12 December 2017 at 9:49 pm #573904
I’m preparing to use an 11″ Celestron RASA (as in balanced on mount today) but as I am normally a Newtonian or RC observer, I wondered what sort of dew heater strap or shield is generally used by people using SCTs of this sort of aperture?
Am thinking of an 8″ long polystyrene shield (black lined) and a standard Kendrick 11″ heating strap. Any thoughts?
Also, I am toying with the idea of co-mounting this with a standard C11. Does anyone make brackets that could handle that? Was intending to ask round at Astrofest.13 December 2017 at 11:30 am #578852Jeremy ShearsParticipant
I use a standard dew heater band on my C11, near the corrector plate. I used to have a dew shield as well, but it was a pain as I kept dislodging it as I moved around the obsy. Moreover, if I needed to cap the telescope during breaks in observing, or to produce CCD dark frames, I had to take the shield off, put the cap on and then reverse the operation to start observing again. Too much of a hassle!
I look forward to seeing your results with the RASA.
Jeremy13 December 2017 at 11:56 am #578853Dr James DawsonParticipant
I use a dew shield with a built in dew strap on my C11. I can’t remember where I got it from, but looking online it seems they cost in the region of £100 now!
No idea about the dual mounting for such big [heavy] scopes, but clearly can be done:
James13 December 2017 at 1:39 pm #578855
Have seen some C11 size heaters for £50 but it didnt quote the power…
Useful webpage! Followed a link on that site and have asked them for a quote on the co-mounting bars.13 December 2017 at 1:40 pm #578854
Will post something here once I have it. Its going to be a stupidly large field of view compared to the f8 Altair RC.
From your picture you use a dome. Does that save you some of the dewing grief?13 December 2017 at 2:05 pm #578856Jeremy ShearsParticipant
possibly, Grant. But the corrector plate is obviously still exposed as it looks out of the dome slit.
Jeremy13 December 2017 at 2:35 pm #578857Andrew SmithParticipant
Grant, I suspect the bracket will be the least of your troubles with a dual mount. That is a big load for an EQ6 if that is the mount you intend using.
Regards Andrew13 December 2017 at 3:19 pm #578858
The C11 RASA and C11 SCT come in at about 32kg, with CCD and filter wheel, call it 35kg. Allowing 50% margin, ~50kg. Thats less than half the system rating of 109kg. Am hoping we are okay, though I would like to be able to slew at 1 deg/sec to keep up with things like the ISS.
A Planewave L600 would be nice but without a Lottery win…..13 December 2017 at 10:28 pm #578859Callum PotterKeymaster
W&W sell dew heaters, and have attended our B2B workshops and other meetings, and were at the IAS.
Website is https://dewheater.com
No connection except I bought one for a camera lens.
Callum14 December 2017 at 10:13 am #578860
I’ve not come across that company before. Prices seem reasonable/good.12 February 2018 at 1:25 pm #579108
Finally got some quality time with the Celestron 11″ RASA. The attached image is M81 and M82 – though I rather suspect you all knew that. I used a venerable Starlight H18 for 15 unguided 60s frames last night. The focus was a little off – my fault not the scope – and also the camera body is not properly aligned with the focal plane after some changes made to ensure it worked with a Canon lens adaptor. So, this pretty much suggests that someone doing the job properly will get quite a nice result.
I liked the focuser, the quality of the images and the speed of the system, but it has made it very clear I need to sort out the cable mess I have before I go much further. I’m not used to wires coming of the scope front – not after using a RC for a while.
I tried deleting one of the images but lost both, so two it is…12 February 2018 at 7:46 pm #579112Nick JamesParticipant
Grant – Nice. This Celestron astrograph seems to perform very well at f/2.2 although your H18 isn’t that much of a test for a thing that is supposed to have a 40mm diameter well corrected focal plane! Are you going to try it with a bigger sensor? I’d certainly be interested to see how it performs in real life with a full-frame (36x24mm) sensor.12 February 2018 at 9:19 pm #579117
A friend has tried a Canon 450 on it but nothing bigger yet. The Starlight 56 has too big a body – the issue being the shutter. The Starlight 25 would be a nice start or better still a 35. However, I don’t have access to anything bigger and so cannot oblige. Anyone want to lend me one? 🙂
I think Celestron have said that wings of the focal plane should run off to 60-70% of peak. The 14″ version is better controlled and is corrected better for wavelengths beyond 800nm.14 February 2018 at 11:24 pm #579126
Just an update: I have been out with the RASA on two nights so far – 11th and 13th – and not yet had trouble with dew forming on the corrector plate – even in the absence of a shield. Hiwever, neither was a wet night (it was -1C and -5C respectively) but there were hard frosts (really hard last night). Will update again when I have used it on a night when water is running off the mount or pier.
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