Eyepieces.. :(

[AweSIM]

@nick.. brother you nailed it!!!.. .. thanks a million.. so the basis of imaging comes down to the fact that YOU NEED AS SHORT FOCAL LENGTH AS YOU CAN.. the shorter the focal length.. the wider the FOV.. the lower the magnification.. and hence the less obvious star trails would be due to mount imperfections.. .. at the expense.. that if you use a camera for instance with the same chip dimensions.. the shorter FL will capture a greater area of sky and hence would lose detail..

i would interpret your explanation as follows..

- if you use a 1000mm FL scope with a camera.. the image captured on the 20x20 mm chip would be lets say 5 degrees across.. a little mount movement would register as lets say 5 pixels blurring..

- if you use a 500mm FL scope with the same camera.. the image captured on the same chip might be 10 degrees across.. and the same mount movement might register as 2.5 pixels blurring..

- hence apparently.. its preferable to get as short FL as possible if DSO imaging is my priority..

- but if i use the 1000mm FL scope.. the take 4 images.. each one 5 degrees across.. i can stitch them together, resize it back to the same size, and id have a 10 degrees across image.. the same as with the 500mm sope.. so its actually the same..

- the downside is that the 4 images might have different exposures.. color hues due to atmosphere.. etc.. which would ruin the mosaic.. and would require 4 times as much time as would the 500mm scope..

have i got it correct? =).. if yes.. then yay.. i can still enjoy 200PDS.. =).. i have all the time in the world to make mosaics even if another scope can do it 4 times faster and with zero hue shift, etc.. ..

thanks a lot brother.. you explained it beautifully.. ..

asim sohail..

[EDIT].. so basically if you can get hands on a coma-free newtonian of 500mm FL.. thatll work as better as the refractor ollypenrice has for instance.. .. just trying to make an analogy..

[AweSIM]

@merlin66, ronin and nickk.. thank you guys.. you guys finally answered my sub-question in this thread.. what determines how big an image forms on the camera sensor where there is not EP to tell you what the magnification of the system is going to be.. .. thanks..

[NickK]

In addition to your description, the 500mm would put more light per minute onto each pixel. So the resulting imaging would be faster.

If you were to make a list of objects, look at their size in arc-seconds. You'll find that you'll need to mosaic some, some will fit perfectly and others will just be small without increasing pixel resolution or increasing focal length. The important bit is picking the focal length that gives you the most targets you want to image.

Mosaics are very possible with a little knowhow - Olly and others has shown this but 500mm is quite wide.

Personally I use my kit for three things:

1. Imaging galaxies and DSOs, over time I'll focus on detail. I have a desire for a 24" behemoth but it's ~2400mm focal length will need a good mount.. so financially it may not happen!

2. Using the imaging to explore beyond visual (the eye limits). Pictures may not be pretty but I go "wow look at that, I would love to get a picture of that".

3. Using the scope visually for the moon, and possibly planets - although 675mm is a short focal length for planets, so it tends to be lunar using my 3.5mm EP. My budget priority next is a replacement laptop and camera - then I'll worry about a 2-3x barlow for planets.

[AweSIM]

In addition to your description, the 500mm would put more light per minute onto each pixel. So the resulting imaging would be faster.

i didnt understand this at first.. i thought you were erring.. but now i realize youre right.. a 500mm scope will create an image of 1"x1".. a 1000mm scope will create an image of 2"x2".. the aperture being the same, the 1"x1" will be brighter than 2"x2".. well said.. and well explained..

If you were to make a list of objects, look at their size in arc-seconds. You'll find that you'll need to mosaic some, some will fit perfectly and others will just be small without increasing pixel resolution or increasing focal length. The important bit is picking the focal length that gives you the most targets you want to image.

yes.. youre right in that too.. i now understand it wont be possible to work with DSOs and planets with the same scope.. you gotta pick either one of the two.. or pick two different scopes..

btw.. how many people out here have two scopes for DSOs and planets?

and just when we're on the course of discussing EPs.. since im going for 200PDS 1000mm FL f5 scope.. would it be possible to get some good views of the planets later on when i buy short FL EPs and maybe 4x barlows? the NEQ6 mount might handle short exposures at high mags.. but can the optics deliver? or would a 3mm EP at 2x barlow be pushing the limits of the optics?

[NickK]

I think you'll be fine.

well with a barlow, 1000 / 3 = 666x magnification. The limit of the atmosphere in the UK is about 270x in perfect seeing!

I have a 3.5mm EP but I'm using it with a 675mm scope normally for planets. I think I could do 2.5mm (275x) but the number of targets and number of times the weather would allow it could be quite small. My main workhorse is a 13mm which means working at 52x magnification.

Your EP requirements will differ from mine. For 1000mm between 1000/250 = 4mm EP for 250x and 1000/50 = 20mm EP for 50x.

I would try the EPs you will get with the 200PDS. Give it some time until you think "I'd like to see the sky about X (ie 2/3rds) of the way between these two EPs to give me a good FOV". Then you can work out the magnification you want and from that work out the EP focal length to get it.

[NickK]

i didnt understand this at first.. i thought you were erring.. but now i realize youre right.. a 500mm scope will create an image of 1"x1".. a 1000mm scope will create an image of 2"x2".. the aperture being the same, the 1"x1" will be brighter than 2"x2".. well said.. and well explained..

I should say that comparison is with the same aperture!

Focal Length / Aperture = F-ratio

Faster scopes will result less time needed to image. However the amount of light entering the scope is defined by the aperture. Additionally resolution is related to aperture.

For example - looking at M51 through my pentax visually I can just about see it (although f6.38 it's only 105mm aperture). Through a 16" dob I could see detail such as spiral arms. However if I stick a camera on pentax, over time I can pull out more detail than the 16" dob can visually. If the dob was build to withstand being mounted on an EQ mount with a camera it would put my pentax to shame in terms of detail - although the cost would be astronomical, both for the mount and the scope!

[AweSIM]

@nickk.. thanks mate.. that was pretty useful info.. as your advice also conforms with what everyone else is saying.. i guess its in the best interests of me (and my pocket).. that i refrain from getting too many cheap EPs at this moment.. i guess i should stick on to the plan..

- 28mm 2" stock EP..

- 2" 2x barlow..

- 1.25" 11mm Plossl EP..

thatll give me good variation.. hi.. mid-hi.. mid-lo.. and lo mags.. without touching either extremes of the scopes capabilities.. and save cash to later spend on other EPs that i may find missing.. .. thanks guys..

[AweSIM]

Hi, I was just visiting this thread again and I just felt compelled to thank all you guys again. You guys have been awesome! I actually love you guys for being so awesome! Thank you so much for having taken time out to guide me. =)

[Starlight 1]

I do not think I got time to read all the post. So if may have been put in here.  You see this in FLO top of page under help & sopport.