Anybody using a Baader Alan Gee Flattener/Reducer with an SCT?

[Tim]

I have read some good reports about them, but not seen anything to put me off.

Anybody on SGL have any info or feedback about them?

Thanks

TJ

[MartinB]

I had one, never used it, then sent it to a fellow SGL member courtesy of the good ol post office and it never arrived. So that was that. It did look a good piece of kit giving good focal reduction without vignetting and I've seen good results on CN. I found it quite fiddly to site properly in the scope and might need a bit of playing around with to get it sorted. I tried it one night, the first time out with my QHY8. I had an incredibly frustrating evening faffing around not sure whether the reducer or the camera that was stopping me getting an image. Eventually realised the QHY8 was turning into a glacier field.

The reason I never tried again was because I decided to go down the AO route with my SCT and it wouldn't have been possible to get the spacing right.

[Tim]

Thanks Martin. Regarding the spacing, can you remember what it should be between reducer and chip?

I will also want to use it with one of my OAG's if it is possible. Also, does the camera attach to the body of the reducer or the sct? It's not too clear how the reducer attaches after going into the baffle.

Thanks Martin.

Tim

[MartinB]

I don't remember it being a flattener Tim so I don't think spacing is critical for field flatness, might be wrong on that. All the spacing does is change the amount of focal reduction. With my AO unit the reduction was going to be far too much with massive vignnetting.

Take out the visual back, insert reducer then replace the visual back to hold it in place. The camera then fits into the visual back. Not a happy arrangement if you are using a crayford or similar.

[SteveL]

I have one (also unused so far). It fits inside the baffle tube, inserted via the SCT threaded hole on the back. If I remember, I will try to dig it out for back spacing details.

[SteveL]

Dug it out this morning, only to discover that I had put the instructions for it somewhere safe (i.e I couldnt find them).

According to >> THIS <<, the optimum distance is 121mm for visual work. Unsure if that would be affected when used with a CCD. 121 mm sounds a lot, but if you remember that the telecompressor is shoved inside the SCT, you will need to remove the length of that from the overall required length. Of course, I didnt measure the length of the telecompressor <sigh>

[SteveL]

and another mention of ~123mm

Accessories

[Tim]

Thanks both. I think the thing is a flattener Martin, but again, might be wrong. I'll look into it a bit more.

Cheers

[SteveL]

If its as clear as it forecasts for the weekend, and not much of a breeze, I`ll see if I can get some Ha filter time with the C11 and the Alan Gee, plus the Celestron OAG. Nice big M25C chip will show if it flattens or not.

[blinky]

Did you get a chance to try this Steve?

[SteveL]

I havent had a chance, sorry! Will need some time to get everything arranged for back focus (Alan Gee unit + OAG + Filterdrawer + M25C) and then get the Lodestar in the OAG parfocal with the M25C. Thats a daytime job for when the skies are rubbish...

[SteveL]

OK, spent some time today installing the Alan Gee into the C11 and getting back focus distance somewhere right. The lack of proper documentation on the distance doesnt help much, no idea if the distance of 121-123mm is to the front ibjective lens, the back lens, the end of the tube, etc. Oh well.

Anyway, also got the M25C to focus, and the lodestar is now parfocal. So the next time its clear overnight, I will give it a go with the Ha filter and point it at something. Not really expecting great image quality due to the lack of real dark skies, but it should provide enough information about FoV and flatness.

[SteveL]

OK, some kit images (sadly no sky images yet). The Alan Gee fits INSIDE the baffle tube. If you look closely inside the SCT thread, there is an inner lip, and the lip on the Alan Gee reducer fits flush with that

It comes supplied with a washer to fill the distance to the back, which also allows whatever is attached to the back to push against in to keep it central and flat:

(Yes, thats a fingerprint on the lens... oops!)

This is the entire imaging/guiding unit, which is now parfocal:

And now all the bits broken down:

Left to right:

Alan Gee unit* (see below)

Washer for AG

Celestron OAG

15mm T-Thread extension tube

6mm T-Thread extension tube

T-Thread to M48 adapter for filter drawer

Gerd Neumann Filter drawer

Starlight Xpress M25C

Above the Celestron OAG is a 1.25" to C-Thread adapter, with a parfocal ring attached. This screws into the Starlight Xpress Lodestar guide camera. The focus point for the kit above was JUST at a point where I couldnt hold the lodestar without a tube, and too close to use a tube, so I had to add the 15mm extension to the imagin train length so I could get the lodestar to focus as well.

* - The Alan Gee comes with several lengths of extention tube, and if you had just a DSLR on the back of the SCT, you would probaboy be somewhere near the 121-125mm back focus with that kit, but with the Celestron OAG and the Gerd Neumann filter drawer system I had to use just the AG unit and the tube with the "lip" on it.

I also applied a new finder to my QHY8 + Nikon lens setup. I havent aligned this yet. It is attached to the filter drawer with "no more nails" strip.

[Tim]

Thats very useful Steve, Im still considering one of these Alan Gee jobbies, for the alleged flattening as well as the slight increase in speed. Will be very interested to see the ccd inspector results.

Cheers

[SteveL]

As soon as I get any images with this setup, I`ll post them and the CCDI report on here for inspection

[riklaunim]

Bumping the thread. I'm thinking about getting the Alan Gee reducer for the C11 and DSI III Pro. And as the CCD isn't quite big (2/3") I would want to push the reduction more. Unitron Italia PDF shows some examples about doing 0,4 - 0,5x with it. Anyone have tried that? And does this reducer adds some noticeable chromatic aberration?

Or maybe I'll first try the f/3.3 reducer at shorter distance to get ~0,4x? OAG won't fit but maybe it will work good.