M51 - The Whirlpool Galaxy (34 min. exposure time)

[Hughsie]

I own an RC8 which has been a source of frustration for over 4 years. When I first bought the scope I tinkered with the collimation. What was I meant to do when a regular feature in the forum topics is the heading 'Ritchey-Chretien Collimation....', I thought everyone enjoyed collimating them. How wrong could I be.

So I tinkered so the collimation got bad, then tinkered some more until it was horrendous and finally when I thought I could tinker no more.....I tinkered again!

The RC8 was like a loveless marriage but we both needed each other. The RC8 needed me to be able to see again and I needed the RC8 for the challenge of getting the poor thing fixed. So, last week I stripped it down to its bare bones (no I didn't remove either mirror, I am not that stupid) and started again. Having gone backwards and forwards between aligning the primary mirror with the optical axis then adjusting the secondary and back again I reached the point where I was only making the tiniest of adjustments. Time for a star test!

Now, there is nothing I enjoy better than a dark night with two tiny Allen keys, a scope pointing at the Zenith, me laying under it figuring out which part of my varifocal lens glasses gives me the best focus and finding that I have just crushed two amorous snails. Seeing wasn't great and common sense said that I should wait for a better night but they are few and far between in the UK. I restricted myself to a loosening tweak here and a tightening tweak there. 

The out of focus star looked......good? Never a word I have used with the RC8. Anyway, I slew the scope to M51 as it was due to cross the Meridian to the South and rattled off 17 subs at 2 minutes each. I actually captured more calibration frames than lights so to be honest but I wasn't expecting much other than good looking flats.

Here is the result. As I say, it's just 34 minutes integration time and I was very surprised. Still, just a few more tweaks here and there and ..........

 

 

 

Edited April 30 by Hughsie

[Backyardscope]

The only way to learn is to tinker, tinker and tinker, and when you still can't get it right, have a rest, then tinker more. I often wonder about getting an RC for smaller object DSO imaging. That's one scope I have not tried yet!

Even so, a good image you have there, may I say.

[RobertI]

That looks excellent John, I think you might have finally nailed the collimation. 

[Hughsie]

26 minutes ago, RobertI said:

That looks excellent John, I think you might have finally nailed the collimation. 

Thank you Rob. Just have to check the secondary one last time then, fingers crossed, we get some clear nights.

[Rallemikken]

Very nice picture. I'm considering one of those for next season, most likely the StellaLyra 8" f/8. What camera did you use on this?

[Hughsie]

2 hours ago, Rallemikken said:

Very nice picture. I'm considering one of those for next season, most likely the StellaLyra 8" f/8. What camera did you use on this?

I used the ZWO ASI294MC camera with a UV/IR cut luminance filter. The camera sensor has a 4.63um pixel size which leads to a slight over-sampling. I have also used the ZWO ASI1600mm which has a smaller pixel size. When using this I will still use bin 1x1 and then resample the image to 2x2 when processing the data to see if there is an improvement or not.

The scope I used is the same one you are looking to purchase. Don’t be put off, the poor collimation was all my doing and a lack of understanding on my part on the mirror dynamics. Collimating these can be a challenge but it just requires a methodical approach and a star test. Worth researching online to see what method may best suit you. Having said that, once collimated they hold very well and probably need checking once a year.

The final thing I would add for you to consider is the stock focuser that comes with it. In summary it’s pants. Build into your budget the cost of a new focuser, the Baader Diamond Steeltrack is available through FLO along with the adapter to attach it to the RC8. I bought this and it’s a marked improvement. You pays your money and takes your choice when it comes to accessories but I would hold fire on things like flatteners, reducers and tilt adapters. This image was a bare bones camera/filter/native 1600mm image with the Baader DST. 
 

I hope this helps.

John

[RobertI]

Interesting to see the image was at the native F8, not bad for 34 minutes and better image scale too I guess? Just realised my RC6 is an F9, but the RC8 is an F8. Wonder why?

[Hughsie]

Morning Rob. Having run the image through nova.astrometry.net the image scale comes in at 0.589”/pixel. With the sensor pixel size at 4.63um this equates to a f/l of 1621 mm whereas the advertised f/l is 1600 mm. Having read numerous user reports on RC’s this is not uncommon for these scopes and if I was going to sweat on this I would be looking for a Ronchi eyepiece to verify if this is correct, too short or too long a f/l. But I’m not going to tinker down this rocky road 🙂

As to the focal ratio, I read somewhere that the Primary Mirror on an RC is somewhere in the f2 to f3 range and the secondary has a magnification of 2.5x to 3.5x. May be the difference is down to the specs of the mirrors.

But the main issue here is you only live 5 mins from me and it should have been you on the ground with the Allen keys and me watching the laptop screen!

[RobertI]

1 hour ago, Hughsie said:

But the main issue here is you only live 5 mins from me and it should have been you on the ground with the Allen keys and me watching the laptop screen!

Ha ha, well I’m more than happy to help out for the “final tweaks”, I might learn something for when I collimate my RC6!