1.25" Focal Reducer with guide camera on OAG

[GraemeH]

I occasionally have difficulty in locating a guide star using my Edge HD 8 with an OAG and QHY5Liic - my limiting magnitude for guide stars seems to be between around 10 and 10.5, and the very narrow field of view when imaging at f10 means that I sometimes can't find one that gives good framing of my target object.

If I were to attach a 1.25" focal reducer (e.g. https://www.telescopehouse.com/revelation-0-5x-focal-reducer-1-25.html ) to my guiding camera, would this work to increase the FOV of my guider or does the prism size in the OAG mean that this wouldn't work?

Hoping some of the experts here can tell me if my thinking is fundamentally flawed.

Edited December 13, 2018 by GraemeH
URL correction

[barkis]

You would be better off with a independent guide scope would you not?
F10 is going to be problematic  trying to pick a guide star off the edge of the primary mirror via your OAG.
The link to TH doesn't work, must be a small error in the URL In your post perhaps.
The reducer may be an option, but I think someone who has used that method would be better positioned to
answer that question.  
What targets are you looking to image at that f/ratio Graeme?

[Davey-T]

I have the same problem especially when imaging out of the galaxy, it can be difficult to find a guide star.

Have to forget perfect framing and nudge around to try and find a guide star

I have a 100mm refractor piggy backed on my SCT and occasionally have to resort to using it as a guide scope.

You'd probably be better off getting a more sensitive guide camera, something like a Loadstar, not cheap though, I'm using one and I still have problems sometimes.

Dave

[GraemeH]

Thanks Doug - I've fixed the URL.

In terms of separate guide scope, I'm pretty convinced by the many threads on here on the subject that suggest long focal lengths really need off-axis guiding to perform well.  I don't think I want to introduce the complication of differential flexure.

My preferred targets are generally galaxies, but I'm still very much a beginner and trying to understand how to tackle lots of different objects.

Dave - unless someone indicates that there's no way to make the reducer work, I may just give it a try anyway.  It's only around £20, so a much more financially sensible option than a Lodestar if I can make it work.  Thanks for taking the time to reply though.

[vlaiv]

It will not work (at least I think so, below is why).

All it can do is shrink available field onto smaller region of sensor, but ultimately field of view is limited by pick-off prism and more importantly opening in prism holder.

I currently guide with a bit larger sensor - ASI185, which has diagonal of ~8.6mm at 1600mm FL - F/8 beam. I get vignetting on this setup, here is screen shot:

Other things that might interfere with using focal reducer are:

- do you have enough back focus to fit body of reducer between sensor and T2 connection of OAG? Additional distance must be mirrored between OAG and main camera as well. Focal reducer moves focus point inward - this means another change in imaging sensor position vs OAG (not sure if this will bring it back forward or does it need still being pushed further out - probably this second point).

What about fixing problem of guide stars in another way? Things that you can do to guide on fainter stars:

1. Make sure your OAG is focused properly. Although you can guide on slightly defocused star, this is not a good thing if you want to use faint star for guiding. You want star light to be concentrated in smallest possible area to maximize star profile and SNR.

2. Position prism as close to light beam aimed at the sensor - just barely avoiding prism shadow on your imaging sensor. This means rotating prism so it sits next to longer sensor edge. Further out in the field stars become more distorted due to different aberrations like coma or astigmatism - look at difference between stars in above image - in right part they are more concentrated than in left part of the field. You want to pick up stars in the least distorted part of the field - again has to do with star light concentration into smallest region and SNR

3. At longer focal lengths you can use a simple trick because with small pixel guide camera you have enough precision - bin your guide camera output. It does not matter if it is true CCD hardware binning or CMOS software binning - it will increase SNR of stars and make it possible to guide on faint stars.

4. You can always increase guide exposure (up to mount limit), if you are used to guide at 1s or 2s exposures and can't find guide star - why don't you try 3s or 4s guide exposures? Sometimes in poor seeing I go as long as 6s or more with my HEQ5 (it is tuned and belt modded, and that is above max exposure that I would otherwise use on it but if seeing is poor, guide performance is not going to be the best possible anyway).

 

[vlaiv]

Forgot to add, if you can, guide with ASCOM driver and higher bit depth, rather than native drivers and 8bit.

[GraemeH]

Thanks vlaiv - you've given me a couple of things to think about.

I think I probably can improve the focus on my guide camera.  It's well enough focussed for reasonable guiding, but I do think it could be improved a little which may allow me to pick up fainter stars.

I'm already set up with the prism on the long side of my sensor (full frame DSLR), but I might be able to push it in a tiny further in to the light path.

I already use 2x2 binning for guiding, and do use the ASCOM driver at 12 bit.

I mostly use 3s guide exposures but there's no reason I couldn't try going a bit longer to see if it helps.

The one part of your reply that confused me a little was your comment about back focus effects on the imaging sensor - what I was proposing was to put the reducer between the prism and the guide camera so this should have no effect on my imaging camera in my mind.  I do understand what you mean about being limited by how much light can be picked up by the prism though.  My prism is 12.5mm, and the QHY5Lii chip is 4.8mm x 3.6mm but I haven't tried yet to calculate the size of chip that could be illuminated by that size of prism.

[vlaiv]

I'll try to explain with diagrams, first just reducer physical length:

Regular diagram:

Diagram with reducer in place:

As you see, in order to focus both guide camera and imaging camera at the same time, because you added focal reducer before guide camera and thus pushed it back, you need to add optical path between OAG and imaging camera.

Next diagram will try to explain additional distance needed because of the way simple reducer works:

This one is trickier to understand because reducer bends rays - this is why we need inward travel of focuser for simple reducers like mentioned two element 1.25" x0.5 reducer. Actual sensor will lie where bent rays meet to be in focus, but at the same time imaging sensor needs to be at a distance equal to where guide sensor would be without bending of the rays - further out.

Hope this makes sense

[GraemeH]

Thank you vlaiv - I think that's clear now.  If I understand correctly, then in order to keep the optimum backfocus distance to my imaging camera, I'd have to be able to position the guide camera much closer to the prism turret in the OAG.  I think this could be possible with the Celestron OAG.  I currently have a 6mm T spacer, then T-C adapter to attach the QHY5Lii to the guide port - I could remove these and insert the camera body into the 1.25" holder part of the OAG to push the camera sensor closer to the prism with the reducer in place.

The diagrams certainly help with understanding.

[vlaiv]

I have that GSO (Revelation) x0.5 1.25" focal reducer, and here are some things that you should know about it:

1. Focal reduction depends on distance between focal reducer and sensor - greater distance, greater reduction - use this in combination with above diagrams - start with close distance and small reduction to see if you are getting usable field at all - then extend distance up to point where it still works.

2. I had to reverse lens in mine - it was set one way at factory but I found that for imaging it works better by reversing the lens in holder (don't ask me how I got idea to try it out - probably read something somewhere). You might want to try which way it gives you better image, by undoing retaining ring and just flipping it around. Use some sort of soft cloth to handle the lens - don't touch it with your fingers to avoid staining it. You can do this at prime focus of scope with your guide camera to avoid any OAG related complications.

[GraemeH]

Just in case anyone else has similar thoughts about this I thought I'd confirm my results.  I have been able to get focus on my QHY5Lii with the 0.5x reducer screwed to the C mount extension supplied with the camera.  I suspect with other OAG designs this may not be possible because of the need to move the camera closer to the prism, but with the Celestron OAG I had enough room.  Platesolving an image from the guide camera has given me an effective focal length of 1141mm with my Edge HD8 (my imaging camera shows an effective focal length of 2140mm with this scope), so the reducer is giving me ~0.53x.  The wider FOV on the guide camera is great for picking up more stars for guiding.

[ArEnJee]

Really useful thread - I have an edge HD and Celestron OAG and was also wondering whether I could use a 1.25" reducer with my guide cam.  It seems as through I can :)

[michael8554]

A colour QHY5 is going to limit  you.

Use a planetarium to find a good guidestar.

With the Canons, and particularly with the Full Frame 6D, you will probably be cropping, so allow some prism shading, that will be corrected by Flats.

This is my 6D at 1400mm FL.

The prism position is accurate and does shade a little: