Trouble fitting the Distance for 0.7 Reducer - Forums

Topic starter

Mar 18, 2021 · Edited: Mar 18, 2021

Hi all,

I am using an C8 EHD with a Celestron 0.7 Reducer. In the manual i read the distance should be 105mm from the end of reducer to the camera chip to reach the exact focal length of 2032mm * 0.7 = 1422mm. But my problem is currently, i use astrometry.net to get the arcsec/pixel to calculate the focallength. I use this formula (camera pixels * 206,265) / arcsec/pixel
And with this formula i don't reach the focallength with 105mm distance. Currently I am at 126.5mm and still not at 1422mm.
With 126.5mm I am at 1433mm. Its next to 1422 but far over the 105mm from the manual.

Here is a Link to my last Upload to astrometry.net. There is the arcsec/pixel at 0.666.

This is my setup. Celestron 0.7 Reducer | Celestron OAG | filter drawer | ASI 294MC

Can someone help me? Maybe I use a wrong formula, or doing something else wrong.

Also curious for me is the amount of stars on my Image i use for platesolving. 30s exposure with Gain 270 an just a few stars on it.

But this is maybe a second Problem.

Help is extremly appriciated,
Thanks,
Daniel

P.S. Excuse my poor english.

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Haramir

Mar 18, 2021

Forget the exakt focal lenght. Use the distance they Guide in the Manual. Dont try to reach the exakt focal lenght.

Daniel

Topic starter

Mar 18, 2021

With other words, don't trust the analyse of astrometry.net. Use the provided information from the manual and use the 105mm distance.
I will do but then always there is such a feeling of insecurity. All time thre will be the question, Am I doing something wrong?
It's just a kind of lack of understanding. Analysing an image taken without reducer, then the calculated focal length is correct at 2032mm. That makes me think, the analyse is doing well.

Tim Hutchison

Mar 18, 2021

You should absolutely be at 105mm. Don't be distracted by the focal length calculations. What you should be looking at is the sharpness of your image and the shape of your stars. If you deviate even 2mm from 105mm you will negatively impact your image. Here are some documents to help you understand.

Read this white paper from celestron

And the specifications for the focal reducer
I have the EHD8 with a focal reducer and found it to be very sharp when properly collimated, placed on a good mount, used with the mirror locked, and proper back focus.

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Haramir

Mar 18, 2021

No the focal lenght calc is correct. Your System is Not exactly at F7. And this is normal.

NighttimeskyGuy

Mar 18, 2021

Hi, I might be missing something, but when I do the math 2032 * 0.7 = 1422, not 1244.
I have this same scope and reducer and it works great at 105 mm of back focus. Always good to check collimation too.
CS

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Rodolphe Goldsztejn

Mar 18, 2021 · Edited: Mar 18, 2021

Hi
In my experience, if you're not close enough to the 105mm distance (in or out), you'll face issues with much elongated stars seen by your guiding camera connected to the OAG, up to a point where guiding is almost impossible.
I use a 55mm backfocus camera + OAG (Moravian) and connected + a Moonlite motorized focuser, the setup is very tight but gives great results,
HTH
Rodolphe

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Mark Wetzel

Mar 19, 2021

I have a Celestron 9.25 Edge HD SCT. I used a 1 1/2" Blue Fireball SCT spacer near the scope to get to the 146mm back focus required. This way, the guide scope could be pushed nearly all the way in to the helical focuser on the OAG and be par focal with the imaging camera. The large SCT space also provides more clearance with the knobs and focus motor. Note that the ASI 1600 camera has a 7.5mm separation to the sensor (in the camera specifications) less a 1mm offset.

Mark W

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NighttimeskyGuy

Mar 19, 2021

Hi,
I found that I needed to add back space because of the filters if using a mono camera. About 1/2 the thickness of the filter used should be added to your spacing. Someone please comment if not accurate.
CS

Ian Dixon

Oct 9, 2021

I will chime in - I have a C8 edge (2020) and use a CHL Moonlite focuser with it. The beauty of the focuser other than having good fine control in the field, is that you can dial in the backfocus on the bench, then lock it down. This allows some flexibility for different additions to the optical train.

But my point is that I second the approach to focus only on the proper backfocus of 105mm. Try to stick to this as closely as you can. I have had good luck with the use of the reducer and at the native FL.

Best luck,

Ian

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