prime focus imaging with nikon d90 - magnification?

[AweSIM]

hi..

ok i think this might be a dumb question afterall.. but i cant seem to find a good enough answer to it..

ive tried prime focus photography.. removing the camera lens and fixing it to the focuser of my SkyWatcher Explorer 200PDS.. the thing is.. i cant seem to figure out a way to change the magnification..

ive captured images of the moon.. and the moon fills up less than a quarter of the image.. i wanted to try making a high-res moon mosiac.. and theres no point in doing that if the image isnt larger than the camera sensor.. also, even though i can use different EPs to magnify jupiter for visual observation, it appears as merely a couple of pixels when prime imaged..

thanks..

awesim

[Merlin66]

you need a barlow lens in front of the camera body to increase the "effective" focal length and hence the size of objects on the CCD chip.

A x2 to x4 barlow would be a starting point...

[AweSIM]

oh.. so id need to have a 2" x2 barlow.. that leaves me with a couple of follow-up questions..

1. i'm ordering a T2Nikon adapter to connect my Nikon D90.. i think i dont need anything else but if i'm wrong (extension tube, some other adapter, etc), please do correct me..

2. even if i use a x2 or x3 or x5 barlow, that still won't show jupiter's disc clearly.. ive seen people post captured pics of jupiter, saturn etc.. how do they do that? is DSLR imaging not the way to go for planetary imaging?

thanks..

awesim

[Merlin66]

IMHO, for planetary imaging a webcam or one of the Imaging Source "industrial strength" equivalents is the way to go....you can quickly collect an AVI file which maximises the chances of recording that moment of good/ great seeing....

DSLR's are OK of the bigger stuff like the moon and wide field objects.

[AweSIM]

what exactly are the pros and cons of DSLR vs webcam imaging? the way i see it.. a webcam like Celestron NexImage CCD Lunar and Planetary camera also needs to be plugged into the EP holder.. so i cant use it to image planets either.. since the image size will be fixed as a function of the focal length of my scope and the CCD chip size..

[edit]

posted at the same time i guess.. =).. anyways in reply.. yes im aware of the fact that if you record an avi, you maximize your chances.. but the image size is still fixed isnt it?

when i use my DSLR to image using SkyWatcher Explorer 200PDS, i can see jupiter as a mere pinpoint.. hardly a couple of pixels across.. same will be true with a webcam.. right? and even if i use barlows, i can magnify that image to maybe 50 or so pixels across.. not more..

i guess my question is.. how do you people capture high-res images of the planets?

[Merlin66]

With the smaller LPI, webcam and the DSI etc you can "plug" them into a barlow like you would an eyepiece to give a greater focal length and large images.

[AweSIM]

hi merlin.. im sorry but im still stuck.. if the image of jupiter i capture with just a webcam is around 10 pixels across.. using a x2 barlow will just make it 20 pixels across.. i may have great EPs with focal lengths of the order of 10mm or so that can magnify upto to 100x or so.. but all those are of no use in imaging.. am i right?

[Merlin66]

You can use a x4 or x5 barlow to get even more image scale....

An eyepiece projection adaptor can also be used, but the limit will still be around f 25 - such is life......

(All the great images you see of Jupiter etc are taken around f25 to f30 - just a benchmark)

[Merlin66]

Say the size of Jupiter is 60 arc sec, and your typical seeing is good at 2 arc sec; this means that the "blocks" which make up the image will give you 60/2=30 across the image. No matter what else you do, this will never change...if you increase the magnification the size of Jupiter's image will increase, but so to will the size of the Lego block...there will always be a resolution of 30 across the image...

Now consider the focal length and the camera.

If your effective focal length, with a x4 barlow is say 1000 x 4 = 4000mm, the physical size of the image of Jupiter at focus will be 1164micron (just over 1mm).

Now if your camera has a pixel size of 5.6micron (typical webcam), due to the sampling criteria this gives a resolution of 11.2micron. So according to the camera, we can get 1164/11.2 = 104 camera "blocks" across the image of Jupiter...three times better than the best seeing conditions.

This allows us to record the maximum amount of detail available to us....

HTH

[JohnnyD333]

Im just curious, with a D90 and 13 megapixels, that shot you get of the moon could produce a high res mosaic. MPs dont mean as much in the DSOs and Planets, but you should be able to create a great image of the moon with the images your taking already. Then again, maybe you want a giant poster of the moon so ignore my comment if that is the case.

If it makes you feel any better, my prime focus magnification with my Nikon is only about 25x, so its the moon or DSOs for me. Luckily, if you get frustrated with all the barlows and T-Adapters webcams are relatively cheap and give surprising results. Also, does you camera have a video mode, maybe you could record some videos and try to stack them. Worth a shot.