Multiple Focal Reducers with C6 and Lodestar x2 mono

[Jim Smith]

Last night I tried stacking 2 Celestron f/6.3 focal reducers on my C6. I calculated that I should get about f/4, but processing one of the images in astrometry.net showed that the new focal ratio is about f/3.4. I might try reducing the spacing between the double-stacked focal reducers and the sensor and see what that gives me.

The first image below is M10 from a previous session using one f/6.3 reducer. (I include it so that the fov can be compared with last night's M10 shot) The others are with two reducers. There do seem to be a few funny shaped stars, reflections and uneven field illumination present, but for live viewing I think it's OK. 

[alphatripleplus]

Jim,

Nice to see you trying this. I've been using twin stacked Meade f/6.3 reducers with my C8 for a while, and it works pretty well at f/4 down to about f/3.3 with more aberrations the faster you go. I ran out of focus travel before I could get to f/3.0.  From my testing I think that if you decrease the sensor to reducer distance by 10-11mm from your current spacing, you will probably go from f/3.4 to f/4. Look forward to seeing more of your results with the C6 and twin reducers!

 

 

[arkosg]

What sort of spacing are both of you using between the reducers (and between the reducer and image/sensor plane)?

Cheers,

- Greg A

 

[Jim Smith]

One reducer is directly screwed to the back of the scope. The second reducer is screwed directly onto the first. The sensor is about 80-90 mm behind that.

[Jim Smith]

Another clear night! Here are some shots using less spacing between the reducers and the sensor. I think the spacing is now about 60-70 mm (~20 mm less). Astrometry.net processing and checking in Stellarium suggest that I am now getting almost exactly f/5.0.  I think, with the adaptors I have, I can adjust the spacing to something between my previous two configurations. It will then be pretty much as alphatripleplus suggested.

The field of view at f/5.0 is clearly narrower than at f/3.4, but is anything else different?

 

 

[stash_old]

Jim ,

 

Didn't you have an Hyperstar fitted to your C6 - dont you use it now - if so why? - Just interested ! 

[Jim Smith]

Yes, I did have a Hyperstar and it worked very well. After using it for a while, I decided I needed a smaller field of view for most of the objects I was interested in. I can now, fairly easily, swap from f/10 to f/6.3 to f/4(or nearly!). Changing from Hyperstar mode to non-Hyperstar mode during an observing session was just a bit too time consuming and fiddly for me.

Having said that, I am now toying with the idea of getting a wide field electronic finder of some sort. The go-to on my mount is not very accurate. Finding objects at f/1.9 was really easy and I do miss that!

[stash_old]

"The go-to on my mount is not very accurate" - Assuming you use the EQ mode of your AZ-EQ5,why not use Plate solving attached to your mount such as Astrotortilla (local none networked version of Astrometry.net) which places the object dead centre 8 out of 10 times. I use Plate solving ,via APT/EOS , and can find and centre any object within 60s. Also to improve ,if needed , your accuracy perhaps you need to Polar Align (if you dont already do it). Sharpcap has a fine Polar Alignment routine and even at the non free price of £10 is still cheaper than "other" forms of Polar alignment kit and just as accurate - only takes 1-2mins and if your mount is fixed you dont need to do it every time.

The AZ-EQ5 should be easily up to the job.

Good luck with whatever route you take. I shall be following your reducer on reducer reports. Clear Skies !

[Jim Smith]

I have been swapping between ALT-AZ and EQ modes to see which works best for me. At the moment I am favouring ALT-AZ for two reasons. Firstly, I seem to be able to grasp much more easily the orientation of my on-screen live view relative to the sky; so even if I don't know what exactly I am looking at, at least I know which way up it is. Secondly, in ALT-AZ mode I can easily mount my 80 mm refractor parallel to the SCT. Through it, I can see what I'm pointing at much more easily than through the 50 mm finder on the SCT. Also, when I press the arrow keys on the controller, up is always up and left is always left. 

I have found that in EQ mode (even after doing a polar alignment or two) or in ALT-AZ mode, the go-to rarely places the object within the admittedly small field of view of the Lodestar when used at f/10 or f/6.3.  I assumed that this was normal for these mounts!

I might take a look at Astrotortilla, but I think I rather fancy the video finder option. Thanks for the suggestions.

[stash_old]

You are correct in all you say - I forget I am fully automated and haven't used a hand controller ,eye piece or a view finder for 2 yrs now - beats being cold !

 

Hope you find your own method - as one size does not fit all in Astro !

 

[alphatripleplus]

On Wednesday, July 05, 2017 at 16:41, arkosg said:

What sort of spacing are both of you using between the reducers (and between the reducer and image/sensor plane)?

Cheers,

- Greg A

 

To get f/4.0, I have  a distance of  63mm from the sensor to the back of a T-adapter that is screwed into the back of the rear of the two f/6.3 reducers. I haven't measured the thickness of the T-adapter, but I think it adds perhaps 15mm. So a total of about 78mm from sensor to the rearmost reducer to get f/4. As Jim mentions, the two reducers are screwed into each other.

[Jim Smith]

I am still experimenting on my C6, but I think the sensor to rearmost reducer distance will be around 75-80 mm for f/4.0.

[Jim Smith]

I think I've now got to about f/4.2. The sensor to rearmost reducer distance is now ~80mm.