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QHY OAG-M parfocalising 268M with 5III-174

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#1 RaulTheRat

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Posted 14 January 2022 - 01:13 PM

EDIT: If the main body of this post isn't useful to you, I'd appreciate if you'd check the bottom 3 paragraphs for a question I have to see if you have any input. Thanks.

 

This is just a quick post to help anyone else looking to put the following setup (which I imagine must be reasonably common, or at least something with the same or similar backfocus) parfocal so that the guide camera and imaging camera are both in focus.

 

I just set up the following (roughly, in daylight, on a distant terrestrial object):

 

QHY 268M (17.5mm backfocus)

QHY CFW3M-US (7x 36mm unmounted type (17mm backfocus)

QHY OAG-M (13mm backfocus when including the 3mm thick front M54 adapter)

 

So the above were bolted together for a total of 47.5mm of consumed backfocus.

 

Initially, I expected, that since I have a short backfocus (approx 11.5mm) QHY guide camera (5iii-174), it would come

to focus in the OAG-M's helical focuser with no further adjustment. Wrong. In fact, the guide camera needed to be moved "in" (further forward in the light path), such that it would bottom out in the OAG when the main camera was focused without getting anywhere near focus.

 

I do, before anyone asks, have the OAG stalk fully retracted to be as short as possible and it is right at the edge of the field (which is ideal anyway, since my calculations showed this would give enough clearance for light cones as fast as around f/3 without the prism casting a shadow on the main sensor, and I wanted the imaging train to be capable of that in case I buy a hyperbolic newt like an epsilon at any point).

 

I worked my way up gradually, and eventually have needed 8mm of spacing between the filter wheel and the OAG (you could alternatively put this behind the filter wheel between it and the camera, but then you'd be increasing the chance your filters would vignette) to get both cameras to focus at the same point. Remember that is with the OAG stalk fully retracted, as far out to the edge of the field as it will go, if you wanted it further in, you'd need more spacing.

 

Hopefully that number might help save someone some of the trial and error I went through.

 

Now I have:

 

268M 17.5mm

CFW3M-US 17mm

Spacing 8mm

OAG-M 10mm

M54 adapter 3mm

 

TOTAL: 55.5mm

 

This bolts together well with the 23.5mm/24mm M3 bolts that come with the 268M. You must take care to ensure your bolt ends do not penetrate too far into the CFW, otherwise you will hit the filters, or jam the wheel with the tips of the screws. I measured the front-plate of the CFW as 3mm thick, you can afford your bolts to go 1mm or so beyond (into the wheel) but no more before they would interfere with something.

 

Given that a frequent focal reducer backfocus is 55mm, and I'm aiming for 56mm allowing for filter thickness, that is useable, but I had hoped when I initially bought this setup that I would not need the spacing, and would be able to squeeze more into the imaging train. For example, I have some manual rotators (about 12.5mm thick) that I might just have got away with (with a flattener that said 55mm but really needed more like 58mm) if I didn't need the spacing. 

 

In my opinion, QHY ought to provide better technical drawings so this can be determined in advance. I'm not trying to have a bash at them specifically, since it's normal with any companies system for this to require some tinkering, but especially since a selling point of the system is that it bolts together with no wasted backfocus, and that I have paired it with the shortest possible backfocus QHY guide camera, I had been expecting it to reach focus without additional spacing.

 

With the 8mm of spacing, although I have not yet made a final nighttime adjustment of focus, on a distant daytime target I need about 1mm of extension of the helical focuser on the OAG to focus the guide camera. That's ideal and will give me a little room to adjust either way tonight for final focus.

 

I have one question, if anyone got this far - the 5III-174 has a threaded-on IR cut filter. Removing this would have got the guide camera 6mm deeper into the OAG, hence reducing the spacing required to only 2mm. I have not done this, because I'm concerned that letting IR get to the sensor will mean my guide stars will bloat, leading to worse guiding performance.

 

I am actually wondering if I'm thinking about that wrong, since bloated guide stars aren't always such a bad thing - centroid calculation can deal with very defocused guide stars, the 174 is extremely sensitive and has big pixels (so I would still expect enough SNR to guide on) etc.

 

Has anyone actually tried removing that window to get their guide camera further in and reduce the required spacing? Does anyone have much to offer regarding my thoughts above about removing the IR cut?


Edited by RaulTheRat, 14 January 2022 - 01:14 PM.


#2 ChrisWhite

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Posted 14 January 2022 - 01:44 PM

I did skip to the end and will read more later to see if I can help otherwise. .. but while phd can deal with slightly defocused stars, it will always do better with a well focused star. Especially now that msg exists.
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#3 rgsalinger

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Posted 14 January 2022 - 01:56 PM

Moreover, the problem with defocused stars is that they are dimmer than focused stars. Using an OAG means that you don't have a nice fast F4 guide scope and so with a long scope you'll definitely have trouble. I suspect that with a short refractor that won't be an issue. 

 

You don't mention what scope you have and how much back focus you have to work with. However, if you are using a focal reducer/flattener that requires 55mm, just add spacing between the OAG and the filter wheel and you'll be able to achieve guid camera focus focus - 8mm is a lot of travel. 

 

Apologies if I misunderstand. I have 4 OTA's and 4 OAG's and every one of them required me to add spacing between the imaging camera and the OAG to get focus. It's also been useful in tuning the the flattener on my AP155 as sub mm spacers are readily available. 

 

My practice is to place the OAG prism as close as possible to the long side of the main camera chip until I get a shadow and then back it off. I'd advise you to worry about the F3 light cone when that actually comes to pass. 

 

Rgrds-Ross


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#4 RaulTheRat

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Posted 14 January 2022 - 04:59 PM

Moreover, the problem with defocused stars is that they are dimmer than focused stars. Using an OAG means that you don't have a nice fast F4 guide scope and so with a long scope you'll definitely have trouble. I suspect that with a short refractor that won't be an issue. 

 

You don't mention what scope you have and how much back focus you have to work with. However, if you are using a focal reducer/flattener that requires 55mm, just add spacing between the OAG and the filter wheel and you'll be able to achieve guid camera focus focus - 8mm is a lot of travel. 

 

Apologies if I misunderstand. I have 4 OTA's and 4 OAG's and every one of them required me to add spacing between the imaging camera and the OAG to get focus. It's also been useful in tuning the the flattener on my AP155 as sub mm spacers are readily available. 

 

My practice is to place the OAG prism as close as possible to the long side of the main camera chip until I get a shadow and then back it off. I'd advise you to worry about the F3 light cone when that actually comes to pass. 

 

Rgrds-Ross

 

Hi Ross,

 

Thanks for the reply. Yeah, I have a few OTAs, so part of the reason for wanting to keep it short is to make maximal use of the available backfocus on any OTA, so on an SCT where I've got lots to play with I want to get a motorised rotator and an external focuser in the room left, and on a refractor with shorter backfocus to get in under 55mm.

 

I did play around some more, my 8mm of spacing works, giving me 55.5mm (aiming for 56 with filters), so that pretty much works. I explored removing the IR cut filter from the guide camera (saving 6mm on the guide light path), but don't have the right bolts yet to put the main camera on at the right spacing for that. I'll probably give it a try just to try to eke out the last few mm, but what I have now works.



#5 Monkeybird747

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Posted 16 January 2022 - 11:06 PM

It looks like you are more or less needing B1-3 configuration from the camera guide. Since you are using the ultra slim version of the filter wheel it is not surprising you need some spacer between the OAG and the wheel. According to the QHY diagram they show a 10mm spacer between the OAG and wheel, and mention this is to reach guide camera focus. Also, you mentioned 17.5mm for the camera, but the diagram shows 12.5 assuming you are bolting it to the wheel as shown. I could have misunderstood your configuration, but just thought I'd post the diagram for reference.

 

You definitely don't need the IR cut filter for guiding. Don't worry about IR star bloat as it just doesn't matter for guiding, and PHD2 doesn't care.

 

MB

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Edited by Monkeybird747, 16 January 2022 - 11:12 PM.

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#6 RaulTheRat

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Posted 17 January 2022 - 09:41 AM

It looks like you are more or less needing B1-3 configuration from the camera guide. Since you are using the ultra slim version of the filter wheel it is not surprising you need some spacer between the OAG and the wheel. According to the QHY diagram they show a 10mm spacer between the OAG and wheel, and mention this is to reach guide camera focus. Also, you mentioned 17.5mm for the camera, but the diagram shows 12.5 assuming you are bolting it to the wheel as shown. I could have misunderstood your configuration, but just thought I'd post the diagram for reference.

 

You definitely don't need the IR cut filter for guiding. Don't worry about IR star bloat as it just doesn't matter for guiding, and PHD2 doesn't care.

 

MB

 

Thanks, that's really useful, the various configurations are a bit confusing, and indeed I was probably a bit hard on QHY as I hadn't seen that technical drawing before which is useful. 8mm seems enough to get parfocal, but obviously the overall backfocus consumed is shorter than I had noted previously by a full 5mm which is nice.

 

Thanks also for the IR pointer, I seem to be reading that elsewhere as well, so I plan to shorten the setup and remove the IR cut.



#7 BenKolt

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Posted 17 January 2022 - 09:50 AM

Raul:

 

With the exact same equipment, I ended up using about the same spacing between the FW and OAG as you with the exact same equipment.  I contemplated removing the front filter from the guide camera in order to recover the back focus, but I wasn't sure how to make that work well without exposing and possibly damaging the little sensor.

 

Ben


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#8 rgsalinger

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Posted 17 January 2022 - 12:04 PM

IME, there's just no need to use an auxilliary UV/IR filter on any guide camera. I've been using OAG's with 6 different telescopes over the past 11 years and I've never noticed "bloated" causing problems.

 

It's vastly more important to get the prism properly positioned or spend your money on a better guide camera. Most of the chips (290 and 174 are the ones I use) don't have much QE outside that visual range in the first place. No guiding system I know of cares if stars are slightly larger/smaller either. 

 

Look at the guide camera frequency versus QE graphs and see if you agree. Looks to me as if they reinforce the fact that there's really no point in a high/low cut filter - the QE rolls off all by itself. 

 

Rgrds-Ross


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#9 RaulTheRat

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Posted 17 January 2022 - 01:00 PM

Raul:

 

With the exact same equipment, I ended up using about the same spacing between the FW and OAG as you with the exact same equipment.  I contemplated removing the front filter from the guide camera in order to recover the back focus, but I wasn't sure how to make that work well without exposing and possibly damaging the little sensor.

 

Ben

 

Hi Ben, thanks for the confirmation. Yes, it's a shame they didn't just make that IR cut filter closer to the sensor and let it thread into the camera body without leaving such a big protruding bit. I also noticed that with the IR cut removed, you not only recover it's thickness, but also about another 1mm because without the filter the edges of the camera body get all the way to the bottom of the well of the helical focuser, when the filter is on the camera, it seems to stop at the tops of the screws you can see in the bottom of the well.

 

As everyone is saying, it's redundant in guiding anyway, so apart from the mechanical protection it gives the sensor and cover glass it seems to serve no purpose.

 

I can confirm that without the filter on, no harm will come to the sensor in use on that OAG, nothing in the helical can touch it or the cover glass, it's a good few mm down into the camera barrel.

 

I almost always leave the camera in the OAG unless it's being switched to another imaging train anyway, so protecting it isn't a big deal, but I get what you mean, if you were storing it separately to the rest of the imaging train that way I'd want to at least put a cap over it.


Edited by RaulTheRat, 17 January 2022 - 01:03 PM.


#10 RaulTheRat

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Posted 17 January 2022 - 01:02 PM

IME, there's just no need to use an auxilliary UV/IR filter on any guide camera. I've been using OAG's with 6 different telescopes over the past 11 years and I've never noticed "bloated" causing problems.

 

It's vastly more important to get the prism properly positioned or spend your money on a better guide camera. Most of the chips (290 and 174 are the ones I use) don't have much QE outside that visual range in the first place. No guiding system I know of cares if stars are slightly larger/smaller either. 

 

Look at the guide camera frequency versus QE graphs and see if you agree. Looks to me as if they reinforce the fact that there's really no point in a high/low cut filter - the QE rolls off all by itself. 

 

Rgrds-Ross

 

Thanks Ross, this is exactly what I'm thinking, I've yet to see how well the very edge of the field that I've decided to use illuminates the guide sensor, but I'm hopeful it'll be enough, the 174 is pretty sensitive and the big pixels are perfect for this application IMO.



#11 rgsalinger

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Posted 17 January 2022 - 02:34 PM

The 174 will be prism limited. We use a 174 on a 4.7 meter focal length RC at F8 and get guide stars every time. Secret is the very large prism in the Mega Moag that we use. Prisms rule at some point but few of us have 5 meter fl telescopes. 



#12 RaulTheRat

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Posted 17 January 2022 - 02:36 PM

Another follow up for anyone interested, I carefully measured the exact saving (including the screw heads I refer to above) by removing the IR cut filter on the 5III-174, it sits exactly 6.5mm lower in the OAG according to my calipers.

 

So given that I required about 1mm of helical focuser extension, I'm going to get bolts the right length to reduce my 8mm of spacing to 1mm, which should put the focal plane right at around 0.5mm of helical focuser extension, and result in 43.5mm of consumed backfocus for the whole setup, which is nice and compact, and lets me just squeeze in a 12.5mm thick manual rotator when working with a typical 55mm flattener (ie. 56mm required including filters).



#13 RaulTheRat

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Posted 17 January 2022 - 02:36 PM

The 174 will be prism limited. We use a 174 on a 4.7 meter focal length RC at F8 and get guide stars every time. Secret is the very large prism in the Mega Moag that we use. Prisms rule at some point but few of us have 5 meter fl telescopes. 

 

Yes, at 5M I'd imagine you'd need it. I've already noticed how restrictive the prism is, but it does illuminate enough sensor that I'd expect to be able to find guide stars at my focal lengths (up to around 2M).



#14 BenKolt

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Posted 17 January 2022 - 11:05 PM

I'm curious if this IR window impedes the use of this guide camera with the IF ONAG system?  Recall that this is a dichroic mirror that reflects optical light to the imaging camera but passes IR to the guide camera.  Would this window defeat the purpose of guiding with IR light?  I don't presently have any need to use my ONAG, however it will be important to remember this discussion when I do!



#15 Monkeybird747

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Posted 18 January 2022 - 12:21 AM

I'm curious if this IR window impedes the use of this guide camera with the IF ONAG system?  Recall that this is a dichroic mirror that reflects optical light to the imaging camera but passes IR to the guide camera.  Would this window defeat the purpose of guiding with IR light?  I don't presently have any need to use my ONAG, however it will be important to remember this discussion when I do!

Ben, the optical window of the color version is AR + IR Cut, but the mono version is AR + AR coated only. Unless I misunderstood, the OP is referring to a threaded IR-cut filter accessory, much like one you'd use in a filter wheel, and not the actual optical window.


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#16 BenKolt

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Posted 18 January 2022 - 01:26 AM

Ben, the optical window of the color version is AR + IR Cut, but the mono version is AR + AR coated only. Unless I misunderstood, the OP is referring to a threaded IR-cut filter accessory, much like one you'd use in a filter wheel, and not the actual optical window.

Thanks for the clarification!  That's good to know.  I purchased my mono version with the intention to use it with both OAG and ONAG.



#17 RaulTheRat

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Posted 18 January 2022 - 06:20 AM

Ben, the optical window of the color version is AR + IR Cut, but the mono version is AR + AR coated only. Unless I misunderstood, the OP is referring to a threaded IR-cut filter accessory, much like one you'd use in a filter wheel, and not the actual optical window.

 

Yes, in case I didn't mention, I'm referring to the mono version of the 5III-174, so the sensor cover glass is AR coated, but there is then a threaded-on IR cut filter in front of that which can be removed to save 6.5mm of backfocus.


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