GSO 10" RC baffling and flats

Same problem here with my 2024-bought RC12.  The 1ary mirror baffle is simply not long enough to prevent light from outside the field getting into the beam.  There is a great YouTube video somewhere (I can't remember the link) explaining this in detail.  

I have been the otherwise happy owner of a GSO RC8 and now RC12 since 2020.  My RC8 suffered from the same problem, and so when I purchased my RC12, I asked the shop I bought it from whether this problem had been fixed.  I was assured by the shop that it had been [advice directly from GSO].

This turned out to be incorrect, and I had to purchase an extension to the baffle sold by TelescopeExpress.   It worked and it is simple to install.  It doesn't affect the collimation, and so I take it in and out each night. [Otherwise the 1ary mirror cover wouldn't go back on.  GSO sent me a replacement machined baffle [funny how they had one ready to go…] but I don't use it as the 1ary mirror cover wouldn't fit.

CS Brian

Unlikely or not, it is certainly my experience. 

Before the baffle, I was getting terrible scattered light into the frame.  It showed up as broad arcs of light - like a sector of a very large out-of-focus doughnut across the sensor.  After the baffle - nothing.  I bought but didn't use the tube sock.

The frames are now flatfield-able.  I use flats from the sky.  I never use anything else - otherwise you are asking for trouble. 

You can check out my AB page to see the final results with this set-up.  I am really pleased with images from this set-up, indeed 90% of my images have been lucky enough to be nominated for - or received - Top Pick status.

I thought this issue was solved quite some time ago. But apparently it still shows up. A polish user did not have the top baffle either on her 6' GSO RC she recently obtained. Stray light comes in and spoils the images , and cannot be corrected with flats.
Fortunately mine recently obtained 8" RC from TS, has a top baffle and does not show any issues (at least not sofar)
And even the length of those top baffles seems to be different. I can't pass mine through the spider. But an US user can with his RC 8". Bit weird all this….

Yes my RC8 (purchased 2020) did not show this problem, but my RC12 (purchased 2024) did.

I find it very telling that GSO sent me a longer, machined 1ary baffle within a day of my vendor raising it has an issue.  If GSO maintain this is a mirror separation problem: 1) how come they had a manufactured longer baffle "ready to go" and  2) how did they know how long it needed to be?

I would just use the baffle to see if it works, before trying to change mirror distance….

CS Brian

GSO was so nice to send me the 2D design of the RC10. See the PFL above in this thread. There it can be seen that the BWD needs to be 239.8mm.

It should be easy to understand that when the BWD is much lower, there will be stray light problems. If the BWD is too large, there will excessive vignetting.

The BWD is changed by adjusting the mirror distance. According to GSO when the BWD is not 239.8mm, this is wrong and the mirror distance needs to be adjusted. This is a very clear statement I have received by them.

On the practical side, as I already said, my BWD was below 210mm. I have adjusted the mirror distance to get closer to 240mm. Afterwards I did an accurate re-collimation and tilt correction for my sensor. The resulting test images showed round stars up to the edges of my full-format ASI 6200MM sensor with a small pixel size of 3.76mm. Star sizes were uniform across the image about 2", which is seeing-limited. So there is nothing wrong with adjusting the mirror distance when its necessary. Its logical and consistent with what GSO is recommending, and the results are good.

I havent had the chance to test for stray light and how flat fielding works. Waiting for clear weather…

And of course: nobody is supposed to change the mirror distance when the pics are fine, with of without a baffle. But when a baffle is necessary for flat fielding to work and the telescope is younger than 3 years, it might be worth checking the BWD and considering to change the mirror distance. In my case, TS offered to recollimate the telescope for free if my attempts should fail, which was a nice backup, but finally not necessary.

It is always difficult to do something against the main stream opinion (at least for me), and I have to admit that it caused significant stress to get bashed by various users on CN for going against a public no-go of adjusting the mirror distance. Yet, GSO did agree with my approach, it was a valuable experience and results are worth it.

The PDF sent to me by GSO (RC10A-2DL_Model.pdf) shows that the BDW  should be 239.8mm measured from the rear plate of the RC.

Götz Golla:
The PDF sent to me by GSO (RC10A-2DL_Model.pdf) shows that the BDW  should be 239.8mm measured from the rear plate of the RC.

For some nominal mirror placement, I don't doubt that.  The problem is neither mirror is fixed in place.  Understand that it's possible to move the primary mirror a few mm in or out on the collimation screws, just as it's possible to move the secondary mirror in or out significantly on the mirror spacing adjustment.  The mirrors can be set at 463.6mm spacing but have more or less than 239.8mm to the rear plate.  That's the primary reason why specifying 239.8mm behind the rear plate is wrong.  

It's even more complex when you take off nominal mirror spacing into account.  The relationship between mirror spacing and focal length is about 10 to 1, meaning that a 1mm error in mirror spacing is 10mm in focal length.  Since my 10" RC out to about 2008mm focal length, my mirror spacing is about 0.8mm too long, putting its focal plane at about 248mm without a reducer.  

Adding a reducer further complicates this since it shortens the focal length by the difference between its focal length and working distance.  My TSRCRED has an 82mm BWD and focal length of 102.5mm, so it shortens my BWD by 20.5mm.  From the nominal 239.8mm, this is 219.4mm nominal BWD with the reducer.  Off nominal mirror spacing will skew this offset as well, though I'm not sure how to determine what this would be.  

As an example of this cumulative error, my 10" GSO truss tube RC comes to focus 215mm behind the rear plate, or 4.4mm shorter than that drawing says it should be.  My advice is to use 239.8mm as only a reference, not truth.  You'll be close but give yourself a tolerance either side of that number.  If you're paranoid about correctly setting mirror spacing, get a proper truth source.  Either plate solve against a dense starfield or use a 10L/mm Ronchi Eyepiece.  Otherwise, if you're not seeing any spherical aberration, don't worry about it too much.
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Edited 1st paragraph for clarity

Brian Boyle:
GSO sent me a replacement machined baffle

***Hello, I just found your post and would like to know what you did to get GSO to send you the tube. I have e-mailed them twice through their web page but never got a response. ***

Increasing distance between the mirrors decreases focal length, and it's about 1mm of adjustment for 10mm of focal length. However, I wouldn't touch mirror spacing for a 4mm error. Instead I'd look for spherical aberration under the stars.

Buckey Eyes Stargazer sells a custom 3D printed adapter that extends the baffle and stops direct light entering the focuser.

only reflected light should enter the focuser. Buckey has a web site, and also sells through Agena I believe. 

I feel one of the problems is that people who dont really know what they are doing TINKER with their RCs, and when they still cant make them work properly, they sell them off.

then the next guy buys something with incorrect mirror spacing etc etc…

Back on topic, I had issues with flats, and bright circular rings, and after adding Buckeys adapter things looked good again.