Noob question - Off Axis Guiding

[ollypenrice]

1 hour ago, Robindonne said:

I really hope your bad experience was an unfortunate moment.  Because if it works then i’m gonna spend my money on a very good assembly and safe on a guidescope guiderings tuberings mountingbars etc etc.
Im still in the need of buying most of probably not needing items. so all beginners including me have to decide which direction to go for the guiding.  
Oag seems to be a good (old)new option.

 

Both systems work. I don't want to get in a fight over this! Just one detail: you don't need guidescope rings. In fact they are a potential menace because they are flexure-prone. All you need to do is bolt the guidescope down hard. It does not need to be precisely aligned with with the imaging scope. 

In using an OAG with a small refractor you'll be choosing an unusual path but there's no reason for it not to work with a sensitive camera.* Be sure to set it up so that the prism comes in at right angles to the long side of the chip. That way it will get deeper into the light cone without shadowing the image. You can set the prism depth to the deepest possible (in search of more light) in the daytime by shooting flats. (Not flats that you'll use as flats but they'll tell you when the prism is intruding. When it does, just lift it till it doesn't.)

Olly

* Just one extra thought on this, though. An OAG adds both weight and cable drag to the load carried by the focuser. There are some very bad focusers out there, especially on small refractors where they are quite extended and have short support for the draw tube. The OAG will correct for any deviation this introduces but not for any tilt. Nobody said it was easy! 😁

 

[alacant]

2 hours ago, ollypenrice said:

a specific case

Please share the specific case that we may avoid.

Thanks

[ollypenrice]

Just now, alacant said:

Please share the specific case that we may avoid.

Thanks

From ODK14 to FSQ106N.

Olly

[alacant]

2 minutes ago, ollypenrice said:

ODK14 to FSQ106N.

With a Canon 6d? the OP has a 6d. What were the adjustments you had to make?

Thanks.

[ollypenrice]

1 minute ago, alacant said:

With a Canon 6d? the OP has a 6d. What were the adjustments you had to make?

Thanks.

It was a full frame CCD but so long ago that I can't remember. It was to do with prism depth, not focus.

Olly

[alacant]

Just now, ollypenrice said:

prism depth

I think with the 6d cropped to the size of a circle the 80mm refractor (I think it will cover aps-c with a decent ff)  produces, the OP can safely fix the prism along the long edge. 

HTH

[ollypenrice]

2 minutes ago, alacant said:

I think with the 6d cropped to the size of a circle the 80mm refractor (I think it will cover aps-c with a decent ff)  produces, the OP can safely fix the prism along the long edge. 

HTH

He can indeed but this can be done in the daylight using the 'flats' method. Has the OP said which 80mm he's using? I think there is one which covers full frame. I know from personal disappointment that the FSQ85 doesn't, despite Tak's claim.

Olly

[alacant]

6 minutes ago, ollypenrice said:

this can be done in the daylight using the 'flats' method

Does that mean it's my turn to explain?!

 

[Laurin Dave]

6 hours ago, Robindonne said:

Sounds good.  So theoretically ,when both cameras are focused on one scope, the combination stays in focus on different types of scopes? What is the thing mentioned earlier in this thread where users say that fl can influence the assembly and therefore needs adjustment? Is it safer to start the calibration of both cameras on a particular  focallength or aperture? Like calibration on an 14” is more likely to be spot on focused on an 8” compared to vice versa?

 

Oh and to ask for some buyersadvice, im thinking of buying the attached item from FLO.  Do you think i will get it working when the asi120 will be directly mounted on the oag when using a dslr as maincam? Or do i need a small helical focuser for the guidecam?   Cams used are asi120mc-s and a eos60d.   If it can be answered then thanks a lot.  

You’ll need to check whether with the ZWO OAG you can get the correct back focus if you are using a reducer or flattener..  there are OAGs designed to work with DSLRs and to give the common 55 mm back focus ..  TS optics I believe have them..  with the ZWO OAG I found it much easier to adjust fine focus using Delrin spacers rather than adjusting the guide head on the stalk..  

not sure what scope you’re putting it on but if it’s a refractor then id suggest ( as others have done) that a guide scope will be somewhat easier to use particularly in terms of having a good selection of guide stars 

[alacant]

8 minutes ago, Laurin Dave said:

ZWO OAG

Hi. To be sure, I'd recommend this oag instead. Works right down to only 55mm.

HTH

[ollypenrice]

58 minutes ago, alacant said:

Does that mean it's my turn to explain?!

 

I don't follow. Sorry.

Olly

[Robindonne]

Ceasefire!!

while the op is temporary unavailable due to some time-consuming trouble with adjusting his oag assembly, im soaking up the info and starting to realize both options can cause issues. And because im almost on the same route as the op this discussion is very helpfull.  
 

The TS oag is indeed a very good alternative. Especially when using a dslr.  (And the option to change in future to threaded connections) And other then the op, i use a 60d, so cropped sensor and probably plenty of room to do a 360 with the prism around the fov if necessary.  
 

If i sum up my modest starting tools to maybe help other users of these scopes.  
8” Quattro newton (f/4) (of course tube rings)

8” sct (f/10-f/6,3), No tube rings

5”sct (f/no idea), No tube rings

80mm refractor (f/“”) no tube rings

71mm refractor (f/“”) no tube rings

Will a modest starter-collection like this benefit from a single oag-setup? If not then 4 of the 5 ota’s needs rings and bars and maybe multiple guidescopea due to multiple FL.

All use a 2” connection btw

 

Edited May 6, 2020 by Robindonne

[alacant]

15 minutes ago, Robindonne said:

single oag-setup

Hi

The Quattro -nice- and the refractors will be fine.

My experiences using cameras on scts with or without guiding are not publishiable on a family site. 

HTH

[Laurin Dave]

2 hours ago, Robindonne said:

Will a modest starter-collection like this benefit from a single oag-setup? If not then 4 of the 5 ota’s needs rings and bars and maybe multiple guidescopea due to multiple FL.

 

The Quattro requires an OAG so nothing lost if it doesn't work with the refractors..  either of which you could use as the guidescope for the other 

[dannybgoode]

Just my tuppence worth.  I went the guidescope route with this package and it really is excellent.  You don't need extra rails or anything - the scope just replaces your finder scope and it comes with very solid rings etc.

Given with an OAG you still need to buy the actual guide camera the price is pretty much the same and I have had excellent results with it and it will fit any of my scopes right up to my 2500mm 10" SCT

https://www.altairastro.com/altair-60mm-guide-scope--gpcam2-mono-camera-combo-with-polar-alignment-assist-75-p.asp

Don't see the point of extra layers of complexity an OAG will bring to the party.  Fit guidescope into rings, fit guide camera into scope, focus once and lock it down and you can swap between scopes with ease.

[ollypenrice]

I recently processed some 10 inch Quattro data captured by another member. It was absolutely excellent and, given the large aperture for the focal length, it had gone very deep for the exposure time. The flip side of this is that the same member spent a long time reassembling and perfecting the instrument when he first took delivery of it. It was certainly worth the effort. I don't see why the 8 inch shouldn't be made to work as well as this. 

Olly

[spillage]

12 hours ago, Laurin Dave said:

The Quattro requires an OAG

Sorry I do not understand this? I have the 8" quattro with one of my st80's strapped to for guiding as I do no use an oag.

[ollypenrice]

Just now, spillage said:

Sorry I do not understand this? I have the 8" quattro with one of my st80's strapped to for guiding as I do no use an oag.

The advantage of an OAG on reflectors is simply that they 'see,' and so correct for, any movement of the primary in its cell. You're not certain to have such movement, or not often, but an OAG puts you on the safe side. You're guiding and imaging on the same light cone.

Olly

[spillage]

7 minutes ago, ollypenrice said:

movement of the primary

Oh thanks Olly. I always thought that SCT's had the issues with mirror flop and did not  think that reflectors suffered from it to that extent.

[JamesF]

11 minutes ago, ollypenrice said:

The advantage of an OAG on reflectors is simply that they 'see,' and so correct for, any movement of the primary in its cell. You're not certain to have such movement, or not often, but an OAG puts you on the safe side. You're guiding and imaging on the same light cone.

I was pondering on this the other day.  For reflectors with movable mirrors clearly it's better to guide on the same light cone.  But what about newts and mak-newts for instance.  Why should they suffer from mirror movement any more than a refractor suffers from lens movement?

James

[ollypenrice]

Just now, JamesF said:

I was pondering on this the other day.  For reflectors with movable mirrors clearly it's better to guide on the same light cone.  But what about newts and mak-newts for instance.  Why should they suffer from mirror movement any more than a refractor suffers from lens movement?

James

I was just mulling over the same question! We know that mirrors are usually left slightly loose, or at least not firmly gripped, to avoid pinching - and yet refractor objectives are also made of glass and are held firmly enough not to move. Could it simply be the size of the glass elements, the larger sizes of mirrors being more prone to expansion? I really don't know but I know a man who does. I'll ask him.

Olly

[JamesF]

37 minutes ago, ollypenrice said:

I was just mulling over the same question! We know that mirrors are usually left slightly loose, or at least not firmly gripped, to avoid pinching - and yet refractor objectives are also made of glass and are held firmly enough not to move. Could it simply be the size of the glass elements, the larger sizes of mirrors being more prone to expansion? I really don't know but I know a man who does. I'll ask him.

The expansion issue was the only idea that occurred to me at the time, though I have no idea how much expansion one might actually expect in the average borosilicate mirror over a temperature range of, say, 30C.

James

[Robindonne]

Cant dive this deep into it as you guys. But for a starter like me its clear.  Use both.  Not at the same time of course, i mean have both available.  And as i can see while shopping around you can have en proper guidescope for €£ 150/200,-

the mirror-movement discussion is of course very useful and basically says that oag’s on reflector/sct/mak etc are somewhat adviced.  
 

why did astrocamera manufacturers never made side by side sensors.  Is it just not possible? a small sensor next to the main sensor? Maybe it exists and im not aware of the excisting of some semi-dual mode camera? If it exists than i would really like to know because im still on the point of buying ap-equipment

Edited May 7, 2020 by Robindonne

[Robindonne]

And someone mentioned here about a solid fixing of the guidescope. Is this solid?   It will definitely save me some tubering because my small refractors already have this small mount used by a rdf, but is it solid enough for a guidescope?

[spillage]

17 minutes ago, Robindonne said:

is it solid enough

I guess it depends on your mount and guide scope. If you are near the max limit and you need to keep an eye on adding extra weight then you will have use a small guide scope which I think normally use this type of connection. But I would have thought that guiding stability may be compromised. If you can use scope rings clamped down as tight as possible then its not going to move.