Altair RC250 TT

[BlueAstra]

So went down to IAS last week for a look around. Strict mental instructions not to buy anything. However, still had a hankering for a longer focal length (a hangover from my C8 SCT days) so was very attracted by the RC's on the Altair stand. IK had some PlaneWave ODK's, but they were really expensive. My thoughts were to have a standard long focal length RC for high mag CCD imaging, mount my short tube refractor on it for wide field DSLR imaging, swop out the CCD and barlow the RC for some planetary DMK video stuff. Everything on one mount, choose your weapon depending on target, weather or conditions. Well that's the theory, we have to see if it works out, especially the planetary.

After much haggling and deliberation, went for the above RC. Took it home from the show, now in the process of commissioning. Build quality is very good. I've made a bracket for the motor focuser and red dot finder. I also made one for the Meade ED80, which helpfully clamps onto the top dovetail. Hope to have it in the observatory next week.

[BlueAstra]

I decided to mount the motor focuser from my existing scope rather than get a new one. I got a piece of Al and made a small plate. The plate had to be shaped to go round the curved bottom of the focuser. Drilled two holes and screwed it onto the bottom of the focuser. Had to put a spacer between the plate and focuser at one end to stop it tilting as you tightened up the screws. The focuser then just clamps to the plate with grub screws.

For the red dot finder I took off the supplied bracket, drilled& countersunk two new holes in a white dovetail holder then re attached it to the frame.

I've used double sided Velcro to attach the cloud sensor.

My first effort at focussing took some time. I didn't realise the back focus was so long. I eventually found a reference to back focal distances: 280mm from rear plate, 170mm from focuser. My OAG+FW+Camera has a focus depth of about 60mm so I found I needed the 50mm and 25mm spacers to achieve a mid-range focus on a distant pylon.

Its now on the mount in the obs, and I'm now working on dew heaters and cables. Hopefully next week I'll get some clear intervals for alignment. My master plan to mount the other scope has had to be put on hold since I will need another counter weight to balance it. Otherwise balancing has been a doddle, much more symmetric then the Newtonian.

[Steve-G]

If you listen very carefully you can hear all the jaws dropping. Envious? Oh yes, very much so.

[ollypenrice]

This is an interesting scope. Looking forward to hearing more about it.

Olly

[iksose7]

Envious indeed! Congrats on the purchase. Look forward to seeing what sort of images it will produce.

[Dave_D]

seriously nice scope

[ChrisLX200]

Looks an excellent scope and I guess that means yet more cloud for us   I hope you aren't set up too close to Manchester with that!

ChrisH

[Dave_D]

Looks an excellent scope and I guess that means yet more cloud for us I hope you aren't set up too close to Manchester with that!

ChrisH

Awww hell... i just saw where he is

Explains why there's a column of cloud moving due south over us lol

[ChrisLX200]

I just looked at the PlaneWave ODKs on the IK website, 'Expensive' is an understatement. Sometime next year I will be looking to replace my elderly 10" LX200 but a PlaneWave isn't in the running I think... The Altair 12" might be though.

ChrisH

[Dave_D]

I just looked at the PlaneWave ODKs on the IK website, 'Expensive' is an understatement. Sometime next year I will be looking to replace my elderly 10" LX200 but a PlaneWave isn't in the running I think... The Altair 12" might be though.

ChrisH

don't... you... dare...

[BlueAstra]

I've now added the dew heater control box to the back of the scope and glued on the split ring external secondary heater to the back of the secondary cover. Last night was a good chance to check focus, and I found the focus was a good 16mm closer than the original pylon focus (pylon 24mm focus extension, star 8mm extension). May need to consider removing a 25mm tube extension to get the focuser mid range but will check focus for the DMK option first. This may well need its own extension tube. Here is first light, no flats, no darks, heavy gradient removal and noise reduction, M3 @ 8x60s RGB:

[triton1]

Great looking scope and great pic by the way

[BlueAstra]

I have some more images of the setup. This is a picture of the focuser:

Its OK, but the jury is out on whether it can reliably handle a heavy imaging load. May need to upgrade this part. Crayford or R&P? The focuser interface is an external thread 120mm diameter, with a female chamfered ring to press up against. The focuser has a male chamfer and a floating internal threaded ring to clamp up the interface.

I also glued a Kendrick secondary heater strap to the secondary, and copper strips to take power to it without affecting the diffraction from the spider. Still need to solder the wires onto the strips.

[jaspalchadha]

One of a few images I've taken with this scope.

[derrickf]

Jaspalchadha,

Thanks for posting these images - I'm considering buying one of these scopes so it's good to see what others are producing with them.

The image of M27 is very good with reasonably well shaped stars but the stars in the albireo(?) image are distinctly triangular in shape. Did something change in the configuration between the two images that might explain the difference?

Regards

Derrick

[julianr]

Nice write up Graham and great that you have got first light. Lots of good work done there

Sent from my GT-I9300 using Xparent Skyblue Tapatalk 2

[SnakeyJ]

I also glued a Kendrick secondary heater strap to the secondary, and copper strips to take power to it without affecting the diffraction from the spider. Still need to solder the wires onto the strips.

Great idea for powering the secondary heater via the copper strips - thanks for sharing this tip

[jaspalchadha]

Derrickf

I think focus was out, mount not PA well enough in both images and few other things I cant remember ive got a Tak collimator and noticed immediately its out of collimation

[derrickf]

Derrickf

I think focus was out, mount not PA well enough in both images and few other things I cant remember ive got a Tak collimator and noticed immediately its out of collimation

Thanks,

do you think it "fell out" of collimation because of some mechanical design flaw or did you do some tweaking or make changes to affect collimation?

[jaspalchadha]

I've manged to bang the scope few times..

Hard

[snowy911]

Having very recently purchased a similar setup, 250tt -  moonlite focuser - SXOAG - SX filter wheel - ATIK 383, could you shed some light on how many of the spacers you ended up using to attain focus?  Got it out for the first time last night and struggled to find focus. Takes a bit of fiddling when you had to add or remove a space, including rebalancing, etc.

Tried the suggested 280mm above but didn't seem to find it, although could quite easily be user error!

thanks

[Rockenrock]

That GSO 3.3" focuser is pretty good for medium to light payloads, or visual, but mine could not handle at zenith the weight of SBIG Camera+ filter wheel +AO. For manual focusing near zenith, the fine adjustment knob slips (if rotating knob in up direction) with this weight. I could not focus accurately because I had to use the coarse focus knob. My technique was to use coarse adjustement to be a little above focus position, then gradually turn the fine adjustment down until focus (while watching the star brightness readout from camera). If go past perfect focus, then have to go up again with coarse knob, and down again. The locking knob must be extremely tight to just not slip down after focusing. I would spend 10 to 20 minutes trying to get it focused, then tighten the lock knob, and it goes out of focus some becuause it is pushing on the draw tube. In horizontal position, they have the entire weight of your imaging train hanging on one linear bearing which has a rail (silver piece on bottom of draw tube) that is .55" x .21" cross section. This bearing slides very nice with no play in it, but I don't want to trust my camera/filter wheel on that little rail. And it can bend with the weight.

On the positive side, the focuser has a large dia draw tube and body. The linear bearing is smooth. With visual use, or with DSLR useage the focusing was smooth and reliable in my experience. Is it the right focuser for those situations IMO.

Thanks,

Roger 

in Hong Kong

[jaspalchadha]

I changed mine into a feather touch

Sent from my iPhone using Tapatalk

[DrRobin]

Hi,

Nice report on your experiences of a RC250-TT. I have been thinking about one of these for a while, but have a few questions, if you don't mind passing on a bit of knowledge?

Firstly my obs is short of space. It currently has a C11Edge mounted on a single fork mount, but only just fits, although I have since discovered that the centre of the mount is not quite in the centre of the dome, this will gain somewhere close to 60~80mm and if I am careful about height, I could probably gain another 20~30mm, a bit nip and tuck, but comparing the external size of a C11 and the RC it should fit.

I measured one at Astrofest, from the centre of the mounting plate to the front of the scope it was 500mm, is this about right? If it was any further forwards I am not sure it would fit, or at least I would need to sync the slew of the scope with the opening of the obs to gain a little extra clearance. I suppose I could weight it, but that will add to the weight to the scope and I am going to be close to the 20kg max imaging weight of the mount.

Next at F8 it is still too long for what I want to achieve. I currently use a 0.63 FR on my C11 to get down to F6.3, or there abouts, but it gives a relatively small imaging circle, about 23mm. I don't suppose you have tried a focal reducer have you? Astro Physics make the CCDT67 0.67x FR which is suited to the GSO RC (29mm imaging circle) so should be okay, don't suppose you have seen anyone using one?

Regarding the focuser, I currently have a Moonlight, I think it is a Celestron 3.25" thread, do you know if this would fit, or do I need a different adapter? My Moonlight is motorised/stepper and I really need remote focusing, so it would be easier to swap the Moonlite over.

Sorry for all the questions, thanks in advance.

Robin

Next back focus, can you confirm it is 280mm from the back of the scope?

[BlueAstra]

Here's a picture of my original back focus setup with the stock focuser and tilt adjuster:

It comes to about 275 mm (the 70mm camera distance is an estimate), so the original estimate of 280 mm is close. Regarding the focuser, there is a male thread interface on the scope with a chamfered mating surface (see the earlier picture of the focuser removed). The female thread on the clamping ring engages the male and pulls the focuser into contact, then clamps it tight. I upgraded the focuser with the Altair Astro recommended focuser. I also got the Revelation (GSO) 0.75x FR from Telescope House, but I haven't used it yet.

Graham