GSO Ritchey-Chrétien and those fans...

[xthestreams]

Gday everyone. 
 

I’m thinking i know the answer to my question but now I’d like to know if I’m on the right track and ‘why?’, if that makes sense.

I recently purchased a 12” GSO RC, despite being both new to the hobby and having been warned the were a pile of poop by more than a few people who didn’t own one - but I loves me a challenge and hate mirror flop even more, so RC it was.

having been warned about Collimation I took a few image and noticed slightly rugby football shaped stars and assumed it was time to break out the Howie Glatter and have a crack. Over a period of nights I almost certainly made it worse and better to varying degrees but never got even close to round stars on anything other than the shortest of exposures.  

Analysis with CCDInspector at times indicted that I had a tilt problem but that if anything my Collimation was perfect - but the oval shaped stars persisted, oddly they were all in the same direction, roughly ata 30-45’ angle along my images...

I  assumed it might be down to drift but if anything guiding seemed to make matters worse and my PA was less than a few arc seconds out at best and the EQ8-R was barely challenged by the payload. So back  to the Glatter and starvtests and a growing sense that maybe I’d bitten off more than I could chew.

it was around a week ago that my Pegasus Ultimate Power Box turned up. I’m an automation and integration nut and one of my ideas was to hook up the cooling fans of the RC to the variable power either from the dew heater PWM outputs or the variable voltage out port, and it was this I decided to use. 

in my fiddling I had left the voltage out down around 3V, the fans had gone fro their hurricane like howl to something more sedate and having forgotten all about turning them back up again, proceeded to image away.

it wasn’t until I got an alarm saying I was close to the dew point that I thought”oh no the fans won’t be going fast enough tomprevent dew on the primary” that I realised all my stars had become much rounder than ever before. 

I now suspect the culprit was the OTAs cooling fans All along  and that I should only be using them prior to imaging to bring  the primary into rough equilibrium and then let nature do her thing. I had mistakenlyntho8ght the fans also kept he primary closer to ambient thus reducing the chance of dew forming on the primary, but given that I don’t know where that idea came from and GSOs non-existent documentation has nothing to say on the matter (I literally got a box with a telescope in it) I don’t feel stupid so much as really stupid.

I hope that if I am right in my analysis that I have helped someone and of not, that by being corrected/educated that your answers will not only help me but others as “gifted” as I am 🙂

I assume the answer applies equally to Newts, Dobs, SCT and others with fans.

 

Edited August 17, 2020 by xthestreams
Typos

[vlaiv]

That's how I would see it as well if I had scope with fans on primary (my RC8 does not have those, and probably does not need them for its size).

[xthestreams]

Thanks for giving me confidence! So I should be leaving them off during imaging? 

[vlaiv]

1 minute ago, xthestreams said:

Thanks for giving me confidence! So I should be leaving them off during imaging? 

As far as I know, these are just there to help cool the primary mirror. If you have your scope permanently mounted in observatory at ambient temperature, you probably don't need to use them at all.

[Jkulin]

On my RC10 I have the fans on from the start powered by my UPB, I have a Kendrick dew heater that I have fitted discreetly and I run that from 10-40 on the UPB scale depending on the forecast for dew. be very wary as a warm secondary can really wreck your images.

I have never had any dew on the mirrors or distorted stars from the air currents or fans.

The only thing I did as soon as I bought my RC was obtain a 3" R&P Feathertouch focuser as I previously had had an RC8 and the focuser was poor on that and I replaced that with a moonlite. For the RC10 IMHO it was the best £800 I had ever spent (Purchase+Shipping+taxes).

For the last 3+ years once I fit an OTA to my pier, its stays mounted and I cover it up after each session, until I got my dome. The worst thing you can do to an OTA is keep lifting it on and off, I have never had to adjust my collimation and that included my washing the mirror in situ.

HTH

[xthestreams]

Thanks again for your help John - congrats on the dome, you must be thrilled!

Thankfully my setup is on a "permanent" heavy duty tripod pier in my Nexdome - this is where everyone laughs - on the third floor balcony (so probably not the most vibration proof place for a long FL scope I know). 

I followed your (and Chris Woodhouse') lead and used copper tape to hide my secondary heater as well - right now I am running it on "auto" mode for a test but given it's the middle of the day and it's still running the heater I am guessing Auto isn't that brilliant (hardly surprised, but one can only hope and test).

Assuming per our earlier chat you're running the fans off the regular binary on/off 12V port, they will be moving some serious air so perhaps vibration isn't the problem???

Here are two images - excuse the quality of exposures, as mentioned I've been more concerned about debugging than winning an APOD. They're of different targets, I'll try and get in another of M83 if I can find a gap between the clouds. M83 is the "old collimation, fans running hard", NGC 6744 is my updated collimation with fans running lightly - no processing on either other than stacking - but the subs are all show the same artefacts. 

If memory serves M83 was unguided and 30 second subs to try and eliminate drift as the culprit.

 

 

NGC_6744A_Light_Luma_30_secs_001.fits

[Jkulin]

Hi Chris, like you with the RC I found that running on auto was not a good idea, my mate Peter Shah advised me when I was getting odd shaped stars that the cause was the secondary getting too warm, I now use the barest of warmth to stop dew on the secondary.

Indeed on the primary fans, I just run the power from a 12v output of the UPB, I did try to run it on a dew heater output and I found I had to bump it to full blast, so there was no benefit.

With regards to your stars all elongating NW, I would hazard a guess at two potential problems, tilt and/or Polar Alignment, it could also potentially be that you need to re-run a calibration in PHD.

Hope that helps?

[xthestreams]

Thanks John - CCDInspector suggested tilt, I'll look into a tilt adjuster as everything is screw connected.

As for the heating - good ip, I'll dial it back and see where the lower (versus upper) limit needs to be. I *do* wish GSO had integrated a heater and temp sensor into the secondary (and primary) so that we could keep it in the Goldilocks zone.

Final point, I WAS able to get the rans running nicely through the dew ports and even had a play with the variable port which did reduce the noise significantly - but as you are aware, it's one of the two ports that aren't able to be remotely controlled "on/off" so meant the fans had to run all day. 

Thanks again for everyone's replies - we RC folks need to stick together!