JMI EV3CM and 6.3 reducer/corrector question

Hi all. I posted this earlier in the equipment forum but no one has replied so I thought I'd try here.

I just got a new C11 to replace my C8. I've been using the Celestron 6.3 focal reducer on the C8 and have had no problems. When I ordered the C11, I also ordered the JMI EV-3CM motorized focuser for fine focusing when I image. I need to know if the location of the R/C in the optical path makes a difference (in front of the EV-3CM or after it)? Or does it matter?

Any help would be great

Thanks.

Hi all. I posted this earlier in the equipment forum but no one has replied so I thought I'd try here.

I just got a new C11 to replace my C8. I've been using the Celestron 6.3 focal reducer on the C8 and have had no problems. When I ordered the C11, I also ordered the JMI EV-3CM motorized focuser for fine focusing when I image. I need to know if the location of the R/C in the optical path makes a difference (in front of the EV-3CM or after it)? Or does it matter?

Any help would be great

Thanks.

If you can reach focus with it on the rear cell, in front of the focuser, fine. If not, you'll have to place it after the focuser, using a adapter like Jim Henson sells at scopestuff.com. The problem with that? Less focal reduction.

Kevin,

1) The amount of focal reduction the focal reducer provides isn't dependent on where it's placed but because the C11's focal length changes depending on how far behind the rear cell the focal reducer is placed there will be a small difference in the actual amount of focal reduction provided.

2) Placing the reducer before the external focuser does make it more likely to be able to come to focus but it introduces a couple of problems - the first is varying the focal reduction every time you use the focuser, the second and most importantly is the amount of spacing ... it is possible to setup the correct 105-110mm spacing for this reducer with some cameras and this focuser in between but it requires low profile adapters. However, with other cameras (like a DSLR) and the reducer placed in front of most external focusers the spacing will simply be too much and won't provide anything close to a quality image. I said "most" external focusers for a reason ... I believe some folks have gotten this setup to work with a low profile/short drawtube external Starlight Instruments focuser.

The EV3CM is 2.4" or 61mm long, add 55mm for the depth of the sensor in a Canon DSLR (with a T Ring) and you're already at 116mm and then you still have to account for some of the 2" to SCT adapter and SCT to T thread adapter.

To make your setup work properly in a C11 put the focal reducer after the focuser. Don't use any filters or anything else that will extend the focal reducer any farther back from the focuser. Keep the focuser in it's smallest or lowest position.

Use a simple 2" barrel to male SCT thread adapter to mount the focal reducer in the focuser (I believe one of these comes with the JMI focuser).
Use whatever spacers and adapters are required for your particular camera to maintain as close to 105mm of spacer between the rear or of the reducer lens and the sensor in your camera. With a Canon DSLR that can simply be the SCT T-adapter (which is about 50mm in length).


Edit: Item #1, reworded to add the effect of the moving mirror design.

Kevin,

1) You will NOT get less focal reduction

Well, that would be great if it were true, but it is not. The farther back the reducer is from the rear cell, the smaller the reduction. Don't believe me? OK, try it for yourself.

Mark thanks for all the good info. I will try that out the next clear night ( maybe next month?? Lol). I also have the Orion Deep Space Imager CCD Camera. Am I looking at the same situation there as well??

Again thanks very much. I was beginning to think no one knew anything about this!!

Uncle Rod I think what Mark was trying to say that if the spacing stays the same there will be no loss of focal reduction which makes sense to me but then what the heck do I know

Well I thought I'd try something tonight out rather than rely on my memory.

I shot the moon with my C11 and a Celestron f6.3 focal reducer. The camera is a Mallincam Xtreme. The connection is via a Celestron T-adapter plus a 30mm T spacer for a total of 80mm. That plus the T-C adapter and the spacing within the Mallincam puts the total spacing from the back of the reducer to the sensor at around 110mm.

I used this setup threaded directly onto the SCT adapter on the rear of the scope and then with an older JMI focuser in place (similar but not identical to the cool newer Event Horizon model). The difference in the position of the focal reducer between the plain scope and with the focuser is about 65mm.

I captured a few images of the moon with and without the focuser in place using SharpCap software and then measured the distance between a pair of craters. No processing, resizing, etc. so the images are as untouched as I can make them.

Without the focuser - 28.3cm between the crater walls
With the focuser - 30.3 cm

So there is indeed a difference in size of about 7% between the two setups ... why is that?

Well there is also 65mm difference in the back focus (the height of the external focuser with the drawtube racked just about all the way in). On a C11 that 65mm equates to just over 200mm of focal length (according to Ken Hutchinson's paper on Celestron SCT Vignetting Analysis, Version 1.5).

200mm of additional focal length on a 2800mm fl scope is wonder of wonders about 7%.

Without the focuser the scope's focal length is around 2800, with the external focuser the focal length is just over 3000mm. The focal reducer appears to be providing a consistent amount of reduction in both configurations.

So it would appear that Uncle Rod is correct, the two different positions do have different effective focal reductions and if that 7% difference that was caused by the change in focal length of the scope was what he had in mind I apologize. However, the focal reducer is indeed providing exactly the same reduction factor in both positions which is what I was trying to convey in my earlier comments in this thread. I will amend those comments to more correctly reflect the effect with the moving mirror SCT.

Here is the setup without the focuser in place ...

Just to update my situation, I just came back inside from the ob as it clouded up on me. Supposed to be clear after midnight but we all know how that goes...... Anywho, I discovered that on Saturn anyway, with the FR in front of the EV this scope will not reach focus. The mirror was bottomed out. I moved it to outside the EV, with the EV direct on the rear cell and then the FR, my 2" visual back and 2" diagonal and it focused fine. That's all I had a chance to do before the clouds rolled in.

Hopefully it will stay clear tomorrow night so I can try it with my Canon and the Orion CCD.

I'll keep you posted.

Kevin

Kevin,

Watch your spacing to the reducer. A 2" diagonal is usually at or over 100mm, and with the other adapters you are probably just over 110mm spacing which will be ok for visual. For imaging you may need to be more critical about the distances.

This is also likely why you can't come to focus in the first configuration ... the focuser plus the diagonal (160mm +) is waaaay too much spacing for that reducer.

Mark,

I hope to try it tonight if it stays clear. Will post with progress. Thanks for your help.

Didn't have very good skies last night to image but here is a picture of the optical train with focuser, FR, visual back, and mirror diagonal. Any ideas on how to make it more compact??

Kevin,

Your diagonal nose piece to the SCT threads looks pretty good. I think that's about the best you can hope for with a visual setup using a 2" diagonal. Even with that setup it's likely some 2" eyepieces will come to focus and some won't. You'll probably have to keep the focuser's drawtube racked in as much as possible/practical. When I'm right on the ragged edge of focus travel I'll rack the focuser all the way in, then back out a very small amount, then focus using the SCT's internal focuser, and finally only very small movements with the external unit to fine tune the focus.

You'll have to get rid of the diagonal if you image with it (but you already knew that ... lol).

My older JMI focuser is almost exactly the same setup as yours. It has the large thread connection but doesn't allow me to break it down and put the FR in front of it.

Mark

That piece that allows me to put the FR after the EH is an adapter. It has threads on one end and a smooth 2" barrel on the other. Pretty cool!! It came with the EH focuser but I would think you could find one.

That rig really threw out my balance on the dec axis. I didn't expect that the first time I used the scope after I put it on. I loosened the clutch and the dew shield hit me in the head !!!

It looks now like at least a week or more till I get to play again and try some imaging. What with the moon going full this Saturday and nothing but showers and thunderstorms each and every night.

BTW, check out my pictures of Black Hole Observatory in the observatory forum. It's the thread titled, you guessed it, Black Hole Observatory lol I'd be interested to read your comments/suggestions.

Kevin