Loss of OAG focus changing filters

[tooth_dr]

This is something I hadn’t experienced before but is now a problem.

I fitted an OAG onto my scope, to be used in combination with a 2600MC and filter drawer. I had an IDAS NBZ filter in the drawer, main camera in focus and adjusted the OAG into focus. A few days later I removed the NBZ filter and inserted a UV/IR filter into the drawer to keep it parafocal.  However the OAG was well out of focus.  I since discovered the Astronomic UV/IR filter is only 1mm thick and the IDAS filter is 2.5mm thick.

Given that the 2600MC has an inbuilt UV/IR cut filter should I just buy a clear filter instead of another UV/IR filter?  Are there other brands that are 2.5mm thick glass or is it just IDAS. I’m a Bortle 4 but my south is over a large town 5 miles away so maybe a LPF is a better choice?  Please help me decide 👍🏼

[michael8554]

You will have to refocus the guidecam for every filter change.

Or insert a 2.5mm clear glass filter, not a second UV/IR.

Or arrange 2600MC then OAG then Filter Drawer then OTA, then with or without a filter the guidecam will stay in focus once the 2600MM is refocused.

With the LPF the guidecam may not see many stars, I don't know.

Michael

Edited November 27, 2021 by michael8554

[Dr_Ju_ju]

Can you not fit the OAG before the filter, so that once set / aligned with the main scope, it won't matter then what filter is being used....

[tooth_dr]

4 hours ago, michael8554 said:

You will have to refocus the guidecam for every filter change.

Or insert a 2.5mm clear glass filter, not a second UV/IR.

Or refocus the guidecam after each filter change.

Or arrange 2600MC then OAG then Filter Drawer then OTA, then with or without a filter the guidecam will stay in focus once the 2600MM is refocused.

With the LPF the guidecam may not see many stars, I don't know.

Michael

Cheers Michael. Refocusing the guidecam is a non-starter as it’s done with both thumb screw and allen key bolt, the latter of which is very awkward to access.  I’m more concerned about the NB filter, can’t imagine being that many stars.  I’ll try to get a 2.5mm but IDAS don’t seem to be making one. 

[tooth_dr]

4 hours ago, Dr_Ju_ju said:

Can you not fit the OAG before the filter, so that once set / aligned with the main scope, it won't matter then what filter is being used....

The OAG is before the filter. In my head this was the right way. But when I change filters it puts the guidecam out of focus when I refocus the main camera. This makes sense when I think about it now. 

[vlaiv]

Maybe simplest solution is to get one of these:

https://www.teleskop-express.de/shop/product_info.php/info/p4591_TS-Optics-short-T2-Helical-Focuser---Adapter---camera-does-not-twist.html

On the other hand - how fast is your setup?

Small changes in focus position will have large defocus only on fast system. On slow system defocus will be much smaller. I'm asking this because OAG position should be as close to sensor on fast system to avoid vignetting of prism.

[tooth_dr]

26 minutes ago, vlaiv said:

Maybe simplest solution is to get one of these:

https://www.teleskop-express.de/shop/product_info.php/info/p4591_TS-Optics-short-T2-Helical-Focuser---Adapter---camera-does-not-twist.html

On the other hand - how fast is your setup?

Small changes in focus position will have large defocus only on fast system. On slow system defocus will be much smaller. I'm asking this because OAG position should be as close to sensor on fast system to avoid vignetting of prism.

It’s F4.65 Vlad, pretty fast.  I would like to truthfully avoid any additional focusing but that is a nice solution. 
 

Here is my assembly 

 

[vlaiv]

1 hour ago, tooth_dr said:

Here is my assembly 

That is fairly close already. Not sure if you could even fit helical focuser in there.

[Laurin Dave]

Hi Adam... I gave myself this issue a few years ago by mistake, mono  camera though with an EFW so had to change all the filters.. But I think you can do this (although I may have it the wrong way around)....   with 2.5mm thick filter in place I'd put a 0.5mm spacer (ie 1/3rd of the filter thickness difference) on the OAG stalk to move the Guide camera out and focus ..   when you change filters remove the spacer and it should still be in focus with the main camera (although in absolute terms out of focus so re-focus is necessary)...  You'll have to redo guiding calibration when you change over as the orientation of the camera will change but on the Mesu that takes no time at all..  

Dave

Edited November 27, 2021 by Laurin Dave

[Astroscot2]

Should be workable with following. 

[tooth_dr]

1 hour ago, Astroscot2 said:

Should be workable with following. 

I’m not having an issue getting focus, rather a shift in OAG focus when filters are changed, that means I have to manually adjust the OAG stalk.

My back focus is 75.6mm with a Riccardi 0.75x reducer, not 55mm, so I have a bit of room to spare. 

[Starflyer]

1mm to 2.5mm filter thickness will give you problems, I'd suggest moving to a 2mm UV/IR filter and setting the OAG focus half way between the two.

I had this with the difference between an IDAS LP filter and my Baader filters and focusing the OAG between the two solved the problem.  It's not perfect, but good enough, I have no idea why IDAS chose a 2.5 mm substrate 😶

 

[Whirlwind]

Can't you just get the helical focuser for the OAG to make refocussing easier - only issue is I'm not sure whether you can get it to work with that style of guide camera?

[tooth_dr]

26 minutes ago, Whirlwind said:

Can't you just get the helical focuser for the OAG to make refocussing easier - only issue is I'm not sure whether you can get it to work with that style of guide camera?

I want to avoid refocusing my guidecam. I’ll switch to a finderguider if it can’t be done.

 

36 minutes ago, Starflyer said:

1mm to 2.5mm filter thickness will give you problems, I'd suggest moving to a 2mm UV/IR filter and setting the OAG focus half way between the two.

I had this with the difference between an IDAS LP filter and my Baader filters and focusing the OAG between the two solved the problem.  It's not perfect, but good enough, I have no idea why IDAS chose a 2.5 mm substrate 😶

 

Thank you for your first hand insight!  I thought that 2mm might be close enough. If I can get a 2.5mm filter I will but it’s a bit if a rip off at £120

Edited November 28, 2021 by tooth_dr

[newbie alert]

Hi Adam, isn't your oag before the filters/ camera?

[tooth_dr]

15 minutes ago, newbie alert said:

Hi Adam, isn't your oag before the filters/ camera?

Sure looks like it 😅 but that is the problem.  When the OAG is in focus with the focuser at position X with filter A all is good.  But then I have to refocus to position Y with filter B because it's thicker/thinner, and it bends the light more/less.  When the focuser is at position Y then the OAG is not in focus. Looks like I need two filters with the same thickness so that the focuser is always at position X or very close to it.  Thats how I'm interpreting it.

Edited November 28, 2021 by tooth_dr

[david_taurus83]

16 minutes ago, newbie alert said:

Hi Adam, isn't your oag before the filters/ camera?

When he changes to the thinner UVIR filter he has to move the focuser in for the main camera and it throws out focus for the OAG as he needs to rack that back out by itself.

[Laurin Dave]

41 minutes ago, tooth_dr said:

 I thought that 2mm might be close enough. If I can get a 2.5mm filter I will but it’s a bit if a rip off at £120

A set of T2  Delrin spacers is only £11 ..   thinking a bit more about it though when you change filters your back focus will change by 0.5mm which may give you wonky corner stars with one of the  arrangements with an APSC sized sensor at f4.65... worth checking before committing..  you could move the camera rather than the guide camera with the spacer to adjust for correct back focus but then you'd need to sort both camera angle and guide calibration (although these would reset when you change back so you could use a bit of masking tape to align the camera and use different PHD2 profiles for the different guide camera orientations.. 

A bit of a faff though so same thickness filters and a guide scope would be simplest me thinks as you'll maintain back focus, won't lose subs from the other scope when this one focusses and will have far more guide stars to choose from.

[tooth_dr]

5 minutes ago, Laurin Dave said:

A set of T2  Delrin spacers is only £11 ..   thinking a bit more about it though when you change filters your back focus will change by 0.5mm which may give you wonky corner stars with one of the  arrangements with an APSC sized sensor at f4.65... worth checking before committing..  you could move the camera rather than the guide camera with the spacer to adjust for correct back focus but then you'd need to sort both camera angle and guide calibration (although these would reset when you change back so you could use a bit of masking tape to align the camera and use different PHD2 profiles for the different guide camera orientations.. 

A bit of a faff though so same thickness filters and a guide scope would be simplest me thinks as you'll maintain back focus, won't lose subs from the other scope when this one focusses and will have far more guide stars to choose from.

See I actually thought of this - I knew adding the NBZ would add 1/3 filter thickness which is why I purposely calculated my back focus to this and then added a UVIR filter for when I took the NBZ out to compensate. It never occurred to me that filters came in different thicknesses, I took it for granted that they all were 2mm.

With my dual rig accurately aligned (including rotation), and tilting accounted for using ollies chip tilt rig idea, the idea removing the main camera is 🤯

If a 2.5mm clear glass filter will solve this and let me continue to use the OAG then i would like to do that but does it work like that?

 

[tooth_dr]

Is there much difference in these two?  I’m Bortle 4 but most of my imaging is over my local town (large enough with about 25000 people) which is in the distance to the south

 

 

 

[Allinthehead]

I have this same issue and need to refocus the oag after I change from rgb imaging to duoband. It's not really a big deal for me as I have a zwo helical focuser and Ive marked the two focus points. I did need to add a baader Uv/ir filter for rgb imaging just to get to the proper back focus for my Epsilon. I also considered going with a clear filter and I can't recall why I went with the Uv in end. I wonder now if that was a mistake. 

[nfotis]

that seems to preclude buying filters from different vendors, though 😞

N.F.

 

[Thommy]

9 hours ago, nfotis said:

that seems to preclude buying filters from different vendors, though 😞

N.F.

 

Yes! My RGB filters are Baader, and Ha filter is Astronomics. The RGB filters are roughly parfocal, but the Ha is far off. I too have to manually refocus the guidecam when switching between RGB and Ha. The solution is to buy/sell filters to have a parfocal set. Or to use a guide scope instead, of course, but to be honest I've spend too much time avoiding flexure. 

[nfotis]

8 hours ago, Thommy said:

Yes! My RGB filters are Baader, and Ha filter is Astronomics. The RGB filters are roughly parfocal, but the Ha is far off. I too have to manually refocus the guidecam when switching between RGB and Ha. The solution is to buy/sell filters to have a parfocal set. Or to use a guide scope instead, of course, but to be honest I've spend too much time avoiding flexure. 

 

I suppose that using a guide scope with my Skymax 180 or C9.25 would be  a losing proposition...

N.F.