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Classical cassegrain for visual double stars ..


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Good morning lovely people I have a Dilemma ….

now over the past year or so I’ve been extremely interested in double star observing and more interestingly I’ve been measuring double stars .

I’m part of the Webb deep sky society and as such bought Bob argyles book about the subject .

now I’ve been using a 90/1000 achromat and a Meade astrometric eyepiece with 3x Barlow to achieve the required focal length of 3000mm 

this past year has been training and getting used to the setup measurements angular measurement and of course the calibration of the setup for this work . Now here is my dilemma … to realistically make an impact on the WDS list I need bigger apeture to be able to measure fainter companions and sets im

not looking at buying now but in the near future possibly, now up until this point I assumed that anything with a central obstruction would be a no mo for serious measurements as it may not perform how I want it to and won’t get a second chance at buying something decent .

I will have a budget of around £500 I think and recently went looking down the CAT route as a possible serious alternative being that the increased apeture vs price on a car compared to a refractor (which I love ) may be an option I have been looking at the SL 6” F12 from our guys at FLO and people seem to really love this as it’s different from a mak or SCT as it doesn’t have a front miniscus or lens element I wondered if anyone with one of these classical cassegrains who perhaps (clutching at straws here) loves double stars could give me his or her opinion on them as I’ve never looked through or owned a compound scope how sharp are they ? Is the CC diffraction spikes noticeable ? Thoughts and opinions please guys and gals I’m kind of going round in circles here the bigger the apeture the more targets I will have open to me to resolve but I want to buy new pref from Flo and the weight needs to be under 6kg for my current and prospective new mount any ideas ?? 
thanks for your time with this needy post 

clear skies Phill 

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I'd perhaps throw a few other scopes into the mix

How about something like a 180mm SW Maksutov ? Maks are good at double star viewing with a small CO.

Double the aperture you have and close to that 3 metre focal length in a fairly compact tube.

And you should be able to pick up a good used one for your budget.

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15 minutes ago, Space Hopper said:

I'd perhaps throw a few other scopes into the mix

How about something like a 180mm SW Maksutov ? Maks are good at double star viewing with a small CO.

Double the aperture you have and close to that 3 metre focal length in a fairly compact tube.

And you should be able to pick up a good used one for your budget.

Perhaps that is another option although I don’t want to go down the used route for this I would like new as it will be with me for a long perhaps indefinite period just for this task 🙂

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I have the GSO 8" Classical Cassegrain @Moon-Monkey though its actually 185mm in reality. These are my ad hoc observations in case they're of any use.

Resolution for this aperture in theory is 0.62" (Dawes) and 0.74" (Rayleigh) while my "personal best" so far with this scope is 1.6" (with a 0.5 Mag. difference between components). I haven't deliberately tried to find my tightest possible split on the best nights but i assume it would be better than 1.6" if i did because on a couple of great nights i could have driven a bus through ~2" splits.

Limiting magnitude for this scope is in the range 13.8 to 14.05 depending on formula while my "personal best" dimmest companion so far with this scope is Mag. 11.9. I have a dodgy observation where the secondary was Mag. 14.0 but i this has a big asterix against it in my notes because i'm not at all positive of the observation and need to go back to it.

Yes you do get diffraction spikes on bright stars with a CC. I've never checked at what magnitude they are not visible. i have recent notes that they were quite strong (though not unpleasant or obstructive to the observation) on the primary of Beta Cephei at Mag. 3.2. I'm not conscious of them on the regularly observed double double at Mag. 5 so somewhere between Mag 3 and Mag 5 they must disappear.

The stock focuser on the GSO scope never had image shift though i have upgraded it for other reasons.

Cheers

Joe

Edited by josefk
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Hey Joe that’s brilliant to hear thanks buddy it’s really interesting to hear that the diffraction spikes clear at somewhere between mag 3-5 I was wondering if they would be visible through the range into the faint stuff really good to know they don’t as that’s what I was concerned about (not being able to rotate the OTA being a fixed dovetail) good to hear from someone who has a used and continues to do so thaanks buddy iM thinking of the smaller one I hope the same things apply 🙂🙂 

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No worries at all.

It's also just occurred to me you could check for yourself on diffraction spikes and if they will be impactful or not for you by stretching some thin (but not too thin) wire crossing at 90 degrees in front of the objective of one of your refractors. You could then systematically target a few increasingly dim stars at a magnification that would be meaningful for your musings.  

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