Visual limits of my scope

[MJ_]

Hey guys im still pretty new at this, and i've been doing a ton of reading over the last week and have got my head around alot of things, thanks to this forum for having such a wealth of information.

Anyway I need to ask about how good should i be able to see deep space objects. I have a Celestron C8 SCT and i've been having a great time looking at saturn and jupiter, even more so now that i just learnt how to collimate it. After i did that WOW what a big difference it made to seeing the detail on jupiter, any newbies reading this i highly recommend you learn how to collimate your scope its the best thing ive done so far.

So now i've turned my attention to looking at galaxies and nebula and all i see is pretty much just a faint fuzzy object. Nebulas are even more difficult to see, its hard to make out any definate gas cloud. I just need to know is this the visual limits of my scope? Other then travelling out to the bush i dont know how else i can improve it?

When i see astrophotos i realise with the long exposures it can capture much more detail then the eyes can detect, i just didnt realise it was so significant and if this is normal?

The only high power eyepiece i got is a cheap 10mm plossl that came with the scope which is probably the weakest link here. I do have a baader hyperion 13mm on order so should i expect a significant improvement with that eyepiece?

Also im in the outer suburbs of brisbane and i dont believe light pollution is that significant or maybe im wrong? :shrug:

Cheers
Matt

[coldspace]

Hi Matt,

You will be best to observe the brighter objects like planets,moon, glob clusters, open clusters, planetary Nebs and of course various types of stars with your C8 in the outer suburbs of Brisbane. Nebular clouds, galaxies need alot darker skies to show any detail in the light glow of suburban Brisbane. You have to remember that we have not been having the clearest seeing up here at the moment so this will hamper faint fuzzies visually. Only try to go for the faint fuzzies at new moon and try to set up in the darkest part of your yard, get dark adapted and you should be able to tease out a little bit more detail in the brighter ones.
You need to get that scope out to darker skies and then you will see alot more objects the difference between suburban Brizzy and say Leyburn dark sky site is no comparison. I can see more with my 5 inch Mak at Leyburn than my 12 inch Lx200R in Brisbane.
I have the same trouble with a 12 inch scope in suburban Brizzy but you will start to see things with more experience and the right night.

I ended up getting a good quality astro video system to beat light pollution as I don't have the the time to get out to dark skies that often and this camera has been the best investment other than my scope as it displays almost photo like images on a monitor but in real time.

Regards Matt

[Quark]

Hi Matt,

Glad to hear that you have worked out how collimate your scope.
With a scope of reasonable aperture the seeing from your observing location will almost certainly determine how well objects will resolve.

Unless you can observe at altitude you are not likely to get near the theoretical limit of your scope.

When observing DSO's low to medium magnification is the go and after a few years of observing, you will probably have a whole new outlook on those faint fuzzy objects.

You have to learn how to eek out detail within them, use averted image to observe them with the most sensitive part of your vision, make sure you are dark adapted and that there is no white light in your vicinity.

Definitely don't try and use a laptop computer, even with a red screen and expect to see faint and delicate detail. With the longer focal length of the C8 I would have thought 10 and 13 mm FL eyepieces would be too much magnification for general DSO observing.

I have a 16" F4.5 Newt and most of my DSO work is with a 14mm series 4000 Meade UW, in my scope that delivers 130x, if I want a wider field I use a 31mm Nagler at 59x or if I want a bit more magnification I use a 9mm Nagler at 203x. For high magnification on the planets I use 6mm and 4mm Orthoscopic eyepieces at 304x and 457x respectively.

With many DSO's it is far better to observe a smaller brighter image than a bigger dimmer image.

Just a few thoughts
Cheers
Trevor

[MJ_]

Thanks for your replies. I didnt realise there is such a significant difference in darker skies. I just focused my attention earlier to omega centauri and wow i wasnt disappointed. First time a saw it and after viewing it for about 30mins i am starting to understand what you mean about seeing more detail over time with experience. I guess im just being a bit impatient :)

I have to admit i get distracted a bit and i dont think ive allowed my eyes to adapt the best they can, so im gonna lock myself outside for a couple of hours with no laptop and see how i go.

Another question, what do you think of a UHC filter? I'm thinking of buying a 2" baader one that i can put in my eyepiece and im curious to know if it makes a worthwhile difference for visual use?

Also im a bit confused on how a higher magnification eyepiece yields a dimmer image? What would be the optimal focal length eyepiece to use with my scope for DSOs?

[Quark]

Hi Matt,

It takes about 10 minutes in a dark environment for the pupils of your eyes to fully dilate. Following on from that over about the next 15 minutes a chemical called visual purple is formed in the retina of your eye. When the maximum amount of visual purple that your eye can create has formed, then you are dark adapted. Any white light and that 20 to 30 minutes starts from scratch. Even bright red light such as a red laptop screen will effect your dark adaption.

The reason that a higher magnification eyepiece will deliver a dimmer image relates to the exit pupil, that particular eyepiece, will deliver in your particular telescope.

When the calculated exit pupil of an eyepiece reaches the diameter of the fully dilated pupil of your eye, then the resultant image will be as bright as your system can deliver for you. The diameter of this fully dilated pupil will generally be larger for younger people than for older people and may range somewhere between 5 and 7 mm.

The exit pupil for your telescope is calculated by dividing the FL of the eyepiece by the Focal Ratio of your telescope. For instance in my F 4.5 Newt a 31 mm eyepiece delivers an exit pupil of 31 / 4.5 = 7mm. The same eyepiece in an F 12 scope would be 31 / 12 = 2.5mm.

I am going on 59 and do most of my deep sky observing with a series 4000 Meade UW 14mm eyepiece. In my system the exit pupil will be 14 / 4.5 = 3mm. I find the images this produces to be relatively bright and most pleasing, but my eyes with pupils fully dilated probably don't get to 6mm.

From this you can see that with a slow Focal ratio scope, such as an F 12, the exit pupil produced by any eyepiece will be considerably smaller than with the same eyepiece in a telescope with a fast focal ratio. This is why, generally, fast focal ratio scopes are better for astro imaging as you can capture more photons quicker than with a slow focal ratio system.

Hope this helps
Regards
Trevor

[Quark]

Hi Again Matt,

The UHC filter will help a lot in better defining many of the Emission Nebulae, it works particularly well with the Key Hole Neb. Although I don't have a UHC filter I do have both 1 1/4" and 2" Astronomik OIII filters and the difference with The Keyhole is immense, it about doubles the size of the Nebula, it makes it so large that even my 31mm Nagler doesn't fit the object in the field of view, which in a F 4.5 system is considerable. You would probably get more general use out of a UHC filter than an OIII. The OIII works particularly well with Planetary Nebulae and Supernovae Remnants. However you would probably get more use from a UHC filter.

As a UHC filter will effectively increase the apparent size of many nebulous objects and considering the long FL of your C8 I would think that you would really want a wide field long FL eyepiece to appreciate the effects of the filter.

Regards
Trevor

Regards
Trevor

[MJ_]

I've done some further reading on exit pupil and now understand that due to my scope being f10 that a lower powered eyepiece will yield a brighter image.

I dont mind spending a decent amount of money on a quality eyepiece which would be my workhorse, i just need to know what focal length to get. If i was to get a 25mm eyepiece this would mean the exit pupil would be 25 / 10 = 2.5mm. With a larger focal length eyepiece wouldnt there be a trade off of not having enough magnification?

Im 26 and my eyes are in good shape so im assuming the diameter of my dialated pupil will be somewhere in the region of 7mm. I realise now that for general viewing and the use of a filter will need a longer FL eyepiece as well, what FL eyepiece i should get?

[MJ_]

Also im just wondering about 0.63 focal reducers, i know they are used to decrease the exposure time in astrophotography, but are they also useful in this case to increase the exit pupil length so such a long FL eyepiece does not have to be used?

[Quark]

Hi Matt,

I have attached a spread sheet that I created that may help you.
I have input the data for your scope, ie 200mm aperture with a Focal ratio of F10 and FL of 2000mm.

I have no experience with focal reducers, and the results of using one for observational work, maybe someone else that is an experienced observer with a scope similar to yours could jump in and advise you on that.

Don't be too worried about magnification, it really is irrelevant when trying to observe quite large, diffuse objects. When observing P/N you will likely need more magnification, I find that the 130x with 82 degree fov of my 14mm series 4000 Meade UV does a top job, even on more condensed P/N and galaxies. Occasionally I go to 203x with a 9mm Nagler as some P/N and galaxies seem to take the extra magnification quite well, however I am using a 16" F4.5 primary to collect the photons.

In my humble opinion it is far better to observe a smaller, brighter and more detailed image than a larger less well resolved and dimmer image. I suppose this come back to the aperture of your scope. Your C8 is far more portable and gives you options as to where you can use it, my 16 is permanently mounted in an observatory, definitely not portable.

Matt I have been unable to attach my excel spreadsheet on telescope data, I will attach it to a PM and email it to you.

Regards
Trevor

[Paul Haese]

I can concur with Trevor, for your scope a low power is best. In fact I would suggest that you use a 24mm or 17mm. For suburb viewing an OIII filter is very good, it shows lots of nebulosity. 10 minutes or so outside and the sky opens up. Even make yourself a little list of object you want to view and use a red torch to read your list and charts.

Trevor has given you great advice, but I just wanted to reaffirm what he said.

[MJ_]

Thanks alot, lots of great advice on this forum. Although i am getting poor pretty quick with all these accessories hehe

[Derek Klepp]

Hi Matt Ihave a similar scope to you and was in the same position a couple of years ago.One can go overboard on spending so I recommend patience.A good low power eyepiece will probably be your best investment and a UHC filter will help once you get used to it.Be careful with a focal reducer as they often have a limit to the type of eyepiece and diagonal they work with best for general viewing.Keep a viewing log you'll be amazed how much more detail you pick up as time goes by.
Regards Derek.