Inward focus

[Adidaz 29]

Hi Iam having trouble with inward focus I have a 10" xti Dob with a crayford focursr that has fine focus to it I bought adapters but don't seem to get closer into focus it's ok with moon and larger objects but for stars Iam not getting that sharp image just big round dot I watched nunber of videos on utube but to no avail now is this my last option to bring my mirror forward or is there anything else I can do also tried a barlow that didn't work either but I didn't want to play around with the mirror but I bet that's my only option many thanks adidaz clear skies n pork pies

[Ricochet]

The points at which the moon and stars focus are extremely close, to the extent that many people do not find the need to refocus between looking at the stars and looking at the moon. If you can get the moon in focus then you will be able to get the stars in focus too. What adaptors have you added? Any adaptor would usually increase the length of the focuser, not decrease it. I think it will be best if you can take a photo of your focuser as you are using it and post it here. If you bought the telescope new then it is almost certain that the focus point will be within the travel range of the original focuser and so we may be able to see something wrong with it.

Also, what eyepieces are you using when you can't focus?

Edit: Just noticed this is in imaging, achieving focus is not necessarily possible out of the box with a dslr.

[Nigel G]

Hi.

Your scope should focus with no trouble.

Is it the original focuser ?

 If you can focus on the Moon sharp you should be able to focus stars too, is it with eyepieces or camera ?

Have you checked collimation ?

Nige.

[Knighty2112]

The same focal point you reach for the moon should be the same point to see the stars too. As your scope is a fast f4.7 scope then stars won’t be as sharp as any longer f ratios. Try a bathinov mask with your scope, which will make it easier to get a sharp focus when looking at stars. Any bright star is fine to set the focus. Is your collimating correct on the scope, as this could exacerbate the appearance of the stars too, so check that as well.

[Dave In Vermont]

BORG makes a variety of Helical-Focuser's that you can fit one end into the telescope draw-tube of the focuser, and your eyepiece is fitted (slid) into the BORG. With this then rotated (they're very smooth!), it's simple to 'dial-in' the pinpoint stars by turning the eyepiece. The one I have is only available as 1.25" - but hunt around. Start with 'BORG' Helical-Focuser in your favorite search-engine.

Con-Side: They are rather costly.

Dave

[Adidaz 29]

1 hour ago, Nigel G said:

Hi.

Your scope should focus with no trouble.

Is it the original focuser ?

 If you can focus on the Moon sharp you should be able to focus stars too, is it with eyepieces or camera ?

Have you checked collimation ?

Nige.

Colamation bang on

[Knighty2112]

So when you say that when you focus on the moon and it’s OK, how much room do you have left of inward focus left? Is it a very small amount, or more than 10mm of inward focus left still? 

[ronin]

It is not going to be inward focus or movement that is the cause.

In simple optics the difference between a star and the moon, assumes star = infinity and the moon - 384000 Km the "difference" between the star and the moon focal plane is

0.0000037mm, 3.7 millionths of a mm. How big is an atom ?

So travel is not going to be the cause. If you simply lightly touched on the focuser assembly it would move inwards many, many times more.

At f/4.7 you may need a coma corrector, and in simple truth the mirror is mass produced, the Orion will be a Synta/Skywatcher item. You are not going to get perfection. High(ish) quality mirrors are made mechanically as per Orion/Synta but then tested and then ground by hand noting at each stage where the profile has to be modified and improved, and they cost a lot more.

The mirror may not be one of the best on an Orion scope but it will be within specification, however in terms of focal plane position the focuser movement is not the cause/problem.

[Adidaz 29]

2 hours ago, ronin said:

It is not going to be inward focus or movement that is the cause.

In simple optics the difference between a star and the moon, assumes star = infinity and the moon - 384000 Km the "difference" between the star and the moon focal plane is

0.0000037mm, 3.7 millionths of a mm. How big is an atom ?

So travel is not going to be the cause. If you simply lightly touched on the focuser assembly it would move inwards many, many times more.

At f/4.7 you may need a coma corrector, and in simple truth the mirror is mass produced, the Orion will be a Synta/Skywatcher item. You are not going to get perfection. High(ish) quality mirrors are made mechanically as per Orion/Synta but then tested and then ground by hand noting at each stage where the profile has to be modified and improved, and they cost a lot more.

The mirror may not be one of the best on an Orion scope but it will be within specification, however in terms of focal plane position the focuser movement is not the cause/problem.

Cgreat explanation