I have a whole new level of respect for imagers using SCTs and OAGs!!

[oymd]

I consider myself a beginner in AP, but I have gained some good experience over the past two years using a widefield Williams Optics 71 Star Apo and an EvoGuide 50mm for guiding.

I am fairly familiar with NINA, PHD2 and the quirks and problems with setup for AP.

Last week, with galaxy season arriving, I decided to give it a go with my Celestron C8 and an OAG.

Boy, this is a whole different level of DIFFICULT!!

I need some advice regarding my imaging train, and what am I doing wrong.

I setup the train for 105mm backfocus.

No matter what I do, I cannot get any sort of stars in the view of my 290mm mini on PHD2.

Just to check my imaging train is OK, I took 30 1 minute subs of M51 without any guiding.

Understandably the image is soft and stars are not that great.

 

Anyways, what am I doing wrong with regards to the OAG?

My imaging train is as follows:

C8 ===> Celestron 0.63 reducer ===> 50mm Celestron T2 extension ===> ZWO 42mm to 48mm adaptor ring ===> OVL OAG ===> 21mm ZWO extension ===> 11mm ZWO ring ===> ZWO 294MC Pro

Total imaging train amounts to about 104.5mm, but I added 2 thin spacers.

Issues I have:

1- There is obvious vignetting on the image window in NINA. Is this expected? Is it caused by the 0,63 reducer? Never saw that with any of my refractors?

Here is Arcturus. I used it for the autofocus routine in NINA

Why is there such obvious vignetting?

2- I imaged fine, but guiding was impossible. I CANNOT get the 290mm mini in to focus at night. During the day, I got it into focus on a very distant object, but for some reason, at night I just see GRAY NOISE? I tried Gain 0 and Gain 90. In PHD2 for the guiding focal length, I input 1280mm, which is what the 0.63 reducer amounts to. I realise that guiding is working at an incredibly small pixel scale of 0.5 pixels / arcsec!!

3- The OAG prism is correctly positioned, and I confirmed that by shining a light onto the scope. I placed the prism parallel to the long axis of the sensor, and is NOT covering the sensor.

4- Could it be that the OVL OAG i am using is not suitable for my imaging train?

5- I am aware of how small the 8x8mm prism is, and how borderline the 290mm mini is, and the 174mm mini is a better fit, but overall I am just trying to do a preliminary setup.

6- I am also aware that the Celestron C8SE OTA is not great for imaging, but as I mentioned, I am just dabbing into SCT astrophotography.

7- I have an EDGE 11HD, but I do not dare trying it yet. I want to familiarise myself with the C8 first.

 

Many thanks

[vlaiv]

21 minutes ago, oymd said:

Total imaging train amounts to about 104.5mm, but I added 2 thin spacers.

I was able to find that back focus is 85mm for this reducer - why go with 104.5mm?

Further away you put sensor - more reduction you will get and hence more severe vignetting.

Make distance between SCT corrector camera side thread and sensor about 85mm.

If you don't want prism of your OAG to be effective aperture stop - put it close to sensor. Swap OAG and 21mm extension (or possibly loose 21 mm extension all together as that will take very close to right working distance).

Don't be afraid to:

1) bin your camera

(check out drop downs that I outlined in your version of PHD2)

2) use long exposure with OAG - like 4s guide exposure.

I prefer to use ASCOM driver for my camera rather than native drivers.

[oymd]

7 minutes ago, vlaiv said:

I was able to find that back focus is 85mm for this reducer - why go with 104.5mm?

Further away you put sensor - more reduction you will get and hence more severe vignetting.

Make distance between SCT corrector camera side thread and sensor about 85mm.

If you don't want prism of your OAG to be effective aperture stop - put it close to sensor. Swap OAG and 21mm extension (or possibly loose 21 mm extension all together as that will take very close to right working distance).

Don't be afraid to:

1) bin your camera

(check out drop downs that I outlined in your version of PHD2)

2) use long exposure with OAG - like 4s guide exposure.

I prefer to use ASCOM driver for my camera rather than native drivers.

Thanks Vlad

The 105mm back focus was recommended by FLO when I emailed them?

I have also seen several YouTube videos where 105mm was the recommended back focus?

I will certainly give it a go with 85mm.

With regards to binning, the options on the 290mm mini are either x1 or x2. Should I choose x2? What does that change?

Many thanks Vlad.

Here we go again, after 2 years happily imaging with the 70mm apo, I am back to kindergarden with the SCT!!

 

[vlaiv]

9 minutes ago, oymd said:

The 105mm back focus was recommended by FLO when I emailed them?

I have also seen several YouTube videos where 105mm was the recommended back focus?

I will certainly give it a go with 85mm.

I see. I just checked specs at TS website and there it says:

This is bog-standard F/6.3 Celestron SCT reducer/corrector:

https://www.teleskop-express.de/shop/product_info.php/info/p2644_Celestron-Reducer---Corrector-f-6-3-for-Schmidt-Cassegrains-Telescopes--SCT-.html

Maybe you have EdgeHD version?

That one is 105mm back focus (according to TS website):

In the case you have standard one - then do pay attention that clear aperture is 41mm and with 0.63 reduction factor that translates into 25mm (at about 100mm away from focal plane), so even at 23mm you'll get vignetting.

Putting prism too far out will place OAG sensor in vignetted region - which will of course reduce amount of light it receives.

If you still have issues with your setup after shortening the distance and moving OAG closer to sensor - maybe consider sacrificing a bit of FOV (by cropping images) and pushing OAG prism closer to sensor (even if it starts casting shadow a bit).

 

[oymd]

43 minutes ago, vlaiv said:

I see. I just checked specs at TS website and there it says:

This is bog-standard F/6.3 Celestron SCT reducer/corrector:

https://www.teleskop-express.de/shop/product_info.php/info/p2644_Celestron-Reducer---Corrector-f-6-3-for-Schmidt-Cassegrains-Telescopes--SCT-.html

Maybe you have EdgeHD version?

That one is 105mm back focus (according to TS website):

In the case you have standard one - then do pay attention that clear aperture is 41mm and with 0.63 reduction factor that translates into 25mm (at about 100mm away from focal plane), so even at 23mm you'll get vignetting.

Putting prism too far out will place OAG sensor in vignetted region - which will of course reduce amount of light it receives.

If you still have issues with your setup after shortening the distance and moving OAG closer to sensor - maybe consider sacrificing a bit of FOV (by cropping images) and pushing OAG prism closer to sensor (even if it starts casting shadow a bit).

 

Yes, I definitely am using the standard 0.63 reducer on my NON EDGE C8SE OTA.

I will try 85mm tonight.

FLO definitely recommended 105mm though.

I changed around my train, and now the OAG is IMMEDIATELY after the 11mm ring on the camera side. so around 11mm + 6.5mm from the sensor.

But that will cause a problem when I factor in a filter drawer down the line?

[ollypenrice]

I suspect the 11 inch HD might actually be easier to use than the C8 which needs a flattener...

You have the OAG projecting into the middle of the long side of the chip, which is good. Did you also push it in as far as you could then back it out by the smallest possible amount? You can do this using a light source of the kind you'd use for flats. You just want to see the start of the OAG shadow then raise the prism very slightly so you don't. It's vital to get it as far in as you can.

Binning the guide cam will reduce its resolution but increase its sensitivity to light.  You have plenty of scope for reducing its resolution without its being inadequate for your guiding purposes.

You can measure your vignetting simply by comparing the ADU of a flatfield sub in the corners and then in the centre.  My Tak FSQ with full frame camera saw a 23% drop-off in light in the corners but flats eliminated it. It was absolutely not a problem.

Olly

[vlaiv]

4 minutes ago, oymd said:

But that will cause a problem when I factor in a filter drawer down the line?

It is better to guide on unfiltered light so OAG should come before filter.

Having said that - why are you using filter drawer with ASI294MC?

It is OSC sensor and I'm guessing you'll be using either LPS filter or some sort of UHC/Duo band filter. In any case - such filter won't need swapping mid session and it is sensible to just put it in T2 adapter right in front of the sensor rather than use filter drawer that facilitates easy filter change.

[oymd]

4 minutes ago, ollypenrice said:

I suspect the 11 inch HD might actually be easier to use than the C8 which needs a flattener...

You have the OAG projecting into the middle of the long side of the chip, which is good. Did you also push it in as far as you could then back it out by the smallest possible amount? You can do this using a light source of the kind you'd use for flats. You just want to see the start of the OAG shadow then raise the prism very slightly so you don't. It's vital to get it as far in as you can.

Binning the guide cam will reduce its resolution but increase its sensitivity to light.  You have plenty of scope for reducing its resolution without its being inadequate for your guiding purposes.

You can measure your vignetting simply by comparing the ADU of a flatfield sub in the corners and then in the centre.  My Tak FSQ with full frame camera saw a 23% drop-off in light in the corners but flats eliminated it. It was absolutely not a problem.

Olly

I will certainly try the flats trick!!

I have a geoptic flat panel.

Will experiment tonight.

I am confused however regarding the back focus.

As Vlad pointed out, TS optics clearly states that back focus is 85mm.

FLO and several you tube videos advised that back focus is 105mm?

[oymd]

1 minute ago, vlaiv said:

It is better to guide on unfiltered light so OAG should come before filter.

Having said that - why are you using filter drawer with ASI294MC?

It is OSC sensor and I'm guessing you'll be using either LPS filter or some sort of UHC/Duo band filter. In any case - such filter won't need swapping mid session and it is sensible to just put it in T2 adapter right in front of the sensor rather than use filter drawer that facilitates easy filter change.

Sorry Vlad, I did not follow that?

I have the ZWO T2 filter drawer, which is 21mm. I plan to use the Optolong UV/IR filter in it as I have bad LP.

Yes, I plan to go: Camera ===> 21mm Filter drawer ===> OAG, so that the OAG gets unfiltered light, but that moves the OAG AWAY from the sensor?

[ollypenrice]

Regarding backfocus, SCTs are very tolerant of variation, unlike faster systems. The consequence of changing the backfocus is that it changes the degree of focal reduction. Many SCT imagers plate solve their images to find out the true focal length of the reduced system.

Olly

[vlaiv]

1 hour ago, oymd said:

Sorry Vlad, I did not follow that?

I have the ZWO T2 filter drawer, which is 21mm. I plan to use the Optolong UV/IR filter in it as I have bad LP.

Yes, I plan to go: Camera ===> 21mm Filter drawer ===> OAG, so that the OAG gets unfiltered light, but that moves the OAG AWAY from the sensor?

There is simple T2-1.25" filter adapter that just screws in the camera nose piece (even if you have that 11mm removable T2 extension).

After you put it in - you still have plenty of T2 thread to attach camera to rest of the system.

It saves you using filter drawer and optical path.

[Padraic M]

I spent some time trying to image with the C8 last year but didn't persevere. There's a lot of confusion around what the correct back focus is, as other sources contradict what TS says. Unfortunately the manufacturer is silent on the matter. Incidentally, the TS item is branded TSOptics, but it looks identical to the Celestron one so I assume it is actually the same reducer.

For instance, Agena Astro have information on back focus https://agenaastro.com/articles/a-primer-on-back-focus-in-astronomy#components. This says:

"When the f/6.3 reducer/corrector is used for imaging, we need to work with its back focus - or what’s really its specified working distance (see Section 3.2). This distance is 105mm for Celestron’s reducer, much longer than reducer/flatteners for refractors. To use a DSLR and T-ring (which have an optical length of 55mm), it’s necessary to add a T-adapter tube with a path length of 50mm. The tube threads onto the reducer, then the T-ring of the camera threads onto the other end of the tube."

My plate-solved focal lengths at a range of back-focus distances are graphed here. This implies a .63x reduction at ~120mm (2032mm x 0.63=1280mm).

Focal length at 85mm was 1420mm. The corner aberrations looked like this:

So it looks like either the available information is not reliable, or there is a lot of variation in these OTAs or correctors. I gave up (wasting too many clear nights) but I'd be happy for someone more capable than me to check my homework and comment on other factors like collimation.

[vlaiv]

41 minutes ago, Padraic M said:

My plate-solved focal lengths at a range of back-focus distances are graphed here. This implies a .63x reduction at ~120mm (2032mm x 0.63=1280mm).

Problem with this approach is that focus position alters primary to secondary distance and that in turn alters actual focal length of instrument (it is no longer 2032mm) - so we can't really be sure of those figures.

 

[oymd]

5 hours ago, vlaiv said:

There is simple T2-1.25" filter adapter that just screws in the camera nose piece (even if you have that 11mm removable T2 extension).

After you put it in - you still have plenty of T2 thread to attach camera to rest of the system.

It saves you using filter drawer and optical path.

I get your point Vlaiv. But that will only work with 1.25" filters.

My problem is that ALL my filters are 2"!!

 

[Fegato]

I have the 8 Edge, Celestron OAG and use the 174mm mini as guide camera.

I can't really help with the backfocus. It's strange how little information on this there is. There are some threads on CN like this https://www.cloudynights.com/topic/612053-calculating-back-focus-for-celestron-063-reducercorrector/ . This one starts on C6, but covers other sizes. Anyway, sounds like it's a bit of a variable thing.  Certainly sounds harder than working with the Edge...  sorry, know that doesn't help!

 

On the 290mm stars - I did find that in some parts of the sky I struggled with finding guide stars, even with the larger Celestron prism. So in the end I invested in the 174mm mini, which has a much larger chip obviously, and that problem has gone away. Perhaps try pointing at a fairly starry field (if you can find one at this time of year) at least to see if you can find stars and focus?

 

[DaveS]

17 hours ago, oymd said:

I get your point Vlaiv. But that will only work with 1.25" filters.

My problem is that ALL my filters are 2"!!

 

Those filters are cheap as chips in 1.25", and in stock at FLO

[oymd]

18 hours ago, Fegato said:

I have the 8 Edge, Celestron OAG and use the 174mm mini as guide camera.

I can't really help with the backfocus. It's strange how little information on this there is. There are some threads on CN like this https://www.cloudynights.com/topic/612053-calculating-back-focus-for-celestron-063-reducercorrector/ . This one starts on C6, but covers other sizes. Anyway, sounds like it's a bit of a variable thing.  Certainly sounds harder than working with the Edge...  sorry, know that doesn't help!

 

On the 290mm stars - I did find that in some parts of the sky I struggled with finding guide stars, even with the larger Celestron prism. So in the end I invested in the 174mm mini, which has a much larger chip obviously, and that problem has gone away. Perhaps try pointing at a fairly starry field (if you can find one at this time of year) at least to see if you can find stars and focus?

 

What I find confusing regarding using a guide camera with a LARGER sensor, or using an OAG with a larger prism, is the fact that the actual light beam coming through the OAG is barely 6 or 7mm wide?

What would be the point of getting a larger sensor, if the actual opening in the OAG that send the image to the guide camera is only 7mm in diameter? It barely even covers the 290mm mini's sensor?

[Fegato]

4 hours ago, oymd said:

What I find confusing regarding using a guide camera with a LARGER sensor, or using an OAG with a larger prism, is the fact that the actual light beam coming through the OAG is barely 6 or 7mm wide?

What would be the point of getting a larger sensor, if the actual opening in the OAG that send the image to the guide camera is only 7mm in diameter? It barely even covers the 290mm mini's sensor?

Yes, sorry, it's a combination of the OAG and the guide camera sensor that give you the bigger area to play with. The Celestron OAG has a 12.5mm prism, and the ZWO OAG-L has I think 12mm. You'd need one of these to take advantage of the ASI174mm, so not much use to you if you have the smaller ZWO.

I guess the imaging sensor size and room left for the prism also come into play. I use the ASI294MC pro with the 8 Edge, and this leaves room with the Celestron OAG to take advantage of it. I do get some strange star shapes sometimes, but guiding works fine.

[Same old newbie alert]

Using a oag and a C8 isn't too difficult, the most difficult part for me was finding the correct adapter so I could incorporate my system into the required backfocus.. you would think using a Celestron scope with a Celestron oag and a Celestron branded guide cam would work wouldn't you? But it never did, and asking the brand for specs and advice made me feel like I was the div... Now Celestron say the required backfocus for their brand of reducer/flatener is 105mm so on platesloving this gave me a focal length of 1320mm and my stars wasn't fantastic, ie pointing inwards... So after much too-ing and fro-ing, I decided to adjust it to work at the spec'd focal length, ie 1280... So I increased the spacing which reduced the focal length.. now I'm working at 1280mm and stars look much better.. adding a better collimation helps enormously .. I've seen the 85mm floating around on cloudynights

The reason I think youre getting a vignette look is the size of your sensor, I get dark corners with my atik 460, yours is a 4/3rds size I think, it's all to do with the baffle in the c8

 I have no issues with my qhy 290 finding guidestar, never had to rotate the guidecam to find one, always plenty in the fov.. I just aligned the long side of the guidecam with the long side of the prism and imaging cam.. My procedure was to get the spacing of the imaging camera right,measure the distance from the prism to imaging sensor and that figure must be the same from the prism to guidecam incorporating the backfocus of the guide cam.. in PhD loop the camera and look for seagull shaped stars and fine tune until you get them as round as poss... That's it.. everytime you set up it's done ready for those galaxies, nebulae etc

 

 

[Padraic M]

5 hours ago, Same old newbie alert said:

So after much too-ing and fro-ing, I decided to adjust it to work at the spec'd focal length, ie 1280... So I increased the spacing which reduced the focal length.. now I'm working at 1280mm and stars look much better..

Just to be clear with this - are you getting 1280mm focal length and good stars at 85mm back focus?

[oymd]

1 hour ago, Padraic M said:

Just to be clear with this - are you getting 1280mm focal length and good stars at 85mm back focus?

I platesolved my images that I took over the past 2 nights. I was using 105mm backfocus and indeed I calculated 1280mm focal length. 

[Same old newbie alert]

2 hours ago, Padraic M said:

Just to be clear with this - are you getting 1280mm focal length and good stars at 85mm back focus?

I'm getting decent stars at 1280mm focal length .. I've not measured the spacing I just added spacers and platesolved until I got somewhere near 1280 but I think I'm more than 105mm

I've just landed back in the UK so when I get a chance I measure it and report back

[newbie alert]

On 13/04/2022 at 17:16, Same old newbie alert said:

I'm getting decent stars at 1280mm focal length .. I've not measured the spacing I just added spacers and platesolved until I got somewhere near 1280 but I think I'm more than 105mm

I've just landed back in the UK so when I get a chance I measure it and report back

It's me back on my original account.. measured the distance from the first threads on the reducer to the sensor... And I'm getting 113.30

[newbie alert]

[Padraic M]

Thanks for that. Also good to see where you're measuring from.