I recently purchased a new CCD camera for planetary imaging and yes I am hooked. My only concern is trying to center an object on the CCD chip. I’m sure I am wasting good seeing and time trying to put an image on the chip. Just using the RDF is not enough. What is the best way to put the object in the FOV?
Any suggestions or tips will be greatly appreciated.
I recently purchased a new CCD camera for planetary imaging and yes I am hooked. My only concern is trying to center an object on the CCD chip. I’m sure I am wasting good seeing and time trying to put an image on the chip. Just using the RDF is not enough. What is the best way to put the object in the FOV?
Any suggestions or tips will be greatly appreciated.
Depends how the camera connects to the scope. If it has a normal eyepiece front (1 1/4") then centre the object using a high power eyepiece and replace the eyepiece with the camera.
If it connects using a T mount then I use a slide mirror. Something like this one.
Last edited by Terry B; 18-01-2008 at 05:28 PM.
Reason: typo
I found that you need to use something in the order of a 8 to 12mm EP to get it reasonably close to centre with the DBK31.
I found it frustrating on 2 accounts
1) you need to get the EP parfocal, so get some parfocal rings - you could be out a fair distance - some things I could see in the EP were completely missing from the CCD because of the distance until I got them closer matched
2) the chip is pretty small, so I got a zoom eyepiece (mine goes from 8mm to 24mm). Doesn't even have to be a "good one". Have it way out at 24mm first to get it rough centred, then zoom in right down to 8mm to make some fine adjustments (in steps if you have to if the star is dim or jumps out of the FOV). If you've parfocalled the EP, then you should have your image sitting there when you slip the camera back in, pretty close to perfectly focussed too.
The flip mirror system is great (make sure the cam is in the "clear path", not the mirror path!). For me though, it takes up too much backfocus for my purposes in general for it to be worthwhile.
I have not problem using a 8x50 finder.
Make sure the finder is accurately aligned.
If not, try aligning on the moon, big target and easy to find.
Take an image of the moon and check where what is visible, adjust the finder as needed.
Once close try on a bright star, and adjust again.
This is good enough to get the star on the chip.
After a few minor adjusts the star / planet is easily centred.
I have not problem using a 8x50 finder.
Make sure the finder is accurately aligned.
If not, try aligning on the moon, big target and easy to find.
Take an image of the moon and check where what is visible, adjust the finder as needed.
Once close try on a bright star, and adjust again.
This is good enough to get the star on the chip.
After a few minor adjusts the star / planet is easily centred.
Personally I cann't stand red dot finders.
Brendan
Hi Brendan,
I'm pretty sure the RDF is accurately aligned. When I manually slew the scope to a star and center it in RDF. The star is close to the center of the FOV.
Maybe a 9x50 finder or a Telrad might be a better option. Which one of the two is better?
I think that Brendan was maybe trying to explain that an RDF or Telrad is only an x1 magnification device and trying to centre an object on a CCD chip can be really hit and miss using these unity magnification devices.
Using a 9x50 finder scope gives you a magnification of x9, so makes placing the object on the ccd chips at least 9x more accurate.
With red dot finders your head tends to move around, which changes slighty where the Red dot is pointing. This is ok for wide field eyepeices, but useless for CCDs.
It's a real pain, and a never ending source of frustration for me - especially when you go from a 5mm eyepiece, to 11m focal length with a CCD and small FOV.
I usually put the eyepiece in (5mm), center the planet and then adjust the finderscope (9x50 RA) so that the planet is centered in the crosshairs.
Then I put the DMK/5x powermate in, which takes it well away from where it was due to the weight difference. So then I use the RA finder and get the planet centered again. With any luck, I see it right away as it crosses the field.
I use something like 15fps, full gain, and slightly unfocused planet so that it covers a bigger/brighter area then when it's focused.
But I have had very frustating times sometimes, spending 5 minutes or more trying to find the bloody planet. I then switch to a 2x or 3x barlow and work my way up to the 5x, getting the RA finder accurately aligned each time.
Yes – adding a Barlow increases the challenge exponentially!
I have a 9x50 finder which has a small circle where the cross hairs meet in the middle of the illuminated reticule. This allows me to place the object dead centre of the cross hairs without the cross hairs occulting the planet or star.
For prime focus and x2 to x3 Barlow’s this is a reasonably accurate and reliable operation, but at x5 it is quite frustrating and I centre the object through trial and error.
I have decided to replace my RDF with a 9x50 finderscope. I just have to find one that is reasonably priced. I like how Dennis finderscope has a small circle where the cross hairs meet in the middle. Making it much easier to center the object.
I purchased mine from Ron at Sirius Optics some 3 to 4 years ago – it was quite expensive, well over $300 but it has proven to be one of my best buys in accessories.
It is Japanese made, quality finder scope. The optics and build quality are much superior to my Orion finder scopes and a little better than those in my Vixen finder scopes. The quality seems more on a par with my Tak finder scope.
I’ve dropped in to Sirius Optics several times, most recently just before Xmas and I haven’t seen these items on sale since, which is a pity.
The cross wires and the small circular reticule are very fine whereas my Orion finders have reticules that are more like hawsers in comparison.
My Tak finder has the illuminated cross hairs, but the tiny segment where they cross is “missing” (by design) so you end up with a blank region equivalent in size to the small circles in the centre of the other reticule.
I have an 10mm guiding EP with happens to be parfocal to my ToUcam (orvery close to parfocal!).
So I simply align on my subject with this EP and then swap the EP for the camera, and do a final fine tune on focus with the camera.
I thinks a parfocal EP is the simplest most reliable way.
Al.
I don't really have a EP that is parfocal with my camera, would be nice though. I think my 9mm EP is the closest I am going to get. Except whenever I remove the EP and replace it with the camera, the image is way out of focus and nowhere in the FOV. Maybe a higher power EP might do the trick?
If you are only going to visit a few objects per imaging session, then a par focal ring and eyepiece combo is a great idea, I used to use that technique with my Meade LPI using a Meade 20mm Plossl. When I switched to the DMK, I found that I needed an extension to my 1 ¼ inch eyepiece in order to fit the par focal ring on the 1 ¼ tube.
Also, when flitting between several targets in an evening’s session, such as double stars, I find it more convenient to use the 9x50 finder which always puts the target on the DMK CCD after a GoTo slew.
I would probably find it inconvenient to manually move the ‘scope to a new target and swap between eyepiece and CCD each time to centre the target? I must be lazy, !
I find I can get the object into the tiny field of my SPC900NC webcam while using a 2x Barlow with my C8 (combined focal length 4000mm) just by using the standard 6x30 finder.
Taking the time to accurately align the finder is critical. I've been able to switch between Mars and Saturn multiple times during the night with ease once the finder is accurately aligned.
Of course there is the electronic method of centering the CCD using Plate solve techniques but this assumes a robotically controlled telescope. Typical pointing errors are sub 1' with my system (point, platesolve, repoint as necessary)
if you have a portable TV that you can place next to your imaging lappy or PC you could try what I do.
If I want to look at the Moon or Planets (my v2.5a hand controller doesn't do planets, I have to go look for them), I have an El-Cheapo wireless security camera fitted into my 8x50 fiderscope. I look at the TV and use the slew arrows on the hand controller to centre the object. I have marked crosshairs on my TV and when the object is in centre of the crosshairs it is also centre of the imaging screen on the lappy.
I also use the TV and security camera for initial 3 star alignment on start-up. It makes the job a lot faster
The security camera in the finderscope can be seen here:
Which one of the two is better?
!
