I believe I have a collimation issue with my Tak FS-102 (Fluorite Doublet).
There is a definite blurring/elongation of stars towards the bottom left corner of the frame in my images. It is evident with and without the focal reducer, and can be seen on images taken with my previous DSLR and my new CCD camera. I've run the images through CCD inspector and it says there is a tilt towards that corner of the frame.
It can be seen if you zoom in on the bottom left corner of this high res image of eta Carina - link
Does anyone have any experience with this particular scope? I haven't found anything specific on the web, only general advice about collimating a refractor.
I have attached two pictures of the front element and the adjusting screws - there are three sets of screws around the objective. The black objective housing is hard against the blue front end of the main tube at the top of the scope, but there is a definite gap on the underside.
For clarity, could people refer to the screws as the short screws and long screws when giving advice please.
They are just standard push pull screws Dave. I normally just use a cheshire to check it. You can do it during the day, leave the dust cap on, put in the extensions for eyepiece use and use the cheshire. Turn the cheshire to catch the light and you'll see a black dot or two reflecting back. Adjust the screws to center the dot. It's much easier to see the dots on a bright day, just had a look through mine then and it's a bit dull today to see it clearly. I used to double check it with ccd inspector after dark, but it was always really close anyway.
Sure you don't have droop in the imaging train or similar causing the grief?
David, have you done a star test at all? Pretty simple really dont use a diagonal though. Just find a brightish star not low on the horizon and check the concentric circles inside and outside of focus. You will need a relatively high power EP, and you should get concentric circles with the centre dot exactly centered and concentric circles radiating out from there. John Glossop has collimated one of these and he is a member here.
Are you sure the collimation is out, it looks a lot like a bit of field curvature to me.
Do a star test without the diagonal in, using a reasonably high powered eyepiece, center a 2nd/3rd magnitude star and rack the focuser in and out, see if the circles are concentric, if they are, the problem is in your imaging setup.
If they are not concentric, there are 2 ways you can collimate, using a Cheshire Eyepiece, you can do the adjustment on the bench. or the other way is by using the standard star test and doing your adjustment there.
You mentioned that one side of the cell is against the front ring, you can adjust this by using a thick feeler gauge to stand the cell off the ring, this will mean adjusting all 6 screws then do a star test and collimate. Stick your finger into the light path and move it around until it is pointing at the fattest part of the bulge, that is the screw you adjust in, be aware that it will only take probably about 1/16 of a turn to produce results.
The FS series is quite easy to collimate, just take your time and make small adjustments.
Certainly check it without the diagonal first.
The easiest thing to verify first, before touching the objective, is that the focuser is aligned with the optical axis.
In the write-up mentioned below, it's covered under the section on laser alignment. Worth checking before you go much further.
First I make a paper circle the same dia as the objective with a dot in the exact centre. put an accurate laser on the focuser tube.
Get the focuser centred first before fiddling with the objective collimation.
You can collimate the lens for hours but it is of no use unless the focuser is spot on.
How is the body of the focuser fitted to the main OTA?
On many refractors there's three M4 screws holding it in place... sometimes the final alignment to the optical axis can be slightly out...
means checking with the laser, loosening the screws tweaking the position of the focuser body and then retightening (and rechecking the alignment).
Be interested to hear what you find in the end about this..
It could be tilt of the sensor in the QSI.. (I've had this problem with the same sensor in my QHY9) but at the f-ratio of the FS-102 it would have to be pretty bad to show up?
As a couple of guys have said, the focuser isn't adjustable.
The eta image was shot with a 2 inch nose piece on the QSI into the Tak 2inch adapter (I was waiting in other adapters to arrive).
I have seen some tilt in my DSLR shots that were taken with rigid screw in adapters, hence my suspicion that the collimation is out. It is always in the same corner of the frame and I think I have had that camera oriented in the same direction for all of the shots I took. As the scope would have been oriented in different directions, I would expect that gravity would have caused the elongation to move around the frame if the focuser was drooping.
Regardless, I'll try some star tests when I next get a chance to set it up and go from there.
try rotating the camera through 180 degrees and take a few shots in different positions. if the problem is always in the same corner of the frame then that would suggest sensor problem. if it's always the same stars then maybe collimation. and if it varies all over the shop then your camera connection..?
OK, here's some more data to put up for the group to interpret.
Attached are some CCD Inspector curvature maps - 1 & 2 are taken with the CCD, the 3 & 4 with the DSLR - all of these shots are taken with the reducer. 5 is taken with the CCD, but no reducer.
As the tilt is evident for both devices, I think that rules out sensor misalignment.
I have oriented both cameras with (roughly) the same degree of rotation.
The tilt is reasonably constant (excepting image 5 which is taken through a different optical system without the reducer, and the camera was attached via a 2 inch nosepiece, rather than screw in adapters). As the CCD & DSLR are vastly different in weight - does this suggest that slop in the focuser is unlikely?
Of course the weather won't oblige to let me do the tests you suggested under the stars for a few nights yet!
Interesting, the first four shots indicate to me that there is a little bit of field curvature, this is also evident on your original link to the photo, when expanded there is radial elongation on 3 corners, are you using a Takahashi 35 Flattener and do you have the correct Metal Back Distances.
The last shot shows what appears to be mis-collimation although you say the camera is attached by the 2" nosepiece, I am wondering whether or not you have a bit of slop in the focuser tube, attached is a PDF of how to adjust for that.
I would still be interested in seeing your results using an actual star test, that to me will prove once and for all if the collimation is out.
As I said in an earlier post, this is very simple to fix and does NOT involve taking apart the lens cell.
Looking at the CCD Inspector plots it is indicating your collimation is good but you have tilt. Tilt and collimation are 2 different concepts.
So the source of the tilt would appear to be the focuser as it appears the tilt is always towards the bottom of the image (focuser leaning downwards?).
Can you feel any play in the focuser with the camera installed.
The 2 crosses in the middle of the CCDI plot show collimation. They seem perfectly aligned.
So it would appear that if you played with the lens cell adjustments it would only make things worse. Best to adjust the focuser first.
I recommend you post this on the Yahoo Tak Uncensored site and have Art or Fred comment on it. They are the experts on Taks and are super knowledgeable and helpful.
Or call Art or Fred at Texas Nautical (in Texas USA).
Interesting, the first four shots indicate to me that there is a little bit of field curvature, this is also evident on your original link to the photo, when expanded there is radial elongation on 3 corners, are you using a Takahashi 35 Flattener and do you have the correct Metal Back Distances.
The last shot shows what appears to be mis-collimation although you say the camera is attached by the 2" nosepiece, I am wondering whether or not you have a bit of slop in the focuser tube, attached is a PDF of how to adjust for that.
I would still be interested in seeing your results using an actual star test, that to me will prove once and for all if the collimation is out.
As I said in an earlier post, this is very simple to fix and does NOT involve taking apart the lens cell.
Cheers
Thanks for that John,
I'm using the genuine Tak Reducer for this scope, and the CA-35 adapter. I used the recommended widemouth T-Mount adapter for the DSLR and have had an appropriate length adapter machined up by precise parts to attach my CCD camera.
I've gone downstairs and wiggled the focuser tube - it doesn't seem to have any flex, although I'm reluctant to roll it in and out as I've now attached a Robofocuser and if I move that, I'll have to redo it's setup routine and v-curves. I'll try the star test when next I get it all outdoors - hopefully I'll have some other experienced eyes with me to help!
Thanks for sending through the image about focuser adjustment - my impression is that is to smoothen the movement of the focuser though, rather than change it's angle??
Quote:
Originally Posted by gregbradley
Looking at the CCD Inspector plots it is indicating your collimation is good but you have tilt. Tilt and collimation are 2 different concepts.
So the source of the tilt would appear to be the focuser as it appears the tilt is always towards the bottom of the image (focuser leaning downwards?).
Greg.
Greg - I've been looking for something on the web to aid in interpretation of these diagrams - a lot of the comments in the help file are related to collimation of newts and SCTs, not refractors. At first reading of the help file, I have the impression that they still recommend adjusting collimation to neutralise this tilt. Are they using appropriate the term tilt appropriately?
My only problem with the suggestion that the tilt is always towards the bottom of the frame is how does the data come off these chips. If you think back to the days of film, images on the film are upside down - either the camera or the software flips the image around so what is actually recorded at the top of the sensor appears on the bottom of the image on screen (just thinking about DSLRs here - not sure about CCD chips). Am I actually seeing tilt towards the top of the sensor - which would go against the theory of droop of the focuser due to gravity.
All very confusing.... Will post on Tak Uncensored as you suggest!
