Flip mirror with two stage focal reducer

[Dom543]

Dear All,

Don asked in another thread, what kind of focal reducer I had in my setup. Since this question has been asked several times before, I enclose a couple of photos of my flip mirror with the two stage focal reducer hidden inside of it.

First all the parts separated with a list of the components from left to right.

LEFT

- Meade 6.3 focal reducer/flattener optics in a 2" cell;

- Vixen flip mirror body;

UP

- T-thered spacer;

- Baader 1.25" helical eyepiece holder (with T-thread);

- Stellarvue 25mm crosshair eyepiece;

- Crosshair illuminator;

RIGHT

- TS 15mm T-thread 1.25" eyepiece holder;

- Scopestuff 1.25" x0.5 focal reducer;

- 1.25" nosepiece with 28mm filter thread on the left side and C-thread on the right side;

- camera.

Here is a partially assembled view

Finally the full assembly.

A few comments:

1. I took the glass (four lenses plus spacers) out of a Meade 6.3 reducer/corrector and put it into a 2"cell that has 2" filter thread (M48) on the outside. This way the focal reducer fits inside of the 2" nosepiece of the flip mirror body. This arrangement helps to keep the total length of the assembly at a minimum.

2. The Baader helical focusing eyepiece holder is a luxury. The ep holder that came with the flip mirror could be used just as well but I had the Baader part, so I used it. The width of the spacer may need to be changed if a rather different overall reduction rate is desired.

3. The TS (Teleskop-Service) 1.25" ep holder is only 15mm thick and helps to keep the overall length short, if that is desired. Right now with the Lodestar I use a spacer + the TS ep holder or two TS ep holders in tandem. Because the Lodestar fits inside of the 1.25 ep holder and is relatively long.

4. If more reduction is required a longer C-mount nosepiece or an added spacer can be used between the Scopestuff 1.25 reducer and the camera. Reduction factor can also be increased by moving the camera+nosepiece assembly further out of the ep holder (inceasing the spacing between the first and the second reducer).

5. The camera on the photo is an LN300 that I don't own any longer. Before getting the Lodestar last month, I used a Samsung SCB-2000 that is quite a bit bulkier than the LN300. But in my opinion the Sammy beats the LN300.

6. 2" filters are screwed on the 2" nosepiece of the flip mirror ahead of the focal reducer. 1.25" filters go on the telescop-side threads of the Scopestuff 1.25" second focal reducer.

7. This two stage reducer is capable of speeding up my f/10 C11 to about f/3. For this additional spacers are needed. When used with a Meade natively f/6.3 OTA, then I keep the reduction factor of the assembly at a minimum to get and overall f/3. If I wanted to, I could go down to around f/2.5 without noticeable deterioration of the image on the small 1/3" or 1/2" sensors.

8. One advantage of the two stage reducer arrangement is that the crosshair ep also looks through one part of the reducer. This way the FOV of the 25mm ep is about twice as wide as the FOV of the camera + 1.25" reducer.

9. Dark frames are very easy to take by just flipping the mirror to the eyepiece. In fact, I usually take darks, while doing the fine centering of the object in the ep.

Clear Skies!

--Dom

[Dom543]

The flip mirror + focal reducer assembly has been successfully used with my C6 travel scope, with Meade 8" and 10" f/6.3 OTA's and with a C11 f/10. The overall achievable focal ratio with the f/10 OTAS is about f/3 . With the f/6.3 OTA's one can go down to around f/2.5. The maximum achievable optical speed (lowest focal ratio) is determined by the back focus of the OTA.

--Dom

[Guest]

Hi,

Very good ideas,

I am mainly intersted in putting the focal reducer optics into the 2" cell

Firstly how easy is it to remove them from the existing cell ?

secondly where did you get the 2" cell from to fit the reducer into ?

Thirdly how did you mount the reduces glass into the new cell

Sorry for all the questions, but if i managed to do,this it could save me a huge headache, in always trying to get the correct reducer to focal plain distance that you need with these particular reducers. As I could mount on the front of my canon DSLR with the correct length nosepiece and then the whole lot would go into my 2" focuser.

An absolutely superb idea....

Regards

Olly

[Paul81]

I like the idea of being able to flip between visual and the video view (with a red filter over the display to avoid too much night vision impairment) and the thought of being able to attached a mono and colour camera and switch between the too! Interesting post! 

[Dom543]

Olly,

Please give me a couple of days. I will check if and where those cells are available. It was some time ago, when I bought them. If I cannot find them online, then I probably have an unused one back in Seattle that I could mail to you.

Paul,

It is an interesting idea  to use the flip mirror between a color and a mono camera. Astrophotographers do sometimes add a mono luminance channel to their color channels. They say it improves fine detail. I don't know, if this would work by simply stacking a couple of mono images on top a stack of color ones. At the minimum, LL needed to be able to accept input from two different USB connections. Some means to boost color saturation would probably also be helpful. So that the mono luminance doesn't wash out the colors.

In any event, it is an interesting idea to think about and experiment with. But don't let yourself to be distracted from v,011.

Cheers!

--Dom

[Guest]

Olly,

Please give me a couple of days. I will check if and where those cells are available. It was some time ago, when I bought them. If I cannot find them online, then I probably have an unused one back in Seattle that I could mail to you.

Cheers!

--Dom

That would be fantastic, really appreciate that,

Is it easy to swap,the glass over from the original cell to the other

Olly

[Dom543]

Olly,

PM me your address. I will be in Seattle next week and I have an unused cell there that I can mail to you. I will also send instructions. It is not difficult to move the glass, I have done it several times. The cell is of the right diameter. All you have to watch is that the stack of lenses and spacers doesn't fall apart. This assumes that you have the Meade, Celestron, Hirsch or Antares reducer corrector, which all have the same diameter lenses (but of different FL).

Cheers!

--Dom

[Guest]

Olly,

PM me your address. I will be in Seattle next week and I have an unused cell there that I can mail to you. I will also send instructions. It is not difficult to move the glass, I have done it several times. The cell is of the right diameter. All you have to watch is that the stack of lenses and spacers doesn't fall apart. This assumes that you have the Meade, Celestron, Hirsch or Antares reducer corrector, which all have the same diameter lenses (but of different FL).

Cheers!

--Dom

Hi Dom,

I have sent you my postal address in a PM, and many thanks, I am really appreciative of this.

Olly

[johnno]

Olly I don't know if this is what your looking for, there's a short video half way down the page about removing the cell

http://www.optecinc.com/astronomy/catalog/pyxis/pyxis_le.htm

[Guest]

Olly I don't know if this is what your looking for, there's a short video half way down the page about removing the cellhttp://www.optecinc.com/astronomy/catalog/pyxis/pyxis_le.htm

Cheers pal

Appreciate it

Olly

[engreric]

Dom,

 

I'm loving your setup, this looks fantastic, I'm really debating building up to this and trying it out.

It looks like your taking a n f/10 to f/3, so a total of x0.3 reduction without too much noticable coma/vignetting

I would like to try this on my refractor - omni 120mm f8.3 achro, is it safe to say that I have a chance of getting my ota down to f/2.5-f/3?

I get the feeling that I wouldn't be able to reach focus, but I would be willing to cut down my achro by an inch or 2 to reach it. I may also have to trim the rack and pinion tube to keep from cutting into the light cone. I can test with the distances of the optical pieces in the refractor without the flip mirror by completely placing the camera and lense assembly inside the 2inch focuser tube before committing to cutting the ota.

By any chance, do you know the distance between the meade 6.3, x0.5 reducer, ln300 sensor? If it's too much trouble, don't worry about it, but I figured I would ask. I plan on eventually upgrading my AVS DSO ccd to a lodestar x2c like you as well.

I've been trying to research if there is a good resource for calculations to model the light angles before I start throwing money at it but I haven't found too much, does anyone have any resource recommendations?

 

I've also been looking for a 2" cell for the meade 6.3 reducer/flattener as well, I'm really stuck at this point too. Has anyone found an online source for one of these.

 

Thanks so much for showing off your creation and I appreciate your guy's time! Thanks,

Eric

[Dom543]

Hi Eric,

Thank you for digging up the old thread about the flip mirror. Since then I have switched to using a Lodestar, that has a larger sensor than what the LN300 had. Correspondingly, I also modified my setup and reduction factor. I still have the same flip mirror with the Meade 0.63x reducer but on the camera I now have only a 0.7x reducer. This change was needed due to the larger 9mm diameter sensor of the Lodestar. (As opposed to the 6mm diameter sensor of the LN300.) The original more aggressive setup would strongly vignette the larger sensor and also cause ugly distortions in the outer regions of the field.

I am not sure, if the 120mm achro could be focal reduced down to f/2.5-f/3. Chromatic aberration of refracting optics is getting worse with more bending of the light. And f/2.5-f/3 requires a LOT of bending. Also, you needed to cut a LOT from all tubes. A 120mm f/2.5 refractor is about 1 foot long. 

I am not saying that it is not possible to achieve focal ratios below f/3 with refractors. But I am not sure if an ad hoc home made three stage optics would achieve satisfactory optical quality. I do have a 300mm focal length f/2.8 Nikkor camera lens that works very well. But this lens contains ED glass element and photo equipment guru Ken Rockwell lists it among the "10 best lenses Nikon has ever made". We are not the Nikon optics design team here...

If you are hungry for fast optics, then, rather than starting to cut your OTA, I would recommend to consider used manual focus camera lenses. They are very affordable up to 300 mm focal length. This is exactly what your 120mm achro would yield at f/2.5. I recommend to look at the Pentax M42 scew mount lenses. They are very easy to adapt to astro use requiring only a thread adapter. I have a 300mm f/4 Asahi Takumar lens and I believe that I paid less then $100 for it. (Asahi Optics later renamed itself to Pentax.)

In summary, while my flip mirror focal reducer works well with SCT's, I wouldn't recommend to cannibalize a nice refractor to work with it. A lot needed to be cut from the tube and the resulting optics would probably have lots of aberrations.

Clear Skies!  --Dom

P.s. By the way, the spacing between the two reducers is about 100mm in my setup.

[engreric]

Dom,

Thanks for advice. Your right about the increased CA, I completely forgot that heavy reduction would exacerbate it. That's good to know that you ended up with a 0.7x in the back when you switched to the lodestar, I'm headed in the lodestar direction once I'm ready to drop the $650 for it. I was actually interested in the heavy focal reduction primarily for the field of view, but the 9mm lodestar would be giving me quite a bit more FOV than my current setup anyway. I'm more interested in the flip mirror for the convenience factor of finding objects a bit more easily than what I'm doing now having to switch back and forth each time focusing. Maybe I can get by on my achro with just a flip mirror and less FR. Maybe the lodestar flip mirror and meade 6.3 only. Hopefully I can still reach focus. Thanks for all your help. In regards to the 2" cell for the 6.3, did you ever find another online source for it?

 

Regards,

Eric

[stash_old]

5 hours ago, engreric said:

Dom,

Thanks for advice. Your right about the increased CA, I completely forgot that heavy reduction would exacerbate it. That's good to know that you ended up with a 0.7x in the back when you switched to the lodestar, I'm headed in the lodestar direction once I'm ready to drop the $650 for it. I was actually interested in the heavy focal reduction primarily for the field of view, but the 9mm lodestar would be giving me quite a bit more FOV than my current setup anyway. I'm more interested in the flip mirror for the convenience factor of finding objects a bit more easily than what I'm doing now having to switch back and forth each time focusing. Maybe I can get by on my achro with just a flip mirror and less FR. Maybe the lodestar flip mirror and meade 6.3 only. Hopefully I can still reach focus. Thanks for all your help. In regards to the 2" cell for the 6.3, did you ever find another online source for it?

 

Regards,

Eric

You could drop the Flip Mirror and use the image via plate solving software(e.g. Astrotortilla etc) to find / centre objects  and control your mount -I assuming you use a software controlled mount(e.g. Ascom/Eqmod).  

[Dom543]

12 hours ago, engreric said:

In regards to the 2" cell for the 6.3, did you ever find another online source for it?

Eric,

If you don't find any such cells and are still interested, drop me a PM in December. Years ago, when I saw them, I bought a couple and had a few spares left. I sent one to MagnaMan in 2014 and I probably still have one somewhere that I could send you. The only problem is that I work at various places around the country and right now I am 3000 miles away from home. I should be back home around December 10 or so.

BTW the Vixen flip mirror has an odd M49 thread in its nosepiece. Not the M48 commonly used for 2" filters and other astro accessories. So I needed to add a few set screws to keep the M48 cell firm and straight in there.

Clear Skies!  --Dom

[Ain Soph Aur]

I only have experience with an Orion flip-mirror (long sold). I can't help but wonder how one accurately collimates a flip mirror, especially to the accuracy needed with EAA stacking? 

[Dom543]

30 minutes ago, Ain Soph Aur said:

I only have experience with an Orion flip-mirror (long sold). I can't help but wonder how one accurately collimates a flip mirror, especially to the accuracy needed with EAA stacking? 

Brandon,

The camera is at the straight end of the flip mirror. So the mirror is flipped out of the way and has no effect when the light is directed to the camera.

I use the flip mirror on a daily basis with my SCT's, this is my diagonal.  I always use it to align the mount.  All my posted captures that use an SCT and don't use the 3.3x reducer are through this flip mirror. It is also very convenient for making darks. One can even make darks, while one is centering the object in the eyepiece.

Clear Skies!  --Dom

[engreric]

Dom - Thank you so much for the offer and the heads up about the nosepiece thread. I may try to 3d print something in the meantime to hold the lenses, I've been having pretty good luck 3d printing telescope parts but I'll get back to you in December if I'm still pursuing this =) I can't thank you enough for the advice!

Stash_Old - OMG Plate Solving is amazing... How is this the first I've heard of it! I just had a go at it with 5 hours of research today and trial and error this evening. Your right, I am using eqmod with a homebrew motor drive on a CG4 mount. Yes... It's not the best setup, but it's mostly my own creation and being an engineer I just won't allow myself to throw money at a better mount, though I would like to. I just got back in the house after successfully plate solving a few times with astrotortilla. It does seem quite difficult to get it to play nice with my weird analog eaa camera setup and small fov but when I got it to work, it was absolutely spot on and amazing. Now I want to try automating the entire alignment procedure as that is going to make life sooooo much easier at the outreach events we do every month. Unfortunately, because I'm using a usb frame grabber with the AVS DSO and SharpCap I had to manually save the images and load them into AT, but it's a step in the right direction. I need to figure out now how to get the screen capture feature to work, I tried but was unsuccessful. I also couldn't find any program that natively supports AT and has access to my camera setup.

 

BTW, I just got my first view on M74 from the side of my house on a really busy main street in the red zone of long beach california. The image doesn't look impressive,actually it's terrible =), but I found it thanks to the platesolving.

Thanks everyone for teaching this newbie some new tricks