Is it possible to achieve focus using an OAG and DSLR/DSI in combo?

[Tim]

Apparently not without something extra.

I attached the Off axis guider to the SCT (via focal reducer). DSI Pro in the slot for the eyepiece, DSLR attached to the back via T ring as per normal.

Focussed the dslr, then tried to get the prism to focus on the DSI's chip. It needed more inward travel than I could give it. (Usually focus for the eyepiece in the OAG is found by inserting the EP in to the required depth).

If I altered focus on the SCT so that the DSI had focussed stars, then the dslr was way out.

So I guess my question is this. If I use the SCT to focus on the DSI, would it be possible to achieve focus in the dslr by extending the length between the OAG and the T ring? Or would that need inward travel to achieve focus?

Or is there any way to artificially bring the DSI nearer to the prism, maybe with a barlow or something?

Or would it work better without the focal reducer?

Thanks in advance

TJ

[Psychobilly]

Tj,

HAve you got a variable T adaptor - the one which splits down into segments of different length - it comes with the meade f3.3 reducer?

I take it's an f6.3 reducer your using ? IIRC the ideal reducer to Chip distance is 110mm normally made up of a 55mm sct-t adaptor and a T adaptor (Front of T adaptor to FP of a camera is 55mm)

You now have the path length of the OAG in there so will need a shorter SCT-T adaptor to put the DSLR chip in the right place and hopefully you will be able to get parafocal with the DSI then.

Billy...

[Merlin66]

The answer is Yes.

As Psyhcobilly said try to get the design back focus to the DSLR first ie 110mm, then reposition the guide camera to suit.

I find I need on my similar set up either a 2" spacer or a prism diagonal (to reduce the height) between the body of the OAG and the eyepiece/ camera.

[Psychobilly]

The answer is Yes.

As Psyhcobilly said try to get the design back focus to the DSLR first ie 110mm, then reposition the guide camera to suit.

I find I need on my similar set up either a 2" spacer or a prism diagonal (to reduce the height) between the body of the OAG and the eyepiece/ camera.

Not too bad after being up for 26 hours...so far...

Billy...

[Tim]

ok, will try that if I have the stuff needed.

Thats the first time I ever heard about having 110mm gap between FR and chip.

Thanks

TJ

[Psychobilly]

Hi TJ..

According to a certain M.A. Covington (P67 of his Digital SLR Astrophotography) the Meade and Celestron F6.3 reducers are designed for a reducer (rear element) to Sensor distance of 110mm

The meade f.3 (carp for APS-C) use comes with a variable T adaptor set as it can be used anywhere form 0.3 to 0.5xdependign on which adaptor sectiosn you use...

Billy...

[Merlin66]

It's also well documented in the Meade Instructions for the Reducer. If you don't have a copy you can down load from the MAPUG site.

[Tim]

Thats very interesting. I bought the celestron version (think its the same tbh?) but it didnt have instructions. I'll grab them from that site and see what Ive been missing.

So I take it from that Peter that its possible to vary the focal reduction by the spacing? Would it then be possible to make it be a 0.8 reducer, to image at F8 or so, and would that result in a little less FOV/ greater magnification?

I really wish that maths wasn't involved in so many things (It is well documented that there are only three kinds of people, those that can add up, and those that cant)

[Psychobilly]

Yes and no...

You can alter the reduction ratio by altering the spacing but the field will be far from flat and you'll end up with all sorts of abberations in the image...

Meade get away with it with the f3.3 reducer becasue they only use a tiny part of the center of the field - it's carp on anything much bigger than a DSI

Billy...

[Tim]

Just been doing a bit of measuring;

Distance from CMOS to front edge of T-Ring = 50mm (+/-1mm) (didnt want to jab the measure right onto the filter.

WO 2" adapter/extension tube = 38mm long (threads taken off)

2" T adaptor with m42 thread approx 40mm long

Given those, I would take from the above then that the ideal spacing on the T adaptor would be 21mm, which would give a gap of 110mm from the back of the focal reducer (f6.3) to the CMOS. Is that right? I'll watch for that next time, geez, why dont they teach you this stuff in school?

However, the length of the celestron Off axis guider is 50mm. the T-ring goes right onto the back of it, and the front of it screws onto the focal reducer. Assembled, the distance from FR to CMOS is 100mm. Does this then mean it is on a back foot from the outset? Even if the spacing is ok, it is impossible to bring both DSLR and DSI in the OAG into focus in this configuration.

If the DSLR could be extended back by 10mm, would that make the difference?

I didn't try without the FR last night (and im not going out in the rain now ) but would both cameras come into focus like that? Or will they always be offset by the same amount unless the dslr can go back a bit??

Newsflash:

I have just dismantled the OAG a bit, and it has a spacer ring in the eyepiece holder. Its possible that with out that in place the DSI may come to focus???????? The spacer is 3mm thick. It will certainly bring the DSI chip closer to the prism, so maybe thats the answer. If not I can adjust it again, but may lose the adjustment provided that tilts the prism.

Any ideas???

[Psychobilly]

TJ,

It's a pity your in coventry...

I am assembling quite a collection of bits and pieces and could also prototype you an adaptor if it was required...I know how frustrating these issues can be but have a workshop in the garage which lets me machine up prototype adaptors - I have had to make parofcal tubes in the past when i wanted to get a DSLR ont he rear port of an ETX Parafocal with an EP in the EP port when you flipped between them with the built in flip mirror...

Do you alwasy end up needing to have the dsi further into the OAG? Is there any arrangement position where you can get the dslr focused and have to lift the DSI out of the OAG to achieve focus in which case you can sort it out using parafocal ring on the DSI nosepiece?

The standard distance for a t-mount to the focal plane is 55mm thats why the thickness of T rings for different camera bodies varies so much.

Billy...

[Tim]

TJ,

It's a pity your in coventry...

Tell me about it

[Psychobilly]

These things are a PITA I have got labels on all my various adaptors as to which bits go together for which setup...

Although things have settled down now...

Silly question but what does the manual say about spacings ? and have you tried the OAG directly on the scope without the FR as that may be the problem...

Billy...

Going to hit the sack now as sleep is desperately needed.... will have a think about it in the morning .

[Tim]

I have a feeling that without the spacer in place, the DSI may come to focus now. Wont know until it stops raining though! It also gives twice the area available for finding a guide star, and hopefully will not cut too much into the light path.

If thats 55mm, then perhaps the stuff is right, and with a dslr on the back of the OAG its at 110mm.

Is there a standard measurement for CCD cameras like the DSI too? All the documents im reading seem to be sure that an autoguide camera can be parfocal with the dslr on the radial guider, no problem.

They also say the FR should work the same as at native F10. Get some sleep Peter

[Merlin66]

I'm somewhat surprised you can't get the DSI to focus.

As I said previously I have a very similar set-up and need a 50mm spacer between the OAG body and the guide camera to get it into focus????? I'll post a photo of the set-up, see if that helps/ explains anything.

[Psychobilly]

Ah thats better nicedly refreshed...

pics of both setups would be very useful...

Billy...

[Tim]

Ok great, i'll try and get a pic later, everything is soaked out there atm.

I also just got all the kit to rig up eqmod, so I want to have a play with that in the light too. Looks like no clear nights for a while anyways

[Psychobilly]

I'm wondering if the spacing of the DSI and DSLR changes when you have the FR fitted...hmmmm

[Merlin66]

Shouldn't, unless the FR is behind the OAG ie between the OAG and the camera.....

[Psychobilly]

The celestron radial guider is getting very mixed reviews

[Merlin66]

I've had the Lumicon, Meade and Celestron OAG over the years; my need to try and guide spectroscopes drives me nuts!

I'd say other than the Lumicon GEG (absolutely fantastic!!) the Celestron would have been my choice. Radial adjustement without moving the camera and fine positioning of the prism... what more do you want?????

[Psychobilly]

some people were moaning about flexure... :scratch:

But you practical advice having used them all is probably the most valid :salute:

Might be soemting i'll look at to get nback to the double DSLR ioamgign steup with simultaneous narrow and widefield imaging of the smaller targets - i used to use a 3rd OTA for this but no room anymore...

Billy...

[Tim]

Thats my eventual aim Peter, use two dslrs at once for narrow and wide simultaneous.

One of my dogs has just lacerated an artery in his leg and is at deaths door, so I'm a little pre-occupied waiting for the vet's phone call.

The celestron OAG is a waste of time for manual guiding in my skies, but with a sensitive autoguide camera it just might be useable. Time will tell.

[Psychobilly]

Ohh sorry to here about the dog TJ...

[Tim]

After a little playing I can bring them both to focus using an extension on the DSI which I have made from some old filters, so thanks for the tip!

Just need a clear night now to test with a star.

TJ