16 replies to this topic

[Leo VH]

Hi all, a few weeks ago I received my Celestron Edge HD 800 as an upgrade for my (old) INTES MK69. I also bought the 0.7 reducer. This setup will be used  in combination with my ZWO ASI071MC Pro. Since the weather is really terrible the last couple of weeks I didn't get the opportunity to test this setup so far, but in the few hours that I did I noticed that I couldn't get the back focus right with the 0.7 reducer. According to the Celestron specifications the back focus should be 105mm... but that didn't work at all. I tried to get the back focus on the right spot with several spacers without any succes. Therefor I decided to ask Celestron if the 105mm backfocus in combination with the reducer could be wrong... and this was there answer:

 

Thank you for contacting Celestron Technical Support.
There has been some confusion about the reducer's back focus. The 105mm figure is for the .63X NON-egde HD reducer for the classic SCT telescopes.
The back focus for the .7X reducers on the Edge HD telescopes does not change from the telescope's native back focus without the reducer.
The back focus with the reducer on your 8 Edge HD will be 133.35mm as measured from the back flat flange on the rear of the reducer.
If the Edge HD focal reducer star coma tails point inward, the back focus is too close, if the coma tails point outward the back focus is too far out.
Good luck!

 

I guess this can be helpfull for other users that are struggeling with the same problem....

 

BR

 

Leo

[jonnybravo0311]

I'm pretty sure you got wrong information from Celestron support. Every piece of documentation from Celestron very clearly states the back focus distance with the 0.7x reducer on the 8" EdgeHD is, in fact, 105mm. Here's their own manual for it: https://s3.amazonaws..._inst_sheet.pdf

[AstroVagabond]

Interesting I have mine set to ~ 105.9 and it looks reasonably good in my view.

[idclimber]

Interesting I have mine set to ~ 105.9 and it looks reasonably good in my view.

Assuming you are using a mono camera and a filter (in your sig), that would account for the 0.9mm difference. 

[Oort Cloud]

Celestron, SMH.

105mm with reducer (f/7), 133mm without (f/10).

The larger OTAs are 146mm regardless of whether the reducer is installed.

ETA: this is why the 8" T-adapter comes with a 28mm extension, to go from 105mm to 133mm...

Edited by Oort Cloud, 30 January 2023 - 02:52 PM.

[HubSky]

As others are indicating, that response from Celestron is wrong.  The statement is true for all of the other Edge HD reducers, but not the 800 Edge HD reducer.  

[dhferguson]

Cheers,

 

Consider buying the Agena Astro/Blue Fireball C-04 adapter. This is a thin adapter to attach your reducer to the camera side of the image train, and yields the maximum spacing for your equipment. I use the following three imaging trains at f/7 with my own Edge 8 as-follows:

C8 Edge-->Celestron x0.7 reducer-->C-04 Adapter-->Celestron OAG--> ...

 

... (1) Nikon T-ring

         Nikon D5600 DSLR,

 

... (2) Zwo 16.5mm spacer-->Zwo 21mm spacer-->ASI2600MC,

 

... (3) Zwo 16.5mm spacer-->Zwo filter holder-->ASI2600MC.

 

All these combos give the correct 105mm back focus from the reducer. Many dedicated astrocams, including the ASI2600MC, use a near-standard 17.5mm distance from the mounting plate to the detector. For the DSLR, removing the two Zwo spacers and adding the T-ring yields the correct 55mm distance from the COAG to the DSLR detector.

 

Happy observing always,

 

Don

[Oort Cloud]

Cheers,

Consider buying the Agena Astro/Blue Fireball C-04 adapter. This is a thin adapter to attach your reducer to the camera side of the image train, and yields the maximum spacing for your equipment. I use the following three imaging trains at f/7 with my own Edge 8 as-follows:
C8 Edge-->Celestron x0.7 reducer-->C-04 Adapter-->Celestron OAG--> ...

... (1) Nikon T-ring
Nikon D5600 DSLR,

... (2) Zwo 16.5mm spacer-->Zwo 21mm spacer-->ASI2600MC,

... (3) Zwo 16.5mm spacer-->Zwo filter holder-->ASI2600MC.

All these combos give the correct 105mm back focus from the reducer. Many dedicated astrocams, including the ASI2600MC, use a near-standard 17.5mm distance from the mounting plate to the detector. For the DSLR, removing the two Zwo spacers and adding the T-ring yields the correct 55mm distance from the COAG to the DSLR detector.

Happy observing always,

Don

It should be noted that the reason that adapter gives you the correct back focus is _because_ you are using the Celestron OAG, which is thicker than most others I've seen. If OP was to pair the scope with a ZWO OAG & camera, for example, they would only need to exclude the 16.5mm extension that comes with the camera, because the OAG is 16.5mm. I'm not familiar with eht Blue Fireball adapter you referenced (They make good stuff though!), but I would assume the fact that it yields proper back focus means that it was _designed_ to be used with the Celestron OAG. The model name "C-04" only convinces me further.

[Leo VH]

yesterday evening I tried the 133mm back focus as suggested by Celestron and it soon turned out to be wrong....so I switched over to 105mm again and used spacer up to 10mm (in 0.5mm steps). It didn't made any difference. The images are still bad. Very frustrating...

[HubSky]

You haven't told us what your image train is or if you are using an OAG or a guide scope nor described or provided an image to show what you mean when you say the image is bad. 

[Leo VH]

I'm using a guide scope not an OAG. So it's the 0.7 reducer, the longer part of the T2 adaptor, a ZWO 16.5mm + 21mm spacer ring and the ZWO ASI071MC Pro. The T2 adaptor is this one: https://www.teleskop...-telescope.html

 

Here is a 3min image of Capella. I used Capella because it was very close to zenith and easy to check/correct the collimation. The quality of the image is poor due to the 500kb restriction

[idclimber]

Can you take a photo of the imaging train from the back of the scope to the camera? Something seems odd as you should not have that much vignetting with the Edge even with an APS sized sensor. 

[HubSky]

I'm using a guide scope not an OAG. So it's the 0.7 reducer, the longer part of the T2 adaptor, a ZWO 16.5mm + 21mm spacer ring and the ZWO ASI071MC Pro. The T2 adaptor is this one: https://www.teleskop...-telescope.html

 

Here is a 3min image of Capella. I used Capella because it was very close to zenith and easy to check/correct the collimation. The quality of the image is poor due to the 500kb restriction

Capelle_3min_exp_105mm_back_focus_0.7red.jpg

I don't have any solutions just some comments.

 

1)  I've never seen that weird wave front pattern that is showing on the over exposed Capella.  Don't know what that is.

2)  Your setup should give you a back focus of 104mm which should be close enough, though you should always look at these as starting points if the back spacing is not working out. 

3)  The stars seem to be primarily elongated radially which usually indicates your sensor is too close to the corrector.  They also seem to be elongated differently in the corners which may indicate also a tilt issue.

4)  I gave up trying to guide my SCT with a guide scope.  Too many subframes thrown out due to slight mirror shifts and flexure.  Switched to an OAG and never looked back.

5)  My vignetting with my 2600mc is almost identical to what you have when using the reducer. 

Edited by HubSky, 31 January 2023 - 03:41 PM.

[Leo VH]

Here's a picture of my imaging train with the reducer.

 

[DirtyRod]

Looks about like the vignetting I get with my 071 using my reducer. Flats take it right out. 

 

Agree with Hubsky about increasing the backfocus and a little tilt. I'm on my 3rd reducer trying to get one with less tilt but yours is better than mine based on the stars in that image. 106.3mm is about the right backfocus for my config. 

Attached Thumbnails

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Edited by DirtyRod, 01 February 2023 - 11:21 AM.

[Leo VH]

Looks about like the vignetting I get with my 071 using my reducer. Flats take it right out. 

 

Agree with Hubsky about increasing the backfocus and a little tilt. I'm on my 3rd reducer trying to get one with less tilt but yours is better than mine based on the stars in that image. 106.3mm is about the right backfocus for my config. 

As I mentioned earlier I increased the backfocus in small steps up to 10mm....without succes...

[Leo VH]

small update... the dealer has asked to send the reducer back for replacement.... fingers crossed...