Any advice for illumination improvement - fast newt
I've noted of late my flats highlight some significant vignetting, quite asymmetrical on the chip - images certainly clean up with flat calibration, but with a small chip like mine, I think vignetting really shows I must be off in collimation somewhere.... I've checked with a glatter laser, seems ok... CCD inspector shows around 5-10" of miscollimation.... I've never actually tried the CCDi collimation utility (seems a waste of a clear night! )
This is with the f/4 newt, Atik 420, and Baader MPCC....
Any advice in getting to the bottom of this? With an increasing moon, might be a good time soon to dismantle and check everything....
Attached is a highly stretched flat, and a CCDi flat analysis....
If your imaging circle is offset in your flat it relates to the centering of the secondary under the focuser. Check with a draw tube and it's likely the mirror is offset towards the spider or the primary. It is possible to have the scope collimated with an offset secondary. You'll just get uneven illumination. That's the problem with lasers. They won't tell you anything about offsets but your eyes will.
Thanks - I pulled apart an old Cheshire to get a sight tube... will try it on the weekend... I was reading about offset secondaries a while ago, all seems a dark art!
sort of a bit wary to suggest this, since it is very easy to misinterpret what is going on. However, I find this tool invaluable for setting up the secondary in my Newtonain after major mods. http://www.iceinspace.com.au/forum/s...ad.php?t=82230
If you do use a sight tube for positioning the secondary, make sure that focuser is set so that your eye is in the plane where the CCD would normally be - the standard texts do not make this point, but the procedure does not work accurately without this at f4.
Thanks - I pulled apart an old Cheshire to get a sight tube... will try it on the weekend... I was reading about offset secondaries a while ago, all seems a dark art!
It's pretty straight forward. First you need to center the spider so make sure the four vanes have the same length and the secondary holder is centered in the tube looking from the front.
Second place a piece of white paper (A4) in the tube opposite the focuser then move the secondary sideways, up and down the tube and tilt it until you see concentric circles and the gap around the secondary shape is the same all around. The secondary shoul look like a circle and the reflection of the primary also.
Thanks gents... will be my weekend project.
I've also noted some unusual shapes of stars, like a tiny pacman mouth on one side - doesn't seem to be any obstruction in the bore, and I know a spider vane is crooked too!
It's a big mess really....
I've just ordered a new CCD, so I'd like everything schmicked away before it arrives....
I've certainly seen coma, I don't think this is though - I've attached a strecthed/enlarged crop from a frame showing what I mean....
If you move your corrector too far from your camera you'll get the inverted seagull/pacman shapes. If it's too close you'll get flaring. If you have some kind of misalignment then astigmatism will give you little cross shapes on the edge of your field. So all aberrations compound to give you chonky stars. First step is to get your system aligned, then you can tweak your corrector at focus to fix your field.
Would I expect to just pop in the RCC, space out the CCD to the longer distance and start imaging away, or will I need to start moving mirrors etc to bring things to focus?
Should have read the blurb better first! Might be worth a look if I need to.
" Addition of the RCC-I into the optical train extends the focal point slightly (approximately 5mm, in an f/4 system). This important attribute ensures that your system will be able to reach focus without needing to shift the primary mirror or increase the size of the secondary mirror (of course, this assumes your system will already reach focus without the RCC)."
Should have read the blurb better first! Might be worth a look if I need to.
" Addition of the RCC-I into the optical train extends the focal point slightly (approximately 5mm, in an f/4 system). This important attribute ensures that your system will be able to reach focus without needing to shift the primary mirror or increase the size of the secondary mirror (of course, this assumes your system will already reach focus without the RCC)."
I needed to move my mirror up by 65mm to get the RCC1 to work. The focal plane does not move by much when you put the RCC1 in, but your scope needs to be able to focus a long way out from the OTA in the first place - the standard scopes don't. The focal plane height above the OTA surface needs to be (roughly) the length of the RCC1 plus the RCC1 backfocus distance of 91.5mm. The secondary needed to be at least 70mm diameter with the new mirror position - slightly bigger would have been better.
When I was using the MPCC, it just slotted in and worked straight off without an scope mods, which was a lot easier
I needed to move my mirror up by 65mm to get the RCC1 to work. The focal plane does not move by much when you put the RCC1 in, but your scope needs to be able to focus a long way out from the OTA in the first place - the standard scopes don't. The focal plane height above the OTA surface needs to be (roughly) the length of the RCC1 plus the RCC1 backfocus distance of 91.5mm. The secondary needed to be at least 70mm diameter with the new mirror position - slightly bigger would have been better.
When I was using the MPCC, it just slotted in and worked straight off without an scope mods, which was a lot easier
Thanks Ray - looks like I'll be sticking with the MPCC for some time then!
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