Is 38-40mm EP a complete waste in a Dob

[Bart]

Hi all,

I have a 30mm 82* low power EP for using in F4.5 and F5 Dobs. I noticed that becasue the 30mm gives higher magnification in the F4.5 (1830fl), that the FOV is smaller. So much so that I just about fitted M81/82 in the same FOV, whereas they are niely framed in the F5 (1250fl) along with a nearby galaxy whose name I cant remember. I would miss that frame in the big Dob. I know all the arguments about exit pupil, but wondered can the same FOV be recreated by using a large FL EP with smaller AFOV probably. I know the light would be diminished, but what REALLY would I see. I'm guessing I'd frame the objects I wanted, but would they be dimmer? or what exactly would be the effect of using a too large exip pupil. I wouldn't be using them for very dim objects, for like expansive views of the Sisters, other open clusters and asterism etc

Thanks for any advice.

Bart

[Naemeth]

Hi all,

I have a 30mm 82* low power EP for using in F4.5 and F5 Dobs. I noticed that becasue the 30mm gives higher magnification in the F4.5 (1830fl), that the FOV is smaller. So much so that I just about fitted M81/82 in the same FOV, whereas they are niely framed in the F5 (1250fl) along with a nearby galaxy whose name I cant remember. I would miss that frame in the big Dob. I know all the arguments about exit pupil, but wondered can the same FOV be recreated by using a large FL EP with smaller AFOV probably. I know the light would be diminished, but what REALLY would I see. I'm guessing I'd frame the objects I wanted, but would they be dimmer? or what exactly would be the effect of using a too large exip pupil. I wouldn't be using them for very dim objects, for like expansive views of the Sisters, other open clusters and asterism etc

Thanks for any advice.

Bart

How about a 31mm T5 Nagler?

EDIT: It doesn't really add much onto the FOV... I can see what you mean though.

[swamp thing]

Have a look at this: it's formulated for a 16" lightbridge.

http://televue.com/e...LCULATE#results

You can see there is little with bigger FOV than the Terminagler . Not much point in going with a longer focal length as the gains aren't really worth it IMO

Ps I'm afraid smaller FOV is one of the downsides of a larger aperture.

[Andrew*]

Hi Bart,

My own experience of this suggests you really should try it!

I used a 40mm 70° Aero (8.9mm exit pupil) in my 16" Lightbridge to great effect, managing to fit the entire Pleiades cluster in the field, and even capturing the elusive nebulosity under average skies! This was not possible in the 28mm UWAN I owned at the time. If your eye pupil is smaller than the exit pupil, you are in effect stopping down the aperture. Say your pupil was actually 7mm with this eyepiece, you would get pretty much the same view in a 12.6" f/5.7 scope! However, we usually only use one scope at a time, so, for convenience, why not?

The caveat is that if your exit pupil is really too big, and the field of view is bright, under heavy LP, for example, you may be bothered by a black patch in the centre of the field of view - the shadow of the secondary mirror.

The field of view is about 10% larger with a 40mm SWA than a 31mm UWA, so the field gain is worthwhile IMHO.

Such an eyepiece would double up as a great finder eyepiece - once found, home in on your target with a smaller exit pupil to achieve better contrast.

Andrew

[John]

I find even the 31mm Nagler too low power in my 10" F/4.8 newtonian except on the darkest nights - the 20mm Nagler presents a darker background sky and better views of fainter objects under those conditions. When I've have 40mm eyepieces in my collection I've only found them useful in my slower scopes.

Your mileage may vary though, so, as Andrew suggests, give it a try if you can !

[alan potts]

I don't think they are a waste of time, but as John says it depends on how dark you sky is, or if you can even see stars when you look upwards. I like wide field but on a fast scope you need something good as you will get soft edges and you will also have coma in the mix. I believe exit pupil will be a bit on the large side.

High quality 40mm EPs do cost a great deal. I have 2 and my main tool is the Panoptic though last night I had the SWA Meade out, my it is heavy, come to that they both are. Food for thought on the Dob. I have also owned a 40mm Aero which was soft at the sides but cheaper and ligther, wish I still had it now.

Alan.

[Andrew*]

Since you are stopping down the scope by using a too large exit pupil, you are increasing the focal ratio and can also expect better performance from the eyepiece. In my example, if you have a 7mm pupil, and are using a 16" f/4.5 with 40mm eyepiece, the effective focal ratio is f/5.7 - quite easy on eyepieces! However, someone with a 9m pupil would find this combination shows far more astigmatism and comma as he is using the native focal ratio.

[Moonshane]

personally I feel this depends on your light pollution levels at your main observing site. I sold a 35mm Panoptic in favour of 26mm Nagler mainly for this reason (LP). as others suggest, maybe try a used decent quality like the Aero and then buy a top quality (if you want one) in due course. personally, I'd not bother.

[Bart]

thanks gents. I prob wouldn't get an expensive one, its only really for the odd target. Cheers

[alan potts]

Bart,

I think the Aero would do you, I had one and it really wasn't so bad at F5.2 but you are a bit faster. John had one see what he thinks, he seem to know a great deal about eyepieces, I'm well impreess with his knowledge.

Alan.

[Mr Spock]

I found when I had a f4.7 anything longer than 22mm was too much due to the background becoming too light. Coma also starts to take over and you'll need a coma corrector for the best views.