vibrations or movement at the scope or around the pier will definitely be more noticeable at longer focal lengths. Were you using a diagonal or was the eyepiece connected directly to the scope?

Draco (Niv) · #7 03-07-2014, 10:29 AM

Hi mental

I will try borrowing some eyepieces from the club members at the next gathering to see if I can get a better image.

Thanks for all the info.

Quote:

Originally Posted by mental4astro

Expanding on astro744...

What eyepiece were you using?

One thing that is next to never mentioned about eyepieces is that they DO NOT PERFORM THE SAME IN ALL TELESCOPES!

Telescopes produce a focused image that is not flat. The shape of this 'plane' depends entirely on the optical system. Newtonians produce a concave shaped focal plane. Maks & SCTs a convex shape. Refractors also convex. Then, add to the mix the focal ratio & the radius of the focal length of the scope (yes !), and the ultimate focal plane is one heck of a fruit salad!

Eyepieces too respond best to certain focal planes than others. Manufacturers won't mention this as it could mean to some people that their products are "faulty" when this is not the case.

If the scope & eyepiece are not an optical match, you can get a terrible image, or change the ease of use (less eye relief or more critical eye placement), or see other aberrations such as chromatic aberration or astigmatism, even field flatness.

Even within a model range of eyepieces performance can vary greatly between scope designs.

Big brands are also no garrantee of optical match.

Best thing is to give some examples: Baader Hyperion eyepieces are excellent in SCTs, but very poor in Newtonians. Nothing wrong with Hyperions - I love them in my C8, but no good in my Newts. Only the 5mm Hyperion is good in Newts & SCTs, which is why the 5mm is the only Hyperion in my kit. If you only have an SCT, all the Hyperions will be great.

Pentax XW eyepieces. Many people believe these are the best eyepieces in the world. They are very bloody good. BUT, depends on the scope you use them in. The 14mm is excellent in Maks, but requires a coma corrector to improve them significantly in Newts. The 10mm is exquisite in Newts, but in an SCT, eye placement is really, really fine, which is ok if you are just observing, but for me with my sketching & constant back and forth to the eyepiece, it is a significant drawback.

AFC does not change the shape of the focal plane. The image you see is still due to the optical mismatch between the scope and eyepiece you are using.

Mental.

Eden (Brett) · #8 03-07-2014, 10:32 AM

Quote:

Saturn became quite out of focus.. so much so that I was seeing doubles of it, though they were very close. I found this to be same for Mars as well.

I had this problem recently trying to do planetary work. If the diagonal is off-axis, you end up seeing doubles.

Draco (Niv) · #9 03-07-2014, 10:53 AM

Hi Eden

The interesting thing is that the image is crisp in the center or round abouts the center of FOV. However as I moved Saturn to the edges, it just started distorting. I will try to put the eyepiece in without the diagonal and see if it makes a difference.

Thanks mate.

Quote:

Originally Posted by Eden

I had this problem recently trying to do planetary work. If the diagonal is off-axis, you end up seeing doubles.

AstralTraveller (David) · #10 03-07-2014, 12:06 PM

Quote:

Originally Posted by Draco

Also, while I was using the telescope, if I tried to focus the object, I found that the image would shake quite abit. It will settle a few seconds after but it was quite annoying since I didnt know which way to turn the focus knob since the shakes blurred the object. This was more evident in the 10mm eyepiece than the 30mm. Is this normal as well or is there something wrong with my telescope?

This why motorised focussers were invented. My Moonlite converted my 15cm f/8 refractor from frustration to bliss. I've gained at least 0.5 mag just by being able to get the focus spot on. The dobs is still manual focus because they are more stable but even then you need a very soft touch to keep the vibrations acceptable.

Draco (Niv) · #11 03-07-2014, 04:41 PM

Hi David

Sorry to play devils advocate but wont the motion of the motor cause vibrations that will shake the telescope?

Seems like a great solution though. Haven't used it first hand so don't know much

Quote:

Originally Posted by AstralTraveller

This why motorised focussers were invented. My Moonlite converted my 15cm f/8 refractor from frustration to bliss. I've gained at least 0.5 mag just by being able to get the focus spot on. The dobs is still manual focus because they are more stable but even then you need a very soft touch to keep the vibrations acceptable.