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Old 18-05-2021, 05:39 PM
Graman (GR)
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Collimation humour...

Hi all,

I finally got around to collimate my Meade 8 inch SCT and thought would share my experience, a few questions and tips on what to avoid!

1. I have owned the scope for close to ~18 months and knew that the collimation was slightly off all along. I was nervous about touching the allen screws in the dark, so ordered Bobs Knobs. Installed them over the weekend, and it was relatively straightforward for a newbie.

2. I left the scope out for a couple of hours for it to cool down last night.

3. When I was ready, I strapped on the dew heater (I had a lot of trouble with dew last month watching the "super moon"). In hindsight, I don't think this is such a good idea. I think the dew heater creates internal thermal currents - so my out of focus star images were very wavy. Is my understanding correct? Should I skip the dew heater for collimation, esp. if humidity levels are low, as it is this week?

4. I figured tracking will help me center the star so I have more time to collimate the scope. So I went through the calibration procedure on AVX mount. I am not sure whether this is necessary. If I don't need GoTo functionality, can I just turn on the AVX and slew to a star manually? Will it then track from there? Or do I have to go through the 2-star alignment + calibration stars procedure to enable tracking?

5. After completing 2 star alignment (+ 2 calibration stars), I pointed to a bright star close to the zenith and started collimating. I really struggled to get the in-focus and out-of-focus star images in concentric circles. It was always a bit funny shaped. After close to an hour of frustration in the cold, I realised I had pointed to Regis Kantarous - a bloody double star! Talk about poor selection! I had a wry laugh, located Antares and went through the collimation process. This seemed to yield better results.

I think I have achieved coarse collimation. I could easily split Regis Kantarous by the end of the night. But I feel I can improve it further. Would appreciate any tips/advice!

Btw, what is the best EP focal length to do for collimation to get good results? I used 15mm, 10 mm and 6.7 mm EPs last night. I could get decent results with 15 and 10 mm, but 6.7 mm was just too wavy (may be because of the dew heater).

In summary, happy with progress so far. Bobs Knobs do make things easier. Dew heaters are a bit of a question mark. But most importantly, if you are new to this, avoid bloody double stars to do collimation!

Clear skies!
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  #2  
Old 18-05-2021, 06:45 PM
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What about using a double to know when you are done
Alex
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Old 18-05-2021, 10:40 PM
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Tulloch (Andrew)
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I did a similar thing when I was first starting out. Trying to get focus right on a star, but it came up with a double - took me ages to find out where the "reflection" was coming from, before I realised I was splitting a double
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Old 19-05-2021, 11:49 AM
Graman (GR)
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Yes, will definitely avoid double stars in future!

I managed to get out again last night to fine-tune collimation further. This time I didn't use the dew heater to prevent internal currents. It seemed to have helped a bit. Humidity was a bit higher (78% vs 45% the previous night), but managed to get about an hour outside, before the corrector started dewing up.

I focused on Hadar/Beta Centauri and primarily used the 15mm EP to do the collimation. I tried again with the 6.7mm EP, but found it a bit hard. Is this to be expected? When I read the collimation discussions on Cloudy Nights etc. they all suggest using high power EPs to do collimation. But I struggle with anything under 10 mm. The patterns are simply too wavy/all over the place to make any meaningful tweaks. What am I doing wrong?

After collimating, I had a good look at the Jewel Box (NGC 4755) nearby. Although I couldn't see anything via naked eye (thanks to suburban light pollution in Sydney), I could clear make out the stars via the telescope. There were dozens of stars all clearly resolved and I could easily make out the different colours as well - white, blue and a prominent yellow star in the middle. Is this about as good as it gets with a well collimated SCT? Or can I eek out a bit more performance?

Please let me know if you have any other tips to improve collimation. Looks like I will have one more clear night tonight in Sydney before the clouds roll in for the next week or so!
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Old 19-05-2021, 11:56 AM
Graman (GR)
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Also, what are some good targets to test how well is scope is collimated? Any ideas/suggestions?

Thanks and clear skies!
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Old 21-05-2021, 06:03 AM
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Are you using a dew shield or just a dew heater . I seen on you tube (can't remember the site now) where a guy done a test with a dew shield only and a dew shield plus heater strip and just a heater strip. The best result was just a dew shield to stop the dew forming . He also used a heat measuring gun which showed the outside of the front glass was about 3 degrees hotter than centre. I have just purchased a mak 180 just for planetary images, (yet to ve used) and decided to just go with a dew shield .
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Old 21-05-2021, 07:59 AM
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Avoid Antares, too, it’s also a double star that an 8” Meade should have no trouble splitting under good seeing. It will definitely distort the diffraction pattern.

Miaplacidus (beta Car) in the south is probably a good choice.
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Old 21-05-2021, 10:03 AM
Graman (GR)
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Initially, I used just the dew heater not the dew shield.

I found that attaching the dew shield made it a bit inconvenient to reach in and adjust the collimation knobs. So I removed it.

I will try with dew shield alone one of these days...when I fine tune the collimation further. Weather has taken a turn for worse in Sydney.

Quote:
Originally Posted by Kuz View Post
Are you using a dew shield or just a dew heater . I seen on you tube (can't remember the site now) where a guy done a test with a dew shield only and a dew shield plus heater strip and just a heater strip. The best result was just a dew shield to stop the dew forming . He also used a heat measuring gun which showed the outside of the front glass was about 3 degrees hotter than centre. I have just purchased a mak 180 just for planetary images, (yet to ve used) and decided to just go with a dew shield .
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Old 21-05-2021, 10:05 AM
Graman (GR)
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Good to know, I didn't realise Antares was a double star too. I might try to split it one of these days, to test out how good my collimation is

I ended up collimating on Hadar (which was high up on the horizon). If you have any other suggested targets to test my collimation, pls let me know.


Quote:
Originally Posted by Steffen View Post
Avoid Antares, too, its also a double star that an 8 Meade should have no trouble splitting under good seeing. It will definitely distort the diffraction pattern.

Miaplacidus (beta Car) in the south is probably a good choice.
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Old 21-05-2021, 07:32 PM
gb44 (Glenn)
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Also have a look at Collimation by Thierry. Theres good info on what mags to use.
http://www.astrophoto.fr/collim.html

GlennB
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Old 21-05-2021, 09:20 PM
Graman (GR)
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Thanks for that. I will take a look.

I also found the following guide very useful, especially on which collimation screws to adjust.

https://astromart.com/reviews-and-ar...ct-collimation
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