Focal Reducer for RC 8" - Forums

Topic starter

Feb 27, 2018

Hi fellow RC-Users,

i own a 8" RC and want to buy a focal reducer. This is my current setup:
Setup with OAG
Please notice the OAG, which works fine :-) The idea is to buy this FR:
Astro Physics Reducer
The setup would be like this:
Scope -> FR -> OAG -> Main Camera
| Guiding Camera
Will this work? Will the guiding cam see enough light?
Has anyone experience with this (or a similar) setup? Is this a good idea?

Thanks,
Michael

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Chad Andrist

Feb 28, 2018 · Edited: Feb 28, 2018

Yes this will work, I use the same in my setup. The thing you will have to play around with is the spacing, the AP telecompressor actually does not have an exact backfocus like many other reducers or reducer/flatteners. Instead, you can vary the compression factor by changing the spacing. I currently reduced mine at .71x at a backfocus of about 72mm. You should consider getting some threaded spacers that allow you to adjust for the level of compression you desire. Do note that most do not have an ideal experience reducing at .67x or lower and report field curvature (elongated stars) towards the edges. Your distance from CCDT67 flange to guider chip and imaging chip should be the same for everything to come to proper focus.

More info at Astro-Physics

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Michael Timm

Topic starter

Feb 28, 2018

That is what i wanted to hear

Thank you very much for your explanations and the link, this will help me a lot planning to buy the correct accessoires. Am i right that i have to move the focuser way more inwards to achieve focus?

Chad Andrist

Mar 1, 2018

Michael:
Am i right that i have to move the focuser way more inwards to achieve focus?QUOTE QUOTE SELECTED

I think that I had to remove one of the threaded extension rings from the optical back of my RC8 so that I had enough inward travel. I have a Moonlite focuser, so the mechanics are different from the stock focuser. My drawtube travel is pretty short as it is. When I shoot at f/8, I just add a 2" drawtube extension to give me further back focus.

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Michael Timm

Topic starter

Mar 1, 2018

Okay, thanks a lot. I will try a bit around with the spacers, but somehow i will achieve focus. I've got enough 2" spacers lying around :-)

Jose Carballada

IOTD Staff

Mar 2, 2018

I use this combo now with my RC10 but previously with the 8" too.

I could tell you that: when you find the best distance and reduction you probably will keep it on your train all time, never will shoot at f8.
In my experience after to see the results at f6 (or so) it's so difficult to come back to the f8 again.
In fact now with the 10" I shoot at 1500 but I am afraid that I can not obteing good results at f8 with my setup (ASI1600 has a small pixel size and my sky is like it is)

Michael Timm

Topic starter

Mar 2, 2018

Whoa, i'm curious now. As soon as i have bought and tried, i will report back. I hope that my pictures are going to be a bit sharper at about f/6, because ASI178 has even smaller pixel than ASI16000

On the long run i plan to buy a ASI294 MC Pro, which seems to be the perfect partner for my setup :-)

Michael Timm

Topic starter

Apr 8, 2018

So, yesterday i could check the CCDT67. I believe that there is something wrong with the distance between reducer and the CCD-Chip. Astrometry tells me that the picture i have taken with binning 1 has a pixel scale of 0.622 arcseconds/pixel. This is about twice as with the unreduced setup 0.309 arcseconds/pixel. If i am not mistaken, then the reducer works at about 0.5, which is outside the sweet spot for this piece. Am i right? (Pictures: With reducer | Without reducer).

The formulas from Astro-Physics tells me that the CCD Chip is too far away from the reducer. I will buy a shorter connector between the reducer and the Orion OAG. I'll show a picture of the current setup later. Thanks fpr any advice ;-)

Michael Timm

Topic starter

Apr 8, 2018

This will help i hope (between OAG and reducer):
M42 - M48 Adapter

Michael Timm

Topic starter

Apr 10, 2018 · Edited: Apr 10, 2018

Okay guys, i need your help :-( I am a bit frustrated and confused. This is the setup i am talking about: Reducer Distance
It looks like the distance between the CCD-Chip (where the scale ends) and the back end of the reducer is about 90 mm. This information from Astro-Physics direct tells me that i should reach a reduction of about 0.65. Am i right?

But why do i get 0.5? The images With reducer and Without reducer are plate solved, the pixel scales are:
Without reducer: 0.309 arcsecs/pixel
With reducer: 0.622 arcsecs/pixel
This proves that the reduction factor is 0.309 / 0.622 = 0.499. That does not make any sense. What is wrong with my thinking? Anyone who can lighten the dark in my brain?

Jose Carballada

IOTD Staff

Apr 11, 2018

Michael:
Okay guys, i need your help :-( I am a bit frustrated and confused. This is the setup i am talking about: Reducer Distance

Did you bin in both pictures?

Michael Timm

Topic starter

Apr 11, 2018

Both pictures are unbinned.

Jose Carballada

IOTD Staff

Apr 11, 2018

But the size of the file is different, one is 1920x1304 and the other 2854x1794.

You need to solve your raw images using http://nova.astrometry.net/dashboard/# for example or other plate solve software WITH NO changes on the image obtained from the camera.

Jose Carballada

IOTD Staff

Apr 11, 2018

please, could you publish, or attach two frames from both targets?

Michael Timm

Topic starter

Apr 11, 2018

Okay, thank you for the hint. It was not clear to me that this matters, but you are right. I will take a look and see, if there are raw images available. If not, i will take new ones and try. Thanks.

Michael Timm

Topic starter

Apr 11, 2018

Okay, and here are the results (from 2 bin2 images):
with reducer: 0.928 arcsec/pixel
without reducer: 0.618 arcsec/pixel
leads to 0.618/0.928 = 0.6659.
You may now call me officially an idiot. Reviewing the data i first used, i totally forgot that i SCALED one of them. Of course the results are not comparabale then.
You made my day, Jose. Thanks a lot. That means that the reducer works fine. So there must be another reason why the pictures have so less details. Checking my ASI next. Thanks a lot.

Jose Carballada

IOTD Staff

Apr 11, 2018

I am glad to ear that!!!
Just for your info, I use a RC10 practically all the time with this reducer because the seeing is not good enought to use it at longer focal.
I use an ASI1600 with a little big pixels too, at 0.557 arcsec/pixel with reducer.

Michael Timm

Topic starter

Apr 11, 2018 · Edited: Apr 11, 2018

Okay, thanks for the information. I will try different exposure times as well as other settings. Most of my raw pictures are very light polluted, see this link (folder StackedOnly) for an example of my stacked raws without any editing. They were taken under a good sky with good seeing. The background is nearly so bright than the object, there is a problem somewhere.

Michael Timm

Topic starter

Apr 16, 2018 · Edited: Apr 16, 2018

The problem with the light pollution has a very easy explanation: After several experiments it is clear that the crayford focuser injects light into the system when there is some sort of lighting around. I will post a picture with the problem and my steps to a solution in my gallery soon. Maybe a black cloth or some sort of cover around the focuser will probably do the job.

Edit: Here is the picture.

Juan Pablo

Jul 23, 2021

Hi friends,
Can you help me choose a focal reducer for my rc8 ?, without generating traces in the stars, my main camera is a 294 mm, what do you think of 27tvph ?.

Thank you in advance.

regards

Brian Boyle

Jul 24, 2021

A very interesting discussion.

I have a GSO 0.75 x reducer and the CCD47 (which I think is a clone of the AP CCDT67) . I used rhe 0.75x reducer with a back focus of 80mm and the CCD47 a the recommeded back focus distance of 85mm for the full 0.67x compression.

When i was imagjng with my ASI1600MM Pro, things images looked more-or-less fine. But i upgraded (largely for my Esprit 100) to the full frame ASI6200MM and the images more than 7mm off axis or so looked terrible with the 0.67x reducer.

Part of this could be attributed to the reducer - and particularly pushing it to 0.67x.

But, much could be attributed to the non-flatness of the RC field. So i actually took the reducer out and did some test with out-of-focus star fields at the native f8. I found that even with perfect collimation, non-flattness really starts to make a noticeable difference to images beyond 6 or 7 mm on axis.

So much so, that my priority is no longer a reducer but a flattener. And my site “enjoys” 3-4arcsec at best. So I will end up going back to 1600mm and using binx2. There are some reducer/correctors out there, but they are expensive and need larger focusser than i currently have.

Bottom line is : your reducer will work fine if you stick with small format sensors. But if you go to larger sensors, you may have to re-think.

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Juan Pablo

Jul 24, 2021

Thank you very much Brian for your help. it is clearer to me. so I think I will opt for a ccdt67 + 294 mm.

greetings and clear skies