Baader RCC1 coma corrector and Baader Off Axis Guider

[gazza]

Hi,

just received a new Baader RCC1 Rowe Coma Corrector and Baader Off Axis Guider to use on my 8" f4 GSO Newt. Decided to get these to hopefully cure differential flex problems I had been having on very long exposures.

The coma corrector appears nicely made and the entrance lens is quite large - around 46mm or so. The Off Axis guider came in a Baader box, but is labeled on the guider body as a Celestron Radial Guider, and appears to be exactly that, but without the SCT connection bits. No instructions were provided with either unit.

Will post pics and test results as soon as the sky clears enough to try some test pics, as I'm sure I'm not the only one interested in this combo - especially since it allows a filter wheel to go after the guider, or the use of a DSLR with coma corrector/off axis guider due to its long (91.5) mm spacing. The guider is 36.5mm thick, so allows an extra 55mm to put a t-ringed DSLR at optimum spacing, or a CCD camera + filter wheel up to that thickness. Purchased from Alpine Astro in the US, with usual excellent service....

First trials were limited tonight. The Baader RCC1 coma corrector came to focus in my 8 inch f4 GSO Newt without having to shift any mirrors etc, though with only 4 mm or so in focus to spare. This contradicts Baader's own specs which say that the coma corrector extends focus by 5mm. I had been previously using a Baader MPCC as a coma corrector, and this is specified as extending focal point by 10mm, and it does this - when using it I have to add a spacer to push the corrector further out. This spacer is 35mm long, and the MPCC focus point ends up about 33mm further out than the RCC1. There have been reports of people having to move their primary so that the RCC1 could focus, however most of them appear to not have been using the correct spacing. Tonights test indicates that used per spec, (91.5 mm between coma corrector and camera chip) the corrector should focus without any need to mod a scope if your focal plane is in normal territory.

The Baader off axis guider would not focus with a QHY5, it requires a couple of mm more in focus. This necessitated an 11.00 pm visit to a friends engineering business for a bit of late night lathe work to turn down a 1 1/4" to t adapter so that the QHY5 could get some extra room to focus. Once reassembled both the Main camera (QHY8) and the QHY5 were able to achieve simultaneous focus. At this point the combined effects of a full moon and almost zero transparency meant calling it quits.....

more info to follow......

Gary

[gazza]

Hi,

after having to wait well over a week for a clear night where I was not working, I finally managed to test this combination on my 8" f4 GSO Newtonian. At this point I am unable to recommend the Rowe Coma corrector. It is incredibly (excessively?) sensitive to collimation. Prior to use I collimated the scope using a set of Catseye tools to get what I thought was perfect collimation. This level of collimation is sufficient that with the Baader MPCC I already owned, I get perfect stars in all four corners of a QHY8 frame. At no point was I able to achieve this with the Rowe corrector. At first I wondered if the extra length was causing focuser sag that was changing the effective collimation, so I tried imaging first on one side of the meridian, and then the other. However there was no change in the star shapes between frames indicating that the focuser was not shifting appreciably. At this point, I believe the only way to salvage my whole imaging system is to buy a set of the new dual hole Catseye tools to see if their increased accuracy cures the imaging problems. Not sure yet whether to do that as it may be good money after bad. Imaging with cheap newts seems very problematical, and I guess you get what you pay for..... Imaging with refractors is sooooo much easier and more relaxing.

The Off Axis Guider was much easier to use than I expected. I managed to get a guide star everywhere I pointed the scope using 3 second exposures on a QHY5 - not the most sensitive of guide cams. I was able to get round stars (in the center of field of course!) with a 30 minute exposure, so that at least works.

I haven't posted copies of images since there is nothing to really show off..... Buyer beware.

cheers

Gary

[rfdesigner]

hmmm

I've been trying to decide what camera sequence to go for and the RCC1 is on my list.

Have you got any further with your RCC1 yet?

Derek

[gazza]

Hi,

I kept trying but eventually gave up. The RCC1 is super sensitive to collimation and the GSO 8"f4 had flex in both the steel tube and focuser. My initial tests for flex were wrong, I did not move the scope enough. When I moved the scope more I found positions where the image would change, usually when the camera was sticking out further. My original tests were with the camera in a position parallel with the dec axis, and therefore less likely to shift.I would collimate the scope using Catseye Tools, but the weight of the camera was sufficient to change collimation through flexure enough so that the corrector gave abnormal images. I eventually sold the scope out of sheer frustration and gave the corrector on semi permanent loan to a friend. He at this point has had no luck with it on one of the new 10" f4 GSO units. These are so incapable of holding critical collimation that he is going to do a complete rebuild (he is an engineer with his own machine shop). I did have some correspondence with Alpine Astro about the corrector, and they subsequently changed their description of the RCC1. It was originally somewhat ambiguous, and if I had seen the modified text I would not have bought the corrector. Some people have got them to work, but they are also regularly coming up for sale secondhand from other users. Stick to the MPCC, I should have, it is sensitive to collimation as well, but NOWHERE near the RCC1. If you really want one, I have a mint one I'll let go cheap :-)

cheers

Gary

PS I now image with a refractor, and I love it!!!

[rfdesigner]

Thanks for the feedback

No wonder the super-newts are getting Carbon Fibre tubes.. Seeing as my scope is already modded quite heavily I have no fear in modding it to stiffen the focusser if needed.

Next question... how does one measure scope flexture?

Derek

[gazza]

Have a look through this thread about the travails of a 12" GSO f4 newt. On the third page is a link to a youtube video after tube strengthening

A GSO 12" F4 with 3" focuser - IceInSpace

cheers

Gary

[rfdesigner]

Thanks gazza

Just for reference, how does that compare with your OTA (that you tried the RCC1 on)

Derek

[gazza]

Derek,

the scope I had was an 8" f4 GSO newt. It was the model with a 70mm secondary and 10"1 focuser. This scope is a baby brother to the one in the video. BTW I had a conversation with the friend I lent the RCC1 to tonight as he is having problems collimating his new GSO 10" f4 newt. It seems he has a problem with the 3"linear focuser. It seems that the focuser easily sags under the weight of a Camera (in this case a QHY9 + QHY filter wheel.) Using the locking screw to hold the focuser in place changes the collimation! These "rigid" linear focusers appear to be a very poor replacement for a conventional Crayford....

cheers

Gary

[rfdesigner]

Just as well my scope has a crayford... but then I did buy the focusser with photography in mind.

That thread you linked to is taking about focusser shift being the root of most of the problems.. I suspect I'd need to add tube stiffness although the tube I have is fairly strong (hey it's lasted about 40 years so far)

I'll try and find out just how stiff things need to be.

thanks for the info, very useful.

Derek