New and looking for help!

[astoc]

Hi guys,

Recently myself and my girlfriend were very kindly donated a 70mm f/900mm Helios refractor. Having read astronomy magazines as a kid but without the opportunity to use a real scope I was eager to give this one a go.

It came with three eyepieces. A 20mm and an 8mm, which I assume came with the scope, and a celestron 40mm plossl.

So far i've only been able to use it from the balcony of our flat, which sadly (in stargazing terms) is located in the city centre. We face north-east, with clear views left and right. We're on the 4th floor so have a pretty good view towards the horizon.

Despite the floodlights of an entire city i've so far managed to see praesepe in the plossl (though not many of the stars were visible), and saturn in the 8mm. I've also seen mars a few times in the 8mm but haven't been able to distinguish surface detail. Saturn was by far the most impressive. I was truly surprised when I saw both its rings and titan right there in the eyepiece.

Having consulted stellarium it would seem the NE sky is pretty boring at the moment, but I have been eager to try and spot M81 and M82. Despite my best efforts, I simply cannot locate these objects and would appreciate any help anyone can give. Are there any particularly identifiable stars that I can use to star hop to them from ursa major?

I am also looking for advice on the equipment itself. The celestron eyepiece clearly gives a marked improvement in visibility over the other two, and i'd like to invest in another eyepiece for this scope if it is worth it. An online calculator tells me the max usable magnification for this scope is 150x. At this power though, would the image be so dim as to be almost invisible to me under this light pollution?

Many thanks, I'll leave it there for now i think. lol.

[John]

Hi, welcome to SGL.

Given your viewing location you will find it most rewarding to concentrate on planets, the moon, double stars and the brighter open clusters. Galaxies in particular are adversely affected by light pollution. Under dark skies M81 and M82 should be readily visible with your scope (I first observed them with a 60mm scope) but the light pollution will make finding them a real challenge. This is the way I find them (using a red dot finder and low power eyepiece):

http://www.synapticsystems.com/sky/astroimg/bigdipm81m82.gif

They are not as bright as this picture suggests though - they are magnitude 7 and 9 respectively. You should be able to fit them both into the field of view of your lowest power eyepiece.

[astoc]

Thanks for the reply, I tried one last time tonight to find M81 and M82 to no avail. I think i'll save those until I get a chance to take the scope into some darker areas.

I'll concentrate on clusters for now as you suggest, and binary/double stars. I'll have to have a look at what binary/double star systems are on show. The moon isn't in our view this time of the month, but i'm looking forward to taking a look at it soon.

[Capricorn]

You will not get 150x out of the scope.

You may get 70x, and if the optics are good, the EP good and the conditions good you may get 100x.

All of which will be too much for dim DSO's.

For those I would aim at 30x and 50x magnification. They may show up as a small object then, and you should have a reasonable field of view from the scope to get the right area of sky.

[astoc]

When you say I'll likely not get above 100x, could you elaborate a little? I presume what you mean is that magnifications around and above this value will be unusable due to the low level of light coming into the 70mm objective. I've certainly had 112.5x while observing Saturn (unless I'm working it out wrong), but the image was substantially dimmed and lacking in feature (other then the rings of course - which were awesome). Of note is that I could see titan in the 20mm but not in the 8mm despite being in the field for both EP's.

So if my presumption is right, and the best working magnification for this scope is ~30-50x then perhaps it would be worth investing in another EP. I think the plossl gives me 23x (again unless i'm working it out wrong), so would a good 20mm EP be a bad investment?

You're certainly right about the DSO's. The one cluster i've so far observed was best viewed through the 40mm. The 20mm still offered a good view but i'm not sure the optics of the EP are as good as the celestron.

Thanks again!

[pvaz]

When you say I'll likely not get above 100x, could you elaborate a little? I presume what you mean is that magnifications around and above this value will be unusable due to the low level of light coming into the 70mm objective. I've certainly had 112.5x while observing Saturn (unless I'm working it out wrong), but the image was substantially dimmed and lacking in feature (other then the rings of course - which were awesome). Of note is that I could see titan in the 20mm but not in the 8mm despite being in the field for both EP's.

So if my presumption is right, and the best working magnification for this scope is ~30-50x then perhaps it would be worth investing in another EP. I think the plossl gives me 23x (again unless i'm working it out wrong), so would a good 20mm EP be a bad investment?

You're certainly right about the DSO's. The one cluster i've so far observed was best viewed through the 40mm. The 20mm still offered a good view but i'm not sure the optics of the EP are as good as the celestron.

Thanks again!

That's about it. The maximum useful mag you get on a scope is 2x per mm of aperture (or 50x per inch of aperture), in your case it's around 140x . However for optimum detail/contrast around 60% of that is recommended (1.2x per mm or 30x per inch of aperture).