tim.stephens
14-11-2015, 07:53 PM
Hi everyone,
Thought I'd share some preliminary results collimating my new RC8, perhaps it could help someone in a similar situation in future. This would be the first time I've collimated a reflecting telescope, quite an adventure but not insurmountable with a little patience and effort. I read everything I possibly could up to this point on the topic and received some great advice from Glen (glend) and the supplier here in NZ - many thanks to both.
It's not perfect but probably fairly close, enough for my purposes at least. All I used was a basic cheshire to align the primary and secondary, the hall-of-mirrors effect and a defocused star test for some final adjustment. The trick to getting the primary aligned was to remove the primary baffle tube so the edges of the secondary baffle can be seen directly.
Both the primary and secondary were seriously out of alignment when I tested it out of the box. I started aligning the secondary to limited success and could not achieve a good hall-of-mirrors without having the secondary spot well off centre, the central and corner stars well out of balance on the camera.
The conclusion was the primary probably needed adjustment. I removed the primary baffle by reaching in past the spider veins, and just let it sit gently on the inner tube baffles. Some recommend to remove the spider and secondary completely from the OTA but I found this unnecessary and risky given I wasn't sure the critical mirror spacing distance would not be maintained on reassembly.
It was an iterative process to firstly align the secondary spot with the cheshire and hall-of-mirrors, then the primary to align the secondary baffle reflections, then back to the secondary again. Repeated this about 8 times before I was reasonably happy with the result. Something I noticed was that the number and clarity of the internal reflections when viewing from the front was increased substantially. Perhaps a sign that the collimation process was successful. You have to get up close to the secondary to view and rotate the tube so that your head doesn't block the scope plenty of light enters.
The primary screws require very very small adjustments as many have noted. Also, it seemed to be easier to adjust them together rather than undoing the small locking screw first. If you do this, then tighten the locking screw, the collimation is sometimes ruined and you have to back out the silver push-pull screw and start again. Irritating.
In the end, it came down to a star test and a final tweak of the secondary. Still not 100% and could be improved but pretty close. I'm amazed at how flat the field is with an APS-C sized sensor and was expecting more edge curvature. Tracking on the HEQ5 was spot-on for this photo.
So... I suppose you all will be expecting some photography in the near future?? Well, I hate to disappoint but the cloudy weather has moved in to New Plymouth for the next few days!!!
Thanks everyone.
Tim
edit: added the image used for ccd inspector. One sub of the helix nebula.
Thought I'd share some preliminary results collimating my new RC8, perhaps it could help someone in a similar situation in future. This would be the first time I've collimated a reflecting telescope, quite an adventure but not insurmountable with a little patience and effort. I read everything I possibly could up to this point on the topic and received some great advice from Glen (glend) and the supplier here in NZ - many thanks to both.
It's not perfect but probably fairly close, enough for my purposes at least. All I used was a basic cheshire to align the primary and secondary, the hall-of-mirrors effect and a defocused star test for some final adjustment. The trick to getting the primary aligned was to remove the primary baffle tube so the edges of the secondary baffle can be seen directly.
Both the primary and secondary were seriously out of alignment when I tested it out of the box. I started aligning the secondary to limited success and could not achieve a good hall-of-mirrors without having the secondary spot well off centre, the central and corner stars well out of balance on the camera.
The conclusion was the primary probably needed adjustment. I removed the primary baffle by reaching in past the spider veins, and just let it sit gently on the inner tube baffles. Some recommend to remove the spider and secondary completely from the OTA but I found this unnecessary and risky given I wasn't sure the critical mirror spacing distance would not be maintained on reassembly.
It was an iterative process to firstly align the secondary spot with the cheshire and hall-of-mirrors, then the primary to align the secondary baffle reflections, then back to the secondary again. Repeated this about 8 times before I was reasonably happy with the result. Something I noticed was that the number and clarity of the internal reflections when viewing from the front was increased substantially. Perhaps a sign that the collimation process was successful. You have to get up close to the secondary to view and rotate the tube so that your head doesn't block the scope plenty of light enters.
The primary screws require very very small adjustments as many have noted. Also, it seemed to be easier to adjust them together rather than undoing the small locking screw first. If you do this, then tighten the locking screw, the collimation is sometimes ruined and you have to back out the silver push-pull screw and start again. Irritating.
In the end, it came down to a star test and a final tweak of the secondary. Still not 100% and could be improved but pretty close. I'm amazed at how flat the field is with an APS-C sized sensor and was expecting more edge curvature. Tracking on the HEQ5 was spot-on for this photo.
So... I suppose you all will be expecting some photography in the near future?? Well, I hate to disappoint but the cloudy weather has moved in to New Plymouth for the next few days!!!
Thanks everyone.
Tim
edit: added the image used for ccd inspector. One sub of the helix nebula.