don't meant to detract, but I once observed collimation on an SDM where he placed a yellow magnetic disc under the focuser from inside the OTA and used a laser to illuminate the reflected triangle. can anyone clarify what this is?
I've been thinking of getting the catseye kit for AP.
The secondary offset is 3mm but it should be 5mm. That offset is built into the spider however the 2 short legs, the ones away from the focuser, are 2mm different. I think one has been stretched somehow and I've had to shim the other one by 2mm to get the bolt centered in the horizontal plane.
Hi Robin,
Those two short spider vanes should be exactly the same length. Unless the spider vanes are partially rotated for some odd reason. To be honest I really hate spiders with a built in offset. There is ZERO need for them. With enough clearance at the side of the telescope tube and a spider whose vanes are not too long, it is an infinitely better system to use a spider which has 4 vanes of equal length and then to adjust any required secondary offset by proper adjustment of the spider vane mounting screws only.
Quote:
Originally Posted by Tandum
Is this discrepancy in the offset going to cause me grief? I seem to be having a lot of trouble keeping the secondary centered and round in the sight tube and pointing at the primary center at the same time.
Generally the secondary offset does not cause any optical problems unless the secondary is almost undersized to begin with and then only with the longest focal length eyepieces, where you "may" notice some very slight vignetting. Most scopes used for imaging generally use an oversized secondary to begin with. In these cases the secondary offset is needed for mechanical reasons moreso than optical reasons. The need for an offset increases as the aperture increases and the F-Ratio of the telescope gets faster. With most scopes the advantage of an offset is to properly align the mechanical axis of the telescope with the optical axis. This affects pointing accuracy with DSC's and ongoing tracking accuracy. With short exposures the 2mm offset error would be immaterial. As exposure times increase it may or may not become noticeable with non circular stars. I am not an imager so don't know all the answers in this reagrd, but an autoguider may of itself compensate for this issue.
don't meant to detract, but I once observed collimation on an SDM where he placed a yellow magnetic disc under the focuser from inside the OTA and used a laser to illuminate the reflected triangle. can anyone clarify what this is?
I've been thinking of getting the catseye kit for AP.
Thanks
Hi,
This is a barlowed laser system. Those supplied with SDM's are from Astrosystems which are very good. However, they are not suitable for adjustement of the secondary mirror in any way. They are only suitable for adjusting the tilt of the primary mirror. They do an excellent job of this and are very fast and accurate.
2mm error doesn't sound too bad. According to the theory, it should be 'easier to collimate' but not improve 'image quality' when you have the right offset. Personally I found my 8" f4 much easier to collimate after the correct offset was applied.
The other thing you can have a look at is the vignetting: test a high quality flat field with CCDInspector, and also have a look at the vignetting impact on defocused off-axis stars.
Cheers guys, I've got it looking pretty good on the bench so back in the dome it goes till the stars show up.
Having a defocused star concentric inside focus and showing coma outside focus is what I saw last time although FHWM took another step down which is good. The secondary looked like it needed to be moved up the tube a little this time, we'll see tonight with a bit of luck.
A bit stop start tonight as clouds passed over but got a result. It seems I may have been chasing my tail a bit. The mpcc appears to be distorting the star shapes and giving the weird in/out of focus results.
First shots showed FWHM under 4 but stars where out of round. I used ccdinspector to tune collmination but it wasn't till I removed the mpcc that it started to come good.
Here is without mpcc and with mpcc, then the 2 central stars without and with plus a tuc shot for colour. 4x120sec on 253 and about 40x10sec on tuc.
I'm pretty sure my distance to mpcc is within 2mm or less so I'm not sure if the mpcc has issues itself or if the low F ratio is a problem for it. I'll have a look at it's lenses in the daylight tomorrow.
Also the primary cell can move on the lock screws, they are a real pain. This scope originally had a cell with 3 captive screws moving the mirror and if you still have that cell Peter, you'll save yourself some hair if you put it back in
It's a bit late to be saving on hair, but OK.We'll whack the captive-screw system back in. Need to replace the screws though. The originals are rubbish - Phillips heads are stripped. Sounds like a new coma corrector too?
Small parts and bearings have moved to the old harley dealers shop at Capalaba. They'll have everything. I'll visually check the mpcc tomorrow. Best to try and borrow one before buying another one though.
This newt seemed to snap into place tonight. I tweeked the primary a bit on a star without the mpcc then put the corrector on and shot a few images. Still a few odd shaped stars in there but the figures look ok.
I thought I'd finish this off instead of starting another thread.
What we found was one leg of the spider had been stretched by 2mm. To offset this I added packing to the other leg to center it in the tube. However, this introduced enough slack in the spider to allow it to twist and this twist changed depending on the altitude and because the secondary was so heavy the twisting then distorted the tube which gave us crazy results.
Peter got a new spider for the newt. Straight away it collminated at elevation and started to work however there is still a flex problem. I stuffed a camera with an OAG down it's throat and the flex issue was gone so it was external. I then removed the guide scope from the top of the newt and put it on a SBS bar with a tak fs60 as a guider and still saw intermittent flex even at 120sec subs. I tried wrapping a strap around the tube and the guider and this reduced flex a lot but it is still there. I think those GSO rings with the single mounting bolts are just not good enough for big newts.
I thought I'd finish this off instead of starting another thread.
What we found was one leg of the spider had been stretched by 2mm. To offset this I added packing to the other leg to center it in the tube. However, this introduced enough slack in the spider to allow it to twist and this twist changed depending on the altitude and because the secondary was so heavy the twisting then distorted the tube which gave us crazy results.
Peter got a new spider for the newt. Straight away it collminated at elevation and started to work however there is still a flex problem. I stuffed a camera with an OAG down it's throat and the flex issue was gone so it was external. I then removed the guide scope from the top of the newt and put it on a SBS bar with a tak fs60 as a guider and still saw intermittent flex even at 120sec subs. I tried wrapping a strap around the tube and the guider and this reduced flex a lot but it is still there. I think those GSO rings with the single mounting bolts are just not good enough for big newts.
Here's a couple of snaps under a bright moon.
The Eta Carina shot seems pretty sharp. I can't see any elongated stars in it. The Omega Cent however is different - there does seem to be an elongation in the horizontal - or is it my astigmatism?
Peter
Yes it's there and it's in DEC. All subs are 120sec, I got none of this with the OAG, not even in 600sec subs. However, I can't run my oag with the mpcc or the wheel. Either the distances are too big or I run out of backfocus
Yes it's there and it's in DEC. All subs are 120sec, I got none of this with the OAG, not even in 600sec subs. However, I can't run my oag with the mpcc or the wheel. Either the distances are too big or I run out of backfocus
Well, my SX rig ought to work OK with the MPCC since I don't have to use any nosepieces on the camera to get the OAG parafocal with the CCD - it is designed to be right without it. I'll go ahead and replace those rubbish GSO rings I think - can't hurt anyway - and see what happens.
