DaveS

Thanks. I may have to use the Ektanon, assuming it has better correction. I'm hoping to avoid having to refocus. I've got some figures to push around, but ATM am in constant pain from what feels like sciatica so finding it difficult to concentrate on the maths.

Thanks Robin, some points to ponder. I just weighed the Ektanon and it comes in at 393g with its flange, a bit heavy I think. I do have a few achromatic doublets available, will do some calculations based on an f/7 telescope, remembering this is a littrow design, not the Lowspec, so my collimating lens needs to be bigger than the bare f/7 figure might suggest. Wondering if I *can* get away with an achro doublet at f/7 without having to refocus, I suspect not.

Gutted that I can't be there, hope it's going well for everyone,may try to make a two day event of it next year.

Fortunately I'm not constrained by a pre existing design, so can design it around the optical elements I already have, or can readily source. At the moment I'm looking at a cage arrangement using lengths of SS studding as a skeleton to hang the optics on. The SX flanges for their filter wheel look to be a good possibility for connecting the spectrometer to the telescope, and the imaging camera to the spectrometer. The guide camera may well end up within the cage.

Thanks, I only just remembered seeing 30 mm gratings in the product matrix. Unfortunately I don't have office or other spreadsheet program on any of my computers. Just going to check Edmund for grating holders, as I misdoubt my engineering (non)ability.

Well, if I go for a littrow design then the collimator and camera lens are the same. Therefore it needs to be faster than the telescope, since the aperture of the input beam will be automatically limited by the geometry of the diverging cone, while the lens will be big enough to catch the diverging rays from the grating. BTW my choice of the Ektanon was determined by Andrew's point about it needing to be well corrected for achromatism, plus I had it hanging around, plus it's fitted to a flange with four screw holes already drilled. But it is a bit of a beast. OK, first bit of arithmetic. (7.5 x 25.4) / 6.8 = 28.0 mm which is the diameter of the beam feeding the grating. Question: Do I go for a grating big enough to take the whole beam, or one covering the diagonal, ie 28mm or 19.8mm? Or more likely go for a 25mm square grating. Will need some dead black around the grating to kill the stray light. Edit: Edmund do 30mm square gratings with 27mm clear aperture. Might be worth the punt.

Thanks John. Hopefully I'll be able to meet up with other SGLers at the next SGL star party. For IAS next year I may book a room at the lodge for one or two nights.

FL is a bit long, but should be doable. No, it's not a doublet, I would guess between 4 and 6 elements. It's a large-format enlarging lens, the last time I was using it was for enlarging 5x4 negs. it will certainly have a big enough corrected image circle to allow a modicum of offset for the right-angle mirror. OK a search shows it to be a Tessar type construction, so a modification of the old Cook Triplet with a cemented doublet field lens.

Cooled astrocam, yes. Probably CMOS unless you can afford £2k+ just for the camera. At the same time GUIDING. You haven't specifically said you have a guiding set up, but if you don't you will find it makes all the difference, even with a basic DSLR. With a 130PD-S you can piggyback it on a second dovetail opposite the main one.. Is there any way you can leave your mount at least set up? Telegizmos 365 covers are Very good and available from our favourite supplier. Although I don't have direct experience of it (Yet) the ASIAir looks to be a very good semi-remote solution that will (Supposedly) allow you to control mount and camera from a smartphone or tablet. The only negative is that it appears to be locked to ASI cameras, but as first cooled cameras they look to be a good option.

Thanks Ade and Gina, yes going to take it easy and give my backa chance to recover. If you (Or any other SGLers) ere in the Dorchester - Bridport area give me a PM, and if I'm home I'll put the kettle on .

[Lots of very bad removed words] Looks like I ain't gonna make it . Drove into Bridport for Morrisons ( 4 1/2 miles / 10 mins), to do my Saturday shopping a day early and could barely get out of the car, even with a stick. When I got home it was just as bad, or maybe worse . Gutted, as I wanted to meet up with SGLers who I missed when SGL19 got washed out, as well as catching up with the FLO guys and Ikarus. Damn, damn, damn. If my back does get better (A miracle comparable to raising Lazarus), then I'll make the effort, but don't count on my being there.

It's not a specific nebula, but the region around Sadre is a very rich NB target, especially if you can give it plenty of exposure. Somewhat smaller and still (Just) north of Orion, the Rosette is a good few degrees across.

Thanks Gina Backs are cantankerous things, only need a slight wrong move to set them off.

I don't have any white hairs as they've all dropped out

I'm working on it, keeping my back moving and not giving it a chance to seize up. I'll keep an eye on things tomorrow, and it might be a matter of stopping more often on the motorway to unfreeze things. It feels like a trapped nerve very low in my back, just above the sacrum.

Damn, Blast, and Bother. I may not be able to make it as I've stuffed up my back AGAIN . Not sure how, probably sitting too long at the computer bashing my head against a recalcitrant NAS, but when I tried to get up my back froze, sending pains down both legs, almost paralysing them. Got up eventually, but 3 1/2 - 4 hours each way in the car is looking problematic. Damn, damn, damn. There are people I want to meet up with, not just SGL, but also Astrograph and OOUK. Will try to unfreeze things tomorrow. Damn.

I had a look around and found this It should be well corrected, but for finite conjugates. How far off it will be for infinite conjugates is another matter. It's also a heavy lump of brass 'n' glass.

I'll have a look in my collection of camera lenses, though my best lens is probably my 180mm f/3.4 Leitz Apo-Telyte-R. No, it's NOT going in a spectrograph lol. The field lens from a 2" 28mm EP sounds like a good idea, as I think I have one kicking around. It came with the 180 Mak-Cass that I got from FLO in a sale a few years ago.

OK, this is me Amazingly it didn't break the camera lol. I may still have my DaveS badge from Astrofest, which I'll have if I can find it.

Which is why I was looking at off-axis paraboloids. Argh! My head is about to explode!

I had a read of Hartridge's paper, and no realise what the slip of black velvet is for. I thought it was just to protect the grating, but it's to absorb unwanted reflected light. A UV/IR cut filter will also get rid of higher overlapping orders, without needing an order-sorting prism.

Thanks Louise. As it happens I have a spare Skywatcher 6x30 finder kicking around that I can dissect for it's objective lens. I also found This, Hartridge's original paper detailing his prism-grating and its characteristics, Now to get myself an optical bench.

Did a quick google on Hartridge Prisms, and the first entry was This. Damnit, I have one in my box of bits but have banged my head on the wall trying to get it to give clean spectra. Every so often I dig it out and think I've sussed it but just come up against the same problems of overlapping orders.

Thanks for the links. I'm going to have another read of Sidgwick in the section about spectroscopes. He gives some equations for designing Hartridge Prisms. I think using a std 600 l/mm grating and a BK7 right-angle prism should give near DV, close enough anyway to put the spectrum on a sensor. I do have a few achromatic doublet lenses in my "box of bits" which I'll dig out. *sigh* Why didn't I start this while I was working? At school I had a whole load of optical bench stuff I could have used for breadboarding, and a prep room where I could set things up.

Well ATM the NAS is just an expensive paperweight . It doesn't help that the "online help" (Yeah right ) refers to the previous version of the Thecus OS, and to Win XP . Tried using the "bundled" Acronis True Image Personal, but it needs a product key, which Thecus have neglected to include . Perhaps if I were more of a computer geek I could work my way around these hurdles but ATM it might as well be in double-dutch. And this thing is supposed to be "Your personal cloud"? Personal millstone more like . I flippin' hate computers . /rant