Issues with focus...

[siriuslyblack]

Hi all! Newbie here ✌️

 

I recently bought a Celestron Astromaster 70. It came with a 20mm and 10mm eyepiece. Got it all set up, could see the moon really well, couldn't see Jupiter clearly but could see it and it's moons but not in detail. Could just see Saturn and a glimpse of it's rings. I went back to the store today and got an eyepiece bundle and no matter what I do, I can't get it to focus when I'm using the 2x power lense and the 6mm eyepiece.

I took it all apart and back together several times, my husband did too and we managed to lock onto Jupiter again, but once we add the 2x power lense it's just a massive white blur and it won't focus at all.

I'm feeling a bit disappointed because all the research I did says all the pieces I've got give really great views of Jupiter and Saturn (so much so that you can see the shadow of saturns rings on Saturn itself) but the quality is nothing like the instruction manuals and books say.

The focuser moves all the way in and out without an issue - all the way out is blurry, kind of sharpens a bit halfway in but then goes blurry again when it's all the way in.

Everything is brand new, lenses are clean - I just feel like there is something not right!

I haven't been able to test the 2x power lense or 6mm eyepiece on the moon as it's current orbit doesn't place the moon in a visible spot for me until about 2am. But when I first got the scope could see things great and really clear.

Any advice would be greatly appreciated! 

[Cornelius Varley]

Hello and welcome to SGL. The Astromaster 70 refractor has a focal length of 900mm and an aperture of 70mm. This means that the maximum practical magnification for this telescope is about 140x (2x per millimetre aperture) . With a 6mm eyepiece this gives a magnification of 150x, add a 2x barlow as well and the magnification is now 300x, which is beyond the practical limits of the telescope.

[Stargazer33]

Hi, here is a simulation of what you can expect to see of Saturn with your scope and a 6mm ep (as Peter says 150x magnification, with your scope having a maximum practical mag of 140x).

[Stargazer33]

P.S.

As magnification increases the range of outside focus - FOCUS - inside focus gets smaller and smaller until just a 1/4 to 1/8 of a turn, or less, on the focus wheel  goes from being outside focus through focus to inside focus and vice versa.

Also, as you increase magnification, the seeing conditions effect the view more and more. If for example you look at the Moon with a 32mm eyepiece and the view is wobbling a bit and you swap to your 6mm ep then the view would be like trying to see a mosaic on the bottom of a swimming pool. Add in a x2 Barlow and it would be impossible to see anything useful.

Under very good or ideal seeing conditions we all like to push the magnification, especially those who do high frame rate imaging. These cameras can take upto a couple of hundred frames per second. They can therefore capture the fleeting moments of good seeing. These moments captured over a couple of minutes can then be stacked in software to form some of the images you see on this forum and unfortunately on various advertisements for astronomical equipment; especially from manufacturers with lower scruples.

[Mr niall]

What you are seeing is about right. The biggest challenge preventing you from getting clear views of planets is atmospheric turbulence, which causes planets to wobble and smudge in and out of focus no matter how big your scope. 

The planets are really poorly placed at the moment (quite low) and this exacerbates the issue. General Consensus is that 250x is the reasonable max on a big aperture. I used to have the same scope as you and I found 75 to 80x gave the best views. After that the view starts to degrade. 

The best thing to do is to focus on Jupiter's moons not Jupiter itself. When these are tack sharp then Jupiter is in focus too. You should be able to make out at least two bands. You should be able to see the gap between saturn and its rings. 

This photo is pretty close to Jupiter on a really good night

Edit: I was always told about 35x per inch of aperture max on planets and that puts you in the same place as mentioned above. 

Edited August 24, 2019 by Mr niall

[siriuslyblack]

Thank you all so much for your comments! I ended up taking everything back to the store and the manager told me I was sold the wrong eyepieces for my telescope, so they upgraded me to the AstroMaster 130!

Got it all set up now and keen to try it once this torrential rain stops and we have a clear night ....

[Geoff Barnes]

Hi siriuslyblack and welcome from me in Melbourne.

As you are also in Oz we currently have Jupiter and Saturn visible at fantastic high altitudes in the sky which means they should look really sharp and detailed on nights of good steady conditions.

I have a big 12 inch Dobsonian and regularly observe both planets at 300x and more on good nights, which gives amazing views.

Even with your new 130mm scope I would expect you to be able ro use 250x on good nights on Saturn, and 200x on Jupiter.

Good luck!

[siriuslyblack]

3 hours ago, Geoff Barnes said:

Hi siriuslyblack and welcome from me in Melbourne.

As you are also in Oz we currently have Jupiter and Saturn visible at fantastic high altitudes in the sky which means they should look really sharp and detailed on nights of good steady conditions.

I have a big 12 inch Dobsonian and regularly observe both planets at 300x and more on good nights, which gives amazing views.

Even with your new 130mm scope I would expect you to be able ro use 250x on good nights on Saturn, and 200x on Jupiter.

Good luck!

Thank you so much! We are on the Central Coast of NSW and have SO much rain right now, not sure when our next clear night will be but I hope it's soon!

[Geoff Barnes]

Sunday on into next week looks great for clear skies for you.

Weve got a rare clear night here so Im going out shortly to look at Jupiter and Saturn.