Biggest Scope For A Dome

[Grant Privett]

A while back I bought an old Pulsar dome – the 2.1m version.

I have now cleaned it up, set it up and am preparing to put an NEQ6 mount on the pier.

I am wondering what the biggest Newtonian I can fit on it is. Some back of the envelope calcs suggest that a 300mm* f/4 Newtonian would just about fit (tube 1160mm long and 326mm wide – assumed balance point 1/3rd way along tube.) with a few cm to spare but, before buying, it would be nice to test this hypothesis. 

Does anyone know of a site with a 3D model to test this please?

Whatever happens its going to be snug.

*Bigger than 300mm would have been nice, but as a teenager a 300mm (12″ as I would have said then) would have seemed like dying and going to heaven, so I shall go with that if need be.

[Jeremy Shears]

A word of caution, Grant. I have that model and it is superb, but with one limitation. The slit is a bit narrow. They widened it in later models.

Consequence is that when the scope is pointing obliquely across the slit, it doesn’t take very long before you need to shift the dome round a bit. The wider the scope, the shorter the time. Can be a pain if you are doing a long time series run. I’ve long thought my C11 is at the practical limit. I wouldn’t like to use a 12-inch.

[Tim Haymes]

I agree with Jeremy.  I had considered exactly the same proposed setup as yourself, but concluded i wouldn’t fit in the dome with the scope 😉     However i suggest that  a motorised dome rotation would overcome the narrow slit difficultly. It works very well on my 2.7m Pulsar.  I align the aperture on the OTA, and select sidereal rate on the control box. The azimuth drive system can be retro fitted.

[Jeremy Shears]

A motorised Dome facility is not commercially available for the original 2.1m Pulsar dome. A few people have come up with diy solutions but way beyond my level of expertise.

[Grant Privett]

I can certainly see a way of doing it with a couple of stepper motors/drives, a Raspberry Pi and after a small amount of dome bodging. The metal bashing is the tough bit. I think I have a plan for that but I may have to do 10 minute adjustments until then, which are not fun in the winter.

I certainly take the point that 11″ would be better. I rather fancied the RASA but the field of view is perhaps a bit big unless what you want is pretty extended nebulae – which isn’t quite me. I don’t know anyone who makes an 11″ f/4-f/5 Newt and a 250mm took too long to reach mag 20 for my taste.

More important is rats attacked the dome last night and tried to gnaw there way in through my lovingly applied mastic… Ho hum.

[Tim Haymes]

I wasn’t aware there was an earthlier design. The 2.2m Pulsar dome is advertised to have the dome drive – apologies for the miss information.

[Grant Privett]

Not a problem. I made exactly the same mistake myself at one point. 🙂

[Peter Carson]

Hello Grant,
Until fairly recently I used a 315mm f/8 Dall Kirkham astrograph in my 2.1m dome. It was a bit snug!

Jeremy is right to draw your attention to the Pulsar narrow slit and viewing obliquely through it. This is a major concern if you have a German Equatorial mount with a large dec axis offset from the RA axis as it exaggerates the obliquity. 

My dome is over 30 years old and didn’t owe me anything so I took a saw to it and enlarged the slit to one metre wide. I fibreglassed some new slit aperture perimeter upstands and made a new aluminum shutter and it works a treat.

I don’t suggest you try this on a new Pulsar dome though.

Peter

 

[Grant Privett]

An f/8 315mm in a 2.1m dome? That really is tight. Did you add tube counterweights to balance at the midpoint? How frequently did you need to tweak the dome position?

My dome certainly isn’t new, but in pretty good condition, so would prefer to avoid major surgery.

I have a plan for the dome motor – it will probably need a couple of attempts to get it right, but a solution is possible. Finding the time is another. In the autumn perhaps when the gardening duties are slackening off is best bet.

I need to spend some quality time with a Raspberry Pi to and some NEMA 23 motors too.

EDIT: Someone kindly pointed out that the 315mm is a Planewave CDK and comparable in length with a C14. Very nice bit of kit, but a bit out of my budget range. 🙂 

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