help focusing an 8" dobs

[triton76]

Hello all.  New to the group and to the hobby.   I recently purchased an 8" skywatcher dobs.  I'm sure this has been covered countless time here before, but I am having focus issues.  I can focus crystal clear on the moon, but everything else is just a complete blur.  I have looked in the lense and see my eye reflection pretty well centered, turned the focus knob all the way both ways, focused no problem on objects during the day....  I've tried with both lenses with and without the extension, but no luck.  My beginner question is, am I missing something, or am I maybe just not finding anything in the sky that I can focus on?  In other words, have I only found stars and not a planet for example?

[bingevader]

Hello!

Where are you and how is your light pollution?

If you can focus on the moon then there shouldn't be a problem.

You should be able to focus on the stars then, they should resolve to nice pinpoints of light.

If it is a problem with locating objects, then what is it you have been trying to find?

I'm sure we can help you with that.

If you are finding and focussing on the moon, then we can also assume that your finder scope is well aligned too, but the more accurate it is the better.

What are you using to locate objects?

Have you downloaded Stellarium?

The globular cluster M13 in Hercules would be a good one to start with.

Can you focus on and split Albireo in Cygnus?

Another good place to start is the book Turn Left at Orion.

[Ricochet]

Which particular 8" dob do you have? The Skywatcher dob, for example, has separate 1.25" and 2" eyepiece clamps/extensions that you use one at a time, but that are stacked together when you receive the telescope. This catches out many, many new users who then can't achieve focus. This also makes sense with you being able to focus on terrestrial objects. Closer objects require the focuser to be extended further. If you have an extra extension in place then it makes sense that you can focus on things that should be too close for your telescope to focus on but not on things far away that it is designed for. Posting a photo of your focuser will allow us to see whether anything looks amiss in this department.

 Anher thing that sometimes confuses new users is what the focuser does.  Some people turn the focuser to make stars bigger, thinking that it will zoom in, but this doesn't work. All the focuser does is focus the image at the given magnification that your telescope-eyepiece combination provides. At all magnifications that you can use, stars will always be point sources. This means that you should always turn the focuser to make them look smaller. When you are way out of focus all you will see is one big doughnut of light. As you turn the focuser, this should become smaller and you will see a small doughnut for each star in the field of view. Keep going and they will become points (or as close to as your telescope/the atmosphere will allow). If the stars start becoming larger then you have gone past the point of best focus. If you reach the end of focuser travel without coming to focus then you know that you have the wrong number of extensions in the focuser. If you're winding it into make things smaller then you have too many, if you're winding it out you don't have enough.

Finally, you need to make sure that your finderscope is properly aligned with the telescope. During the day, point the telescope on the furthest object you can find, a distant treetop, church spire or TV ariel. Once you have found it in the main telescope, adjust the knobs on the finderscope so that it also points exactly at the object. At night you can then fine tune it on the moon, or a bright star (when you can find one).

[triton76]

I am in the northeast U.S., Connecticut to be exact.  Light pollution isn't bad.  At this point all I am trying to start with is finding planets.  When I try to focus on a star, it never gets to that pinpoint, its just a white blur in the center with some blurry points coming out of it.  I haven't downloaded stellarium, but will look into that this week.

 

I have the Skywatcher 8".  I have tried every combination of lens that it came with as well as trying them individually.  I've got my finder pretty well aligned during the day, but haven't fined tuned it at night.  If the skies clear up this week, I can do that, but weather has been tough this spring so far.

[Ben the Ignorant]

Looks like you're simply seeing air turbulence, it's always there to a changing degree (it's a weather pattern so it changes).

[pregulla]

If you are able to focus on the Moon than it's not a focusing issue. Sounds like either bad seeing, or you didn't let your dob enough time to cool down, or you observe over surfaces that radiate heat, like concrete or roof tops, or combination of these factors.

[rwilkey]

Hi Triton76 and welcome to SGL.  I think Ricochet has the answer in that you are using both 1.25" and 2" eyepiece extensions, this is the most common mistake made with these reflectors.  The way you describe the focus problem points to this, alternatively, if you could provide a photo of the focuser that would be helpful.  You should have only the 1.25" unit on the telescope.

[Geoff Barnes]

I think your 8" Dobsonian is performing perfectly well. My 12" scope shows all stars dimmer than say mag 2.0 as pinpoint, but the brightest stars such as Sirius, Canopus or Rigel all appear a bit flared or spiky, and I don't mean the diffraction spikes caused by the secondary vanes, they are just never pinpoint, it seems to be a feature of reflector scopes and particularly large aperture ones.

This may not be the case with top quality exotic mirrors, I've never looked through one.

Certainly if you can focus sharply on the moon then the stars will be in sharp focus at the same time without adjusting the focuser, all are in infinity as far as focus goes.

[triton76]

I’ll get a picture of the focused posted the next day or two. I’ll make sure I’m not doubling up on the extensions, I didn’t think I was but I’ve been known to be wrong more than my fair share of the time :/. We should have some good skies this week so hopefully I can get out and get it sorted. Thank you all for the help. 

[cletrac1922]

Which eyepiece you using

I use 1.25", and find 17mm to 25mm works perfectly with my 10" Dob

If you trying to use 5mm or 10mm, then could have focus issues

Most SW Dobs come with a 25mm and 10mm eyepieces

Perhaps rock up one night club close to you

Most members only too happy to help out

John

 

 

[Ben the Ignorant]

Since you like to see planets you need to observe when the air is calmer, so check meteoblue.com; you can customize the forecast for your city. On the left side of the page click on "Special", a menu will open with an "Astronomical seeing" option. Click it for a forecast of the air turbulence, I've found the main forecast and the seeing prediction are reliable, and my friends who do imaging rely on it, too.

Remembering Jupiter is around 40" across, when turbulence changes a star point into a 3" disk you know what to expect. Other weather services exist but meteoblue is the one I know, and it's good.