Sky Rover 130mm - First Light(s)

[Atmos]

I guess I cannot really say that it is first light strictly speaking, had it out a number of times before Sunday evening but it was my first actual attempt at imaging with this telescope. The first image is pretty obvious, 20 mins of RGB and most definitely overcooked. Not sure why but a LOT of green came through the channels. It gives it a nice golden colour (which I do love btw) but most definitely not natural.

The following image is just calibrated and stretched, Tarantula Nebula. 33 mins of RGB. Then there is a cropped and stretched 47 Tuc which is currently 60 mins of RGB but I know I'll have to ditch near half of them due to some strange artefact.

When I first ordered the whole setup I was hoping that the standard nose piece that came with it would be within 0.5mm of the back focus of the focal reducer. The first 5s image that I got gave the mosaic which is the following image. This of course lead me going back to the supplier who in turn went back to the reducer manufacturer and got the REAL back focus from thread to CCD which just happened to be near 11mm shorter than the nose piece.

So, did all of the necessary calculations and ordered a 23.8mm spacer from Precise Parts. That arrived late last week and with a clear night the following evening I did a little testing. Found that I was getting ~25% curvature still, although the images looked pretty flat. Went and ran a bunch of individual images through CCDInspector after I'd finished packing everything up, found that every frame was a bit different in the results it was giving. Of course it wasn't until I read the manual that I came to realise that I could actually do batch tests :P

So from the above I do totally agree with what Greg Bradley has mentioned, CCDInspector is a good indication of things but it all has to be taken with a grain of salt. So the following image is a composite of 11 of the best frames, lots of stars evenly distributed across the CCD. Depending on what frames I choose and what target I get anywhere between 16-12% curvature. I think it may be closer to 16% from running some of the Tarantula images through it which have more stars than the NGC 292 one displayed.

I am thinking that my spacer is a tad too short. My reasoning isn't from the curvature map but from my flat fields which are getting slightly more vignetting than when the spacer was too long. With the original noise piece I was getting 736mm focal length (as determined by Pinpoint Astrometry) where as with the new 23.8mm one, its 741.8mm. At 736mm the reducer was reducing exactly 0.8x, with the slight increase I am thinking that it may not be completely converging when it hits the CCD. This may also explain why I am apparently getting ~5 arcsec seeing on a night that didn't seem THAT bad.

My next clear night is going to be spent seeing how the telescope performs without the flattener. It'll at least give me a bench mark to work from, something to compared the actual flattener against. Was hoping to do it tonight but the clouds came over :(

[Atmos]

Well, 60s exposures makes it difficult to pull out too much from underneath of all of the noise!
Colour is off but at least the heavy JPEG compression hides a lot of that ;)

Still waiting to hear back from the manufacturer about back focus requirement :/

[rmuhlack]

There is some tilt there, my guess would be with the camera sensor itself. Have you tried rotating the camera and repeating the CCDInspector test? If the camera doesn't have its own built in mechanism for the user to adjust, it could be that you just need a tip-tilt adapter, like this (http://www.teleskop-express.de/shop/product_info.php/info/p4745_TS-T2-tilting-collimator---compensation-for-field-tipping-in-astrophoto.html) or this (http://www.teleskop-express.de/shop/product_info.php/info/p4744_TS-M48-tilter---Tilting-compensation-for-field-tipping-in-astro-photography.html) to offset the tilt of the sensor itself

[RickS]

Congrats on the first light, Colin. CCDInspector does produce variable results, probably because light frames are quite variable too, but you may get more consistent results if you try fields that have simple, bland star fields only.

Cheers,
Rick.

[Atmos]

I have done a 180º flip on the camera, actually had to so that a USB cable couldn't hit the tripod legs! I found the tilt heading in the same direction, didn't really change. I am a feeling that there may be a small amount of flex in the draw tube on the focuser, I am going to have to give it a more thorough looking over.

Would you have any idea how much of the suggested Curvature would be caused by a little 4" tilt?



I did find it jumping around a bit so I think doing an average of a number of frames should give a more accurate result than any single one. I do take all of the specific numbers with a grain of salt, using it more as an indicator than anything else.

Tonight I am going to do some testing without the Flattener/Reducer. At the moment I need a benchmark, something to use as a comparison. So far I haven't been able to get below 5 arcsec seeing, the seeing hasn't been perfect but it hasn't been that bad. At first I figured it was higher because I was ~11mm too far, now I think I may be a tad too close. In either case, I don't think the light is converging correctly. For bit of playing around I have done indoors, I expect to get ~4% vignetting, have no idea on curvature however.

[Atmos]

Well it has taken a fair bit of mucking around and many emails back and forth with Cris Ellis who is an absolute wealth of knowledge, up there with the best customer service I've ever dealt with :D Almost got my curvature map as good as I am going to get it, maybe some slight fine turning here and there but I don't think there is a lot more I can do at the present stage.

P.S.
Don't mind the strange stacking in some of the corners of the mosaic, using the wrong reference frame :D