Expected image clarity question

[Coconut]

First post here, so this may be obvious....

After murmerings of an interest in astronomy, my wife bought me an eight inch Sky-Watcher dob. After various disagreements as to where I'd be allowed to keep it (she wasn't expecting it to be that big) I've finally been out in the back garden looking into the night sky.

Anyways, I reckon I'm lucky timing-wise as there are plenty of planets to look at just now, which are fairly easy targets.

The 'scope has a focal length of 1200mm and came with 10mm and 25mm eye-pieces. From reading this forum, I understand that this should give me magnifications of x120 and x48 respectively.

I've used the 10mm to view the current planets in view and I can obtain a nice crisp view at that magnification. I did however want to enlarge the image and invested in a Celestron Omni 2x Barlow which I then proceeded to use with the 10mm. I think this should provide x240 magnification (I could be wrong).

The image using the 10mm and Barlow is slightly larger, but I can never recreate the crispness of using the 10mm alone - instead there is always a 'fuzziness' about it. I had read somewhere that my 'scope should provide a maximum useful magnification of x400, but I can't possibly imagine ever getting that far if x240 is already becoming less defined.

Of course, it is very possible that I am doing something wrong or that my expectations of greater magnification are misplaced, but I'd appreciate any wisdom anyone may have to offer on the topic.

[LukeSkywatcher]

Hi Coconut.

It sounds as if you are doing all the right things. Realistically in the UK you will never get 400X out of a scope. You can almost imediately cut that figure in half. On great nights of great "seeing", you can push the scope a bit more.

The reason you are not getting a crisp view with 10mm+2x Barlow could be for a number of reasons:

1/ Bad "seeing" conditions.

2/ The 2x Barlow could be a dud.

3/ Too much magnification under bad "seeing".

4/ A combination of all three.

To obtain the magnification of any EP when used with your scope the formula is:

Focal length of scope in mm divided by the size of the EP in mm....................so:

1200mm/10mm=120X

1200mm/25mm=48X

So you are correct on your maths. If you then add a 2x barlow you increase the magnificaion by a factor of 2....................giving 240X and 96X.

240X is doable but really needs idea "seeing" conditions.

So it seems that the problem is with either "seeing" or the barlow. If you can see the planets in a nice crisp way without the barlow then the "seeing" isnt really the main problem. This leaves the barlow as being the problem. Barlows (not all) are known to degrade image quality. Some makes are better then others. The TAL barlows are simply fantastic in both the views they offer and the price.

Most people (including myself) use an 8-9mm EP for observing planets. I have pretty much given up on barlows now and prefer just to use EP's.

Hope this helps in some way (and i hope i am correct in my ramblings).

Paul.

p.s.~~~if you are looking to get yourself some better EP's then i can personally say that the Vixen NPL range are brilliant. They offer stunning images and are very very well priced (about £40). I use them in my 70mm,130mm and 200mm scopes and they work great in all scopes.

[brantuk]

You're doing nothing wrong - the seeing has been naff recently. Being Earth bound we have to look through a layer of atmosphere before we get out into the void that is space. If you magnify an object you will also magnify atmospheric imperfections.

High and thin cloud is a killer and will make any view grainy at high mag. Hot air rising will make the view appear to boil. Moisture content of the air will also affect what you see. It's a delicate balance bringing it all together in a session.

200x is a practical limit in UK seeing - with maybe 180x average - and 250x plus very infrequent. The crisp clear air of winter is best, high objects are better cos you look through less atmosphere, well cooled tubes help, clear and dew free optics naturally, better quality eyepieces help a lot, and choosing the right objects and magnifications is something that needs to be learned. HTH

[Moonshane]

good advice above and to reiterate one thing Kim (Brantuk) says, it is vital to allow the scope to cool adequately for perhaps an hour before using higher magnifications and preferably before using the scope at all.

[LukeSkywatcher]

good advice above and to reiterate one thing Kim (Brantuk) says, it is vital to allow the scope to cool adequately for perhaps an hour before using higher magnifications and preferably before using the scope at all.

+1 for that.

I didnt think to mention it. An hour should be ok, but as long as you can....................the views are greatly improved.

[Coconut]

Many thanks for taking the time to provide all the advice. Good to know I'm on the right tracks!