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Amajed

Oddly shaped stars and count numbers

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Hi, last night I imaged for the first time using my new EdgeHD 8" but the results wasn't pleasing.

my gear:

Sky Watcher EQ6-R + QHY PoleMaster

EdgeHD 8" + Celestron FR 0.7x

ZWO ASI1600MM Cool Pro

ZWO EFW + ZWO LRGB 36mm unmounted filters

ZWO ASI290MM + 80mm Guide Scope

Bahtinov Mask (I used Sirius star for focusing, I think now it's a bad choice considering it's apparent size)

My back focus was 106mm from the reducer to the sensor. ( the extra 1mm was recommended by ZWO when using filters)

everything is screw in type, so everything is tight together.

my guide RMS between 0.4px and 0.5px.

the next image was shot using the following settings:

120sec exp

1x1 binning (I think I should've used 2x2)

Lum Filter

Unity Gain 139

 

https://i.imgur.com/Qe8Lvrh.jpg

 

the picture looks like a bit out of focus, but the shape of the stars is weird, is that how it supposed to look like if I'm out of focus?

I suspected that I need to collimate my SCT, but when I tested it using an artificial star it was a perfect Donut. so it shouldn't be that.

what do you think?

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I think its a combination of focus a wee bit soft and poor tracking rather than collimation - the weird shapes all point the same way.

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5 hours ago, Skipper Billy said:

I think its a combination of focus a wee bit soft and poor tracking rather than collimation - the weird shapes all point the same way.

Thanks Billy, why would my tracking be bad when I'm using PoleMaster + my guiding RMS is bellow 0.5px (imaging scale is 0.52px). are there other numbers I should be looking at to know if i'm tracking poorly?

for focusing, I'll try to focus on a smaller star next time to make sure I reach the perfect X shape.

 

Thanks

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Your tracking is fine (for the 80mm guide scope) but my guess is that there is some movement (flex) between the two OTA's during the exposures. How long had you left the two scopes to cool to ambient temperatures because again the two tubes cooling at different times (i.e. guider cooled to ambient Edge Hd8 still cooling...). Focusing is off but wouldn't cause the odd shape.

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2 minutes ago, fwm891 said:

Your tracking is fine (for the 80mm guide scope) but my guess is that there is some movement (flex) between the two OTA's during the exposures. How long had you left the two scopes to cool to ambient temperatures because again the two tubes cooling at different times (i.e. guider cooled to ambient Edge Hd8 still cooling...). Focusing is off but wouldn't cause the odd shape.

I'm not sure for how long, but it's pretty long, maybe two hours (also temp is not cold around here, so there's not so much to cool down).

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5 minutes ago, Amajed said:

I'm not sure for how long, but it's pretty long, maybe two hours (also temp is not cold around here, so there's not so much to cool down).

OK. Cooling may not be the issue. Post a photo of the scope/guider set-up that may give a clue.

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2 minutes ago, fwm891 said:

OK. Cooling may not be the issue. Post a photo of the scope/guider set-up that may give a clue.

Thanks fwm891.

these pictrures are from last night. let me know if you need more picture.

 

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Hi. I think for consistent results, you're going to have to guide with an OAG [1]. At that focal length, it's difficult or impossible to eliminate flexure -fractions of a mm, expansion and contraction of different materials, moments...-between the imaging and guide telescopes.

[1] I don't think however you have enough FOV; you would need to lose the reducer.

HTH.

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3 minutes ago, alacant said:

Hi. I think for consistent results, you're going to have to guide with an OAG [1]. At that focal length, it's difficult or impossible to eliminate flexure -fractions of a mm, expansion and contraction of different materials, moments...-between the imaging and guide telescopes.

[1] I don't think however you have enough FOV; you would need to lose the reducer.

HTH.

Hi alacant, I don't understand why if I use a OAG I'll have to lose the reducer? what do you mean by not enough FOV? I used ZWO OAG before with this setup, but decide to use my old 80mm because it will be easier for me to find a suitable star. but for guiding, Reducer + ZWO OAG was working just fine (I think).

 

if you think I should go back to the ZWO OAG (or even get the Celestron OAG) I will if it will help with my problem.

 

Thanks

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11 minutes ago, Amajed said:

not enough FOV?

You say it was working fine but you had difficulty finding stars? Was the guiding better than with the separate guide telescope? If it was, then good.

We tried wit a c8 and reducer and there was not enough coverage around the edge of sensor FOV -where the OAG prism protrudes into the light cone of the telescope. At f10 it was fine.

One thing you could try is to lose the guide telescope rings. Use conventional tube rings and bolt directly to the top rail.

HTH and clear skies.

Edited by alacant
prisim prism
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2 minutes ago, alacant said:

You say it was working fine but you had difficulty finding stars? Was the guiding better than with the separate guide telescope? If it was, then good.

We tried witha a c8 and reducer and there was not enough coverage around the edge of sensor FOV -where the OAG prisim protrudes into the light cone of the telescope. At f10 it was fine.

One thing you could try is to lose the guide telescope rings. Use conventional tube rings and bolt directly to the top rail.

HTH and clear skies.

I see, yeah I had to make the prism almost hit the edge of the imaging sensor for me to be able to get a good view for my guiding cam. when I said fine, I meant it was tracking, but sometimes I had to move the telescope around to get a good star. which was annoying.

looking at the Celestron OAG, its huge, I don't think I can use it with the Reducer, I'll be way past the 106mm required space. so it's ether the ZWO OAG or a Conventional tube rings.. right?

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18 minutes ago, Amajed said:

Hi alacant, I don't understand why if I use a OAG I'll have to lose the reducer? what do you mean by not enough FOV? I used ZWO OAG before with this setup, but decide to use my old 80mm because it will be easier for me to find a suitable star. but for guiding, Reducer + ZWO OAG was working just fine (I think).

 

if you think I should go back to the ZWO OAG (or even get the Celestron OAG) I will if it will help with my problem.

 

Thanks

Thanks for the photos. I think your problem is with the 80mm guidescope. The OAG with the EDGE HD8 is the best solution. You say that it works with the ZWO OAG - I'd be inclined to use the OAG, look at the pick-off prism position and see if you can get further into the image cone without putting a shadow on the chip. Nice set-up and location.

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There's a big difference in the pick-off prism sizes between the ZWO and Celestron OAG's. I don't know if there is a specified back focus with the reducer - your camera seems a long way back (to me) check the specifications.

Spacing from the celestron 0.7x reducer gives a back focus of 105mm ?

https://www.firstlightoptics.com/user/manuals/94242_Edge_8_HD_Reducer_Lens.pdf

Edited by fwm891
text added
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Just now, fwm891 said:

There's a big difference in the pick-off prism sizes between the ZWO and Celestron OAG's. I don't know if there is a specified back focus with the reducer - your camera seems a long way back (to me) check the specifications.

with the reducer, its 106mm for backfocus.

the T Adapter (with the orange ring) is 50mm, after that I followed this diagram to get to a perfect backfocus:

spacer.png

 

 

the issue with Celestron OAG (from looking into the PDF file) it says that the OAG body alone is 29mm, and the SCT Adapter is 25.3mm. so the OAG setup alone is 54.3mm plus T adapter. that is 104.3mm. now I need to figure out how to attach the filter wheel (20mm) and the ccd (6.5). I don't think I have much to work with here :D?

BUT! I do have a SCT to M48 thin adapter (I can ditch the T Adapter) and that will give me a lot of space to work with.... right? I'm gonna have to calculate that... but it might work.

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Does your T adapter not split into two parts?

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Just now, fwm891 said:

Does your T adapter not split into two parts?

it does, the 50mm is only one part, I think the two parts are almost 76mm or something.

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To fit the reducer you had to take the visual back off the scope - will that not fit onto the reducer (camera side) instead of the T adapter and give you more control over back focus (just a thought) ?

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Just now, fwm891 said:

To fit the reducer you had to take the visual back off the scope - will that not fit onto the reducer (camera side) instead of the T adapter and give you more control over back focus (just a thought) ?

I think so, but that will require me to use a nose piece, which I don't want to, I like to screw things together for a tight and snug setup.

But wait, I think I got it all wrong about the Celestron OAG, I think I do not require the T Adapter if I'm using it. it does have a "SCT Adapter" isn't that the T Adapter's job? so I don't need the T Adapter anymore, it's like this:

Reducer > SCT Adapter 25.3mm (came with OAG) > OAG 29mm > M48 Camera Adapter 4.5mm > T2 Adapter for Filter wheel 2mm > Filter Wheel 20mm > CCD 6.5mm

 

I'll need spacers about 18.7mm (I need to figure out how to reach this number :D) other than that, it should work!

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1 hour ago, Amajed said:

ether the ZWO OAG or a Conventional tube rings

If I had to choose it would be the former but without the reducer.

Cheers and clear skies:)

EDIT: I never understood why one pays extra to have a nice aperture and long focal length (great galaxy shots), then shorten it. It collects the same amount of light either way!

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1 minute ago, alacant said:

If I had to choose it would be the former but without the reducer.

Cheers and clear skies:)

EDIT: I never understood why one pays extra to have a nice aperture and long focal length (great galaxy shots), then shorten it. It collects the same amount of light either way!

it is the same amount of light, but you can get shorter exposures, meaning more subs, that gives a better image. right? it's F10 vs F7, right?

and another thing, without the Reducer, I can't fit, let say M101

and one more thing, guiding get's easier (2032mm vs 1422mm)

 

I think the Reducer makes a lot of things better. right?

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1 hour ago, Amajed said:

the Reducer makes a lot of things better

Hi. I'm basing my comments on our experience with a c8 telescope from the same maufacturer. Although our Jupiter images from video were quite good, we never got decent deep sky images. 

I think your verson is superior -better mirrors, focusing etc- so you should be able to go deep sky if you can get the OAG working.

Good luck and clear skies.

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1 hour ago, Amajed said:

I think so, but that will require me to use a nose piece, which I don't want to, I like to screw things together for a tight and snug setup.

But wait, I think I got it all wrong about the Celestron OAG, I think I do not require the T Adapter if I'm using it. it does have a "SCT Adapter" isn't that the T Adapter's job? so I don't need the T Adapter anymore, it's like this:

Reducer > SCT Adapter 25.3mm (came with OAG) > OAG 29mm > M48 Camera Adapter 4.5mm > T2 Adapter for Filter wheel 2mm > Filter Wheel 20mm > CCD 6.5mm

 

I'll need spacers about 18.7mm (I need to figure out how to reach this number :D) other than that, it should work!

The Celestron OAG comes with various spacers (3 I think) plus your camera spacers...

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1 hour ago, fwm891 said:

The Celestron OAG comes with various spacers (3 I think) plus your camera spacers...

True, but 18.7mm gonna be tricky.

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Amajed, The Celestron comes with 11.55mm and 6mm T thread spacers, FLO do a Delrin T spacer ring set: https://www.firstlightoptics.com/adapters/baader-t2-delrin-spacer-ring-set.html

Covering thickness' of: 0.6mm, 0.8mm, 1.0mm, 1.2mm & 1.4mm. There should be enough thread to slot one or more of the spacers and get very close to your 18.7 

11.55 + 6 + (1.15) = 18.7   the 1.0 or 1.2 mm spacers very close....

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3 hours ago, fwm891 said:

Amajed, The Celestron comes with 11.55mm and 6mm T thread spacers, FLO do a Delrin T spacer ring set: https://www.firstlightoptics.com/adapters/baader-t2-delrin-spacer-ring-set.html

Covering thickness' of: 0.6mm, 0.8mm, 1.0mm, 1.2mm & 1.4mm. There should be enough thread to slot one or more of the spacers and get very close to your 18.7 

11.55 + 6 + (1.15) = 18.7   the 1.0 or 1.2 mm spacers very close....

Now that you've mentioned it, I did purchase these couple months ago, gonna have to search for them.

 


Thanks

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