10" Dobsonian

[Maged]

Please take a look at an image took through the scope. please note the terminator were the distortion is most visible.

 

I wanted to know is this only a collimation issue? 

[NGC 1502]

Really tough to say where your problem is.   How did you take the lunar pics ?  If you held a phone/camera up to the eyepiece then if it was not square to the EP, that could be the issue, as could if the moon was away from the centre of the field of view, or maybe you used another method ?

Collimation could be the problem, a pic of an out of focus bright star and details of how you took it would help a lot.

What you said in post # 25 could be astigmatism ( cross shaped ) but hard to be certain, try a higher power, could be astigmatism in your eye rather than the optics.  Higher power helps astigmatism in your eye because the smaller exit pupil only uses the centre of your pupil. I have a similar problem, stars are sharper at higher power.

Hope you sort it, Ed.

[Adrian Condon]

Hi,

Welcome to the DOB Mob!

Some interesting and helpful comments above.

However, what is the make of the telescope?

Secondly, what is the focal length?

and thirdly, what make of eyepiece (s) have you tried.

I have an Orion Optics uk 10", 1600mm focal length, f6.3 1/10 wave dob, I use a Vixen Lanthanum 12mm eyepiece and view the terminator on the Moon. The craters usingthis combo are extremely sharp with excellent contrast.

Is it possible to get a bright star, Vega or Capella, get them into the best focus you can, then de-focus slightly and photo the results. That hopefully should show the collimated status of the scope.

I found that when using my Televue 55mm Plossl that I need an extension on the eyepiece to bring it into crisp focus.

Once you have resolved the focus issue, you will be satisfied with your purchase.

Cheers

Adrian

[Maged]

Hi,

Welcome to the DOB Mob!

Some interesting and helpful comments above.

However, what is the make of the telescope?

Secondly, what is the focal length?

and thirdly, what make of eyepiece (s) have you tried.

I have an Orion Optics uk 10", 1600mm focal length, f6.3 1/10 wave dob, I use a Vixen Lanthanum 12mm eyepiece and view the terminator on the Moon. The craters usingthis combo are extremely sharp with excellent contrast.

Is it possible to get a bright star, Vega or Capella, get them into the best focus you can, then de-focus slightly and photo the results. That hopefully should show the collimated status of the scope.

I found that when using my Televue 55mm Plossl that I need an extension on the eyepiece to bring it into crisp focus.

Once you have resolved the focus issue, you will be satisfied with your purchase.

Cheers

Adrian

it's a hand made telescope, including the mirrors. Focal length is 1,500 mm, F5.6.

I tried 17mm orion sirius plossl  which is not that much magnification but I still see distortions. I have other EPs with similar medium quality like 6mm, 25mm, 32mm.

I have an extension but I don't need them with my eyepieces.

I will image a de-focused star and post it

thanks

[Maged]

I got the Cheshire and I aligned my scope. So now all circles are concentric and the primary mirror center point is centered in the Cheshire collimation cap. But still the image I see is distorted!

For example when I look at craters on the moon I see the crater and another image of that crater slightly above it so the image is not sharp. I'm not using high magnification by the way just about 80 X.

Any comments or advice? I'm really getting frustrated.

[niallk]

If you have defocussed a star at reasonably high magnification and can take a pic, that could help rule in/out a few things.

Did you get a chance to check the mirror clips aren't touching the mirror surface, and perhaps check the lock bolts, if there are any, beside the collimation knobs are not tight (eg loosen them completely)?

How is the secondary mirror attached to the spider? And what sort of primary mirror cell does it use - you said it was hand made - including the primary mirror?

Best of luck, -Niall.

[jetstream]

Is there a chance there are a few different things happening?

The Newt will show diffraction spikes around bright stars, hopefully you only see 4 in an "X" shape. For reasons that don't matter (to me) some eyepieces will ghost with some scopes, usually on very bright targets, like Jupiter or the moon- sometimes a persons eyeball reflects light back to cause the ghost...

If your scope is not well collimated it may show coma and the corresponding coma smear to a much higher degree than normal, making objects look soft. The coma free area of your f5.6 is really not that large- with my f4.8 Newt, well collimated, cooled and with reasonably good eyepieces, it will lose sharpness fairly fast the farther from the center of view I go. Certain eyepieces will also go soft near the edges all on there own....

I must add, a good newt, properly collimated and cooled will show RAZOR sharp images in its coma free area. They sure like having the primary mirror cradled loosely with no pressure on it....

Good luck Maged

[Maged]

If you have defocussed a star at reasonably high magnification and can take a pic, that could help rule in/out a few things.

Did you get a chance to check the mirror clips aren't touching the mirror surface, and perhaps check the lock bolts, if there are any, beside the collimation knobs are not tight (eg loosen them completely)?

How is the secondary mirror attached to the spider? And what sort of primary mirror cell does it use - you said it was hand made - including the primary mirror?

Best of luck, -Niall.

I will try to take an image of a high magnification , but it was cloudy past few days.

Yes it is hand made including the primary mirror.

there is no spider, the secondary mirror is attached through an optical window same as in schmidt cassegrain telescopes

[Maged]

Dear all,

 

I have imaged vega out of focus. please let me know your comment, and if you know what kind of distortion I am getting.

 

By the way the scope was collimated using the Cheshire and everything was in alignment, as far as I can tell.

 

Also an image of vega through the eyepiece. note that it is not a point, the distortion is so apparent !