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Old 08-05-2019, 03:10 AM
Renato1 (Renato)
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Join Date: Mar 2014
Location: Frankston South
Posts: 1,120
Quote:
Originally Posted by bluesilver View Post
So from the sounds of things so far, as a few have advised me, my eye pieces i am trying to use for DSO's are not right, i need wider ones, around the 25 mm.
That helps explain a lot to start with.

The only issue with planets that are letting me down and this could just be my setup / collimation, i am using the laser collimation tool for this.
Adjust the secondary mirror until red dot is in centre of primary, then adjust primary until red dot disperses in middle of collamination tool, back to secondary and make sure it is in centre of primary.

When i view Jupiter, i can't quiet get a good clear image, slightly fuzzy, but can see the moons around it as stars.
With Saturn, it is a little fuzzy and although i can see the rings, i can't really see the individual rings.


Thanks.
Peter.
Hi Peter,
Sorry to come in at this late stage. The image you describe of Jupiter and Saturn is extremely poor.

Your telescope's specs says it is f/4.7.

Firstly, I disagree completely with the notion that for DSOs you need a 25mm eyepiece. That's okay for the bigger DSOs - where you want to get the whole object inside the field of view. Otherwise, for typical galaxies an eyepiece that delivers a 2mm exit pupil is best - which means the eyepiece closest to 4.7x2= 9.4mm. So, a 9mm or 10mm eyepiece would do well.

For the small class of DSOs which are faint open clusters, I personally find a 3mm exit pupil better = 4.7x3= 14.1mm eyepiece.

Secondly, with respect to collimation. If you aim at a bright star and throw it out of focus, you may be lucky and see a bulls-eye pattern. The bulls-eye pattern has to be concentric if your telescope is in collimation, and the telescope is pretty easy to collimate when the bulls-eye pattern is off centre/non-concentric.

But in my 14.5" dob f/4.5, the bulls-eye pattern does not come out. But it can be made to come out by making an aperture mask. Grab a piece of cardboard, or big piece of plastic (like from a large folder) and cut out an aperture mask to fit over your telescope. Draw a 10" circle on it, then draw a concentric circle the size of your secondary mirror on it. Then draw a circle whose diameter is between the 10" mirror and the secondary, and cut it out. Either affix the apreture mask to the front of the telescope with tape, or use lots of tape to make a mounting for it such that it slides over the front of the telescope tube.

Aim at a bright star, throw it out of focus, and you should have a nice bulls-eye pattern which, if skewed to one side or the other, you adjust the three knobs or screws behind the primary mirror to collimate, and get the bullseye pattern centered. The aperture mask will always be useful - on nights when the atmosphere is bad and the planets are not sharp, putting on the apeture mask will sharpen them dramatically (but you lose resolution).

Hopefully you have an annulus or at least a black dot stuck accurately to the middle of the primary mirror - without which your Cheshire eyepiece would be useless.

If your Cheshire says it is collimated, but you don't have a concentric bulls-eye pattern from the aperture mask - that suggests that the centre mark on your primary is in the wrong position.

From your description, your Cheshire has the cross hair on it as well. My one doesn't - instead it just relies on the dark spot of the secondary being perfectly concentric within the black annulus in the middle of my main mirror. Does that happen with your telescope?

Regards,
Renato

P.S. The poor images of Jupiter and Saturn you describe are also exactly the same as what I had the other night with my 5" Mak for an hour or so before it cooled down. Something similar also happened in my refractor when I was viewing Jupiter over my house with the heater going, though there one can see a blurry bit heading off to one direction. So make sure you have given your telescope a chance to cool down, and try avoid looking over your or neighbours' houses with heaters going (or air-conditioners going in summer).

Last edited by Renato1; 08-05-2019 at 11:25 AM.
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