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Imaging with Tal200k


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I've got a Tal200k that I would like to use for imaging.

I think that I need to change the focuser.  I'm really struggling to find a focuser that will fit on the back of the Tal.

 

I used a Borg helical focuser in the past, but I don't think that it is strong enough for the weight of the camera etc.

Has anybody else adapted a Tal for photography, and what did you use?

The camera is an SBIG6303.  I can easily put a 2" nosepiece on it, or I can get to M56 or M54 threads.

The main problem is that most focusers have a drawtube that extends into the OTA when retracted.  The hole in the back of the Tal is only about 40mm diameter.

Any suggestions for a suitable field flattener, or focal reducer would be very much appreciated.

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Thank you both for your answers.

An SCT adapter plate would be excellent.  I'll have a Google, although I am not hopeful.

I do have  a plate with a female M57 (Borg/Hutech) thread.  However, the only focuser that I can connect to this is a helical one that isn't quite able to take the weight of the camera (and  I've managed to break it).

I do have a 2" adapter that I can screw in, so if I could find a focuser that fitted into an existing 2" eyepiece holder, then I would be able to get going.

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I image with my Tal 200K and have recently changed to a Baader diamond steeltrack SCT focuser and this adaptor.   

Be very wary that the back focus is limited. I am using the SX mini filter wheel, incorporating the off axis guider and an Atik 460, and it is very tight. I've had to fettle a little to get it all to focus, with only 4mm of focus movement left to play with.

steve

Edited by sloz1664
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Hi Steve, Thanks for the info but I have got rid my Tal, but will pass the info onto a close friend who has a 200K and has modified it but does not do a lot of imaging.

Regards

George

 

 

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5 hours ago, sloz1664 said:

I image with my Tal 200K and have recently changed to a Baader diamond steeltrack SCT focuser and this adaptor.   

Be very wary that the back focus is limited. I am using the SX mini filter wheel, incorporating the off axis guider and an Atik 460, and it is very tight. I've had to fettle a little to get it all to focus, with only 4mm of focus movement left to play with.

steve

Have you measured the backfocus? 

I'm wondering if the F10 and F8.5 versions have the same backfocus.  Mine is F10, and I have measured it (roughly) at 155mm from the rear plate.

Another question ... do the drawtubes in SCT focusers retract back into the OTA? 

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16 hours ago, johninderby said:

No the drawtube travel on an SCT focuer is short, about 30mm and diesn’t go inside the OTA.

Here’s a dawing of the Badder Steeltrack.

 

Thank you for that.  I hadn't realised that SCT focusers did not protrude into the OTA! 

When I posted my question, I wasn't very optimistic about getting anywhere.  Now, it seems that I have some choices in front of me,

The forecast says that it is going to be clear tonight, so I will see if I can connect up my old wobbly image train and get some accurate dimensions for backfocus.

 

Ideally, I would like to get a field flattener into the system, and if I could reduce the F/L to about 1500mm, then I would be very happy indeed!

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I had to remove the 2" clamp from the focuser and replace it with a T2 threaded plate, a std unit you can purchase from Baader, This helped to reduce the back focus by a further 10mm. Still rather tight though. I persevered with the Tal Crayford for longer than I should have. I was getting inconsistent results from the auto focuser, I tried in vain to buy a Crawmach focuser but they didn't seem interested as they never responded to my emails.

Edited by sloz1664
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14 hours ago, johninderby said:

Crawmach have only just stared to produce some astro bits again.Tube rings. No one seems to know if they will ever make any more focusers. 

I was thinking of ordering one about this time last year.  Unfortunately, Crawmach all seems to have gone very quiet.

 

Edited by don4l
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An update...

 

I needed to get a proper handle on the backfocus, so I had a go at reassembling my old helical focuser.  I'm lucky that I did this, because I have about 16mm less than I thought (139mm Vs 155)!

Everything is now connected up, and working much better than I expected.

I've been practising on M82 with subs up to 10 minutes long.  The attached image was collected over the last two nights.  Unfortunately, I have to be up in the morning for work, so I have only got limited data so far.  This image has no Red data, only Ha, G and B.  It consists of 60m Ha, 35m Green and 25m Blue.  The resulting colour is not great, but I'll add some red and maybe Lum when I get a chance.

To say that I am chuffed would be an understatement!

 

I can now take my time to decide what I need to do.  I'd like to get a focuser that could have an autofocus,  but I woud also like to fit a field flattener.  I haven't seen any focuser that leaves enough space for anything else!

 

 

colorm8260mHa.jpg

Edited by don4l
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I thought that I should update this in case there are any Tal200 owners wanting.

I've had another couple of clear sessions over the last week.  Things are going far, far better than I expected.

My "wobbly" focuser does not seem to be a major problem at this stage.  I guess that these things are not as critical at F10 as at F5!

I now think that a field flattener would be a much more useful use of the limited backfocus.  As far as I can tell, I have an 18mm circle that is fairly flat.  Beyond this, things quickly deteriorate.  My camera chip is  27 x 18mm, so there is quite a bit of data being cropped off.

I've put some images below.

I have added some Red and L (60m)  to M82.  I hope to get another couple of hours of Ha on this.  It is already my best ever Galaxy image.

I also took an hour of L on M51 and added it to an image that I took at 850mm FL.  This has improved the image no end.  It is also the first time that I have mixed data from different scopes.

I had a look at supernova sn2019ein in NGC 5353.  The attached image is 3 x 300s.  at m14.2 it showed up clearly in a single shot.  In fact, it would have been obvious in 60s subs.

I've also had a go at imaging the "jet" in M87, which can be seen clearly in the last image.

M82color scaled v2.jpg

M51testsgl.jpg

sn2019eln.jpg

Sum M87Tal scaled v1.jpg

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On 22/05/2019 at 17:32, don4l said:

An update...

 

I needed to get a proper handle on the backfocus, so I had a go at reassembling my old helical focuser.  I'm lucky that I did this, because I have about 16mm less than I thought (139mm Vs 155)!

Everything is now connected up, and working much better than I expected.

I've been practising on M82 with subs up to 10 minutes long.  The attached image was collected over the last two nights.  Unfortunately, I have to be up in the morning for work, so I have only got limited data so far.  This image has no Red data, only Ha, G and B.  It consists of 60m Ha, 35m Green and 25m Blue.  The resulting colour is not great, but I'll add some red and maybe Lum when I get a chance.

To say that I am chuffed would be an understatement!

 

I can now take my time to decide what I need to do.  I'd like to get a focuser that could have an autofocus,  but I woud also like to fit a field flattener.  I haven't seen any focuser that leaves enough space for anything else!

 

 

colorm8260mHa.jpg

Due to the limited backfocus using a field flattener is not going to be an option I'm afraid. I tried, but got no where, fitted different flatteners and reducers to no avail, I could not achieve focus. 

Steve

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3 minutes ago, sloz1664 said:

Due to the limited backfocus using a field flattener is not going to be an option I'm afraid. I tried, but got no where, fitted different flatteners and reducers to no avail, I could not achieve focus. 

Steve

I thought that might be the case. 

Can you remember if you managed to get close to focus with any of the flatteners?

I've got at least 10mm spare at the moment.    I'm fairly sure that I could conjure up another 10mm by getting a freind to use his lathe on the M57 to 2" adapter.  I could get even more by using a shorter nosepiece on the camera.

I will take some photos this evening which will make things clear.

 

(I'm no longer worried about reducing the focal length.  I seem to be getting along fine with 2000mm.)

 

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38 minutes ago, johninderby said:

Looking good, at least to my untrained eye. 👍🏻

Thank you.

To say that I am pleasantly surprised would be an understatement!

I'm no expert, but these images are way beyond anything that I was expecting.  I've got all sorts of plans for it.

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I've taken some photos, and noted the measurements.  I'm posting this in case anybody finds it useful.  I'v also discovered that I have at least 17mm spare backfocus, and I can get another 25mm at least!!

The first photo shows all the bits labelled with their dimensions.  The 7519 filter holder is just used as a spacer, so I can lose it if I want to add something else.

I can also get a shorter 2" adapter, which could save at least 20mm.  I also have another nosepiece for the camera which uses about 5mm less.

The second photo shows all the bits screwed together ready for the camera.  This setup reaches focus with a few mm either side.

I'm going to investigate field flatteners to see if something exists which will fit.  My big question concerns field curvature.  I really don't know anything about this (apart from the elongated stars). 

 

AllExploded-1.jpg

AllBits+Camera.jpg

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SCT field flatteners usually require around 110mm back focus, but here's the rub, your focal length will not be altered. This means the 110mm back focus has to be taken from your imaging train. Unfortunately Tal owners do not have that luxury.

Steve

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On 03/06/2019 at 10:15, sloz1664 said:

SCT field flatteners usually require around 110mm back focus, but here's the rub, your focal length will not be altered. This means the 110mm back focus has to be taken from your imaging train. Unfortunately Tal owners do not have that luxury.

Steve

That's a pity.  I was hoping to find something that took up less back focus.

 

To be honest, I am very happy with the results that I am getting at the moment.  They are  much better than I was expecting, and anything that I have managed before. 

I also have a much clearer view of the options.  The helical focuser appears to be working far better than I expected.  The Tal seems much easier to focus than my F5 refractor.  I'm quite shocked by the difference.

 

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I'm glad you are happy with the setup you now have with your Tal 200K. I spent a long time trying to find a focal reducer / flattener for the 200K and was always stumped by the lack of back focus. I now use the Atik 460exm and find this eliminates elongated stars. Here is a Ha image I took of Barnard 33 using the my Tal 200k & Atik 460 combo:-

Barnard_33_Ha.thumb.png.477683c3212a37d1070a4697770b078d.png

Steve

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