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Edgehd backfocus, moonlite CHL and visual use

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#1 RaulTheRat

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Posted 19 January 2020 - 01:15 AM


I'm thinking about an edgehd 925, which I would use for both visual and imaging.

I will need a decent focuser so I can lock the mirror for imaging, and although I need to look at the other options as well one that looks good is the moonlite CHL.

I'm curious - when using one of these for imaging, does it need to be removed when using the scope visually in order not to exceed the correct backfocus distance once a diagonal and eyepiece (or binoviewer) is in the light path?

Can you just unscrew the imaging train and stick a diagonal into the moonlite without any issues, or is it necessary to unscrew the moonlite and put the stock visual back and diagonal on when using the scope visually?

#2 james7ca

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Posted 19 January 2020 - 06:09 AM

That's an interesting question and I suspect that it would be hard to use the Moonlite focuser with anything but a very thin Off-Axis Guider (OAG) and I'd probably try to use an OAG for imaging.

 

That said, I've actually calculated whether I could use a motorized focuser with Innovations Foresight's On-Axis Guider (ONAG) and I think the only solution that would work on the 9.25" EdgeHD is to use Celestron's 0.7X reducer (which has the same backfocus as the standard f/10) with one of Optec's new TFC-LEO low-profile focusers (with a mid-range focus thickness of less than 40mm -- and a minimum of approx. 32mm). And you need to use the 0.7X reducer so that the focuser clears the mirror locking knobs on the back of the 9.25" EdgeHD.

 

However, part of the problem is that the ONAG is about 66mm thick which takes up a lot of the EdgeHD's 146.05mm of available backfocus.

 

How thick is the Moonlite CHL and do you plan on using an OAG for imaging?

 

[UPDATE]

According to a YouTube video from Moonlite the spacings on the 9.25" EdgeHD are as follows (camera backfocus will vary with model):

 

Focuser: 2.84" (72.1mm, is this the midrange or maximum given the focus travel?)

Spacers: 1.5" (I'm guessing you could substitute something here, like an OAG, maybe)

T-Adapter: zero (I guess they supply this adapter)

Camera: 1.4" (35.6mm, will vary with camera model and filter wheel)

----------------------------------------------------

Total: 5.74" (146mm)

 

It looks like Moonlite gives you special focus and mirror locking knobs to help with the clearance for the focuser. But, maybe they aren't really needed.

[/UPDATE]


Edited by james7ca, 19 January 2020 - 06:28 AM.


#3 Janco

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Posted 19 January 2020 - 07:31 AM

I've used the 9.25" Edge primarily for imaging with the moonlite focuser and some for visual.

The spacing issue is really something you need to be concerned about when it comes to imaging and not so much for visual. The convenience of having a rotable motorized crayford focuser far outways getting the spacing right for visual use.

Also the 146 mm figure seems to have a rather large error on it, especially true for the reducer. My optimal back focus was achieved at 154mm through trail and error by visual inspection and CCD inspector.

Edited by Janco, 19 January 2020 - 07:35 AM.


#4 RaulTheRat

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Posted 19 January 2020 - 07:43 AM

Yes I wanted to buy a Leo, because I had been planning a C11, but having thought through a few of the pros and cons I'm thinking about a 925 which as you said can't fit a Leo between the lock knobs.

Backfocus for imaging is not really a problem at all, with 146mm to play with there's ample for a typical imaging train. I do agree that it would be nice to fit an ONAG in, which on the C11 with a Leo was what I was planning, but having read some reports of issues with focuslock not finding stars I've sort of gone off the idea, which means not being able to fit the Leo on a 925 isn't such a big deal.

Now in the process of reading some other information last night about available focusers, I have sort of answered my own question about leaving a rear cell focuser installed during visual use, for example here's a diagram using a different focuser https://www.primaluc...escrizione2.jpg

That will be similar to the moonlite chl, ok so the moonlite can sink the reducer inside itself but that doesn't help conserve backfocus at f/10 anyway, so let's say we have about 80mm left after the focuser.

A quick Google days a typical 2" diagonal is about 110mm optical path length.

So I think the focuser needs removing for visual use, albeit the tolerances are rather loose (there's a document out there somewhere by Gaston from innovations foresight analysing how close to the optimal backfocus you must be on an edgehd, and as I recall you can be +/- 20mm without major aberrations), so if the moonlite is a bit shorter you could leave it on maybe, and I'm pretty sure that a Leo on a C11/C14 can be left on as well.

#5 Janco

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Posted 19 January 2020 - 08:14 AM

Yes I wanted to buy a Leo, because I had been planning a C11, but having thought through a few of the pros and cons I'm thinking about a 925 which as you said can't fit a Leo between the lock knobs.

Backfocus for imaging is not really a problem at all, with 146mm to play with there's ample for a typical imaging train. I do agree that it would be nice to fit an ONAG in, which on the C11 with a Leo was what I was planning, but having read some reports of issues with focuslock not finding stars I've sort of gone off the idea, which means not being able to fit the Leo on a 925 isn't such a big deal.

Now in the process of reading some other information last night about available focusers, I have sort of answered my own question about leaving a rear cell focuser installed during visual use, for example here's a diagram using a different focuser https://www.primaluc...escrizione2.jpg

That will be similar to the moonlite chl, ok so the moonlite can sink the reducer inside itself but that doesn't help conserve backfocus at f/10 anyway, so let's say we have about 80mm left after the focuser.

A quick Google days a typical 2" diagonal is about 110mm optical path length.

So I think the focuser needs removing for visual use, albeit the tolerances are rather loose (there's a document out there somewhere by Gaston from innovations foresight analysing how close to the optimal backfocus you must be on an edgehd, and as I recall you can be +/- 20mm without major aberrations), so if the moonlite is a bit shorter you could leave it on maybe, and I'm pretty sure that a Leo on a C11/C14 can be left on as well.

Raul, I honestly doubt if you will see any significant field curvature visually at F10. You are however welcome to test it as it won't cost you anything. I use to split hairs about those figures however in the end it wasn't worth it, specially for visual use.

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#6 RaulTheRat

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Posted 19 January 2020 - 08:20 AM

Raul, I honestly doubt if you will see any significant field curvature visually at F10. You are however welcome to test it as it won't cost you anything. I use to split hairs about those figures however in the end it wasn't worth it, specially for visual use.

Sent from my SM-G975F using Tapatalk


Yes Janco I agree with you, theory and calculation are all very well but I suspect even at +30mm or more any aberrations are minor for visual use.

For sure I'll try it with a diagonal in the focuser, and I wouldn't be at all surprised to find that despite it being over the recommended distance it's no problem.

#7 james7ca

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Posted 19 January 2020 - 08:54 AM

...I do agree that it would be nice to fit an ONAG in, which on the C11 with a Leo was what I was planning, but having read some reports of issues with focuslock not finding stars I've sort of gone off the idea...

Are these reports on CN and how many individual cases have you read concerning this apparent problem (with FocusLock).



#8 RaulTheRat

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Posted 19 January 2020 - 09:17 AM

Are these reports on CN and how many individual cases have you read concerning this apparent problem (with FocusLock).


This was one of the threads I read: https://www.cloudyni...erta-foag-help/

Google will find you some more, I'm not saying focuslock isn't reliable in at least some setups, but it looks like a fair few users have had difficulty and it's put me off a bit. It would be nice to eliminate focusing time from a sequence, as it can add up to quite a bit of time that the system is doing stuff other than imaging, but ultimately with the cost of the ONAG and the fact that it would only be able to guide a scope with long backfocus (meaning a bit of fiddling when moving the imaging train to a different scope without the required backfocus) I don't think it'll be the route I'll go.

#9 james7ca

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Posted 19 January 2020 - 10:07 AM

Okay, thanks.

 

However, that report seems to be about trying to use the FocusLock software (technique) with a standard Off-Axis Guider (OAG), not the On-Axis Guider (ONAG) that is offered/sold by Innovations Foresight. In fact, Gaston seems to be willing to admit that using an OAG may lead to problems. Also, I believe that the ONAG can use full-frame focus with SkyGuard which can't be done with an OAG.



#10 RaulTheRat

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Posted 19 January 2020 - 11:47 AM

Okay, thanks.

However, that report seems to be about trying to use the FocusLock software (technique) with a standard Off-Axis Guider (OAG), not the On-Axis Guider (ONAG) that is offered/sold by Innovations Foresight. In fact, Gaston seems to be willing to admit that using an OAG may lead to problems. Also, I believe that the ONAG can use full-frame focus with SkyGuard which can't be done with an OAG.


Ah yes indeed I seem to have misunderstood that then. I stand corrected.


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