Explore Scientific HR Coma Corrector, focal length changes, 150PDS

[Greg Shaw]

Hello Everyone

After lots for reading through this forum and help from Adam J and Daemon I ended up going for the Explore Scientific HR Coma Corrector over the Baader and SkyWatcher versions so that I would have enough back focus etc.

I was finaly able to get my hands on the Explore Scientific HR Coma Corrector last week and was able to put it to test. I also swapped out the standard focusing tube adaptor with the Baader 2" ClickLock M54 Clamp. I was still able to achieve focus with plenty of travel either side of the focus point.

When I was plate solving in ATP. I did notice that the calculated focal length had changed from 750mm to 800mm. I wasn't too sure why this happened because I didn't think the focal length changed with this coma corrector. 

I have the coma corrector fully inserted into the Baader click lock and focusing tube.

I took a quick 3 minute sub of the PacMan nebula which I has also captured a week ago without the coma corrector so that I could compare and I am definitely closer to the nebula with the coma corrector attached.

This images are roughly processed, no flats, darks, bias. I just wanted to process enough to see the nebula and compare

Without Corrector vs With Corrector (also with more vinetting)

The corrector has done a great job of fixing the coma though

Without Corrector vs With Corrector

  

I did notice that if I zoom in some of the stars are not perfect with the corrector. Is this likely due to the focusing tube sag or do I need to play with how much I insert the coma corrector into the focusing tube?

 

Hope you can help

Thanks Greg

 

 

 

Edited September 20, 2020 by Greg Shaw

[alacant]

Hi

Yes, the HR cc will increase the fl of the telescope so you'll be 'closer'. They either gloss over that information or omit it. 

There's a vague triangular pattern to the stars so assuming that you've nailed the collimation, try slackening the 6 screws holding the primary mirror to its cell.

In the end however, the sw ds focuser is the weak link and no amount of tweaking will cope with a dslr and heavy cc combination. Loadsa ways forward though. I suppose it's a case of what is acceptable for you.

Cheers and HTH

[Greg Shaw]

Hi Alacant

Thanks for taking a look and replying. I will have to update my focal lengths in ATP. I did wonder why it was taking longer to plate solve.

When you say slacken the 6 screws holding the primary mirror do you mean the 3 large thumb screws and the 3 small locking screws?

It was mentioned to me that I might need to look at a focuser upgrade the future. Are there any I should consider?

I was planning to get a OSC cooled CCD next and move away from dSLR

Thanks again

[Second Time Around]

Just to quantify, the ES coma corrector will increase the focal length by 6%.  So your focal length will increase from 750mm to 795mm.  So your figure of 800mm is almost exactly right.

[alacant]

9 minutes ago, Greg Shaw said:

he 3 large thumb screws and the 3 small locking screws?

No. Take out the mirror. There are six screws holding the clips which hold the mirror to the cell. In fact they would be better described as screws which prevent the mirror falling out of the cell instead. They should not bear upon the mirror under load from the screws. I believe current wisdom has moved from a cigarette paper gap, via credit card to a sheet of normal printing paper.

For solutions as to how to fix the focuser, have a search here.

HTH

[Greg Shaw]

Hi Alacant

Whilst reading up on this I saw you also had similar issues with your SW focuser and ended up replacing it. Are you still using that focuser? It doesn't seem to be on sale any longer within Alliexpress. 

From reading the 130pds thread I can confirm that if I get collimation correct with the focus tube fully extended it does drift out slightly as I back focus into the scope, so it looks like I have some focus tube tilt to sort out.

Also I saw on a different thread a technique where if you defocus a star and then move the star around to find the best location in the FOV where the star collimation looks correct, then adjust the collimation bolts to get the star back into the middle of the FOV it will then correctly align the collimation.

This is what my star collimation looked like in the middle of the FOV

Just to see what would happen I moved the star around the FOV using EQMOD and I was quite surprised to see that the collimation was good in the middle but very bad when in the edges

 

 

Any idea why this is happening? I am collimating using a Cheshire and I was using my 600d with the  ES HR Coma Corrector installed

 

[alacant]

That looks fine. The cc is 'crushing' the star correctly. 

Try as one may, I don't think it's possible to remove tilt from a SW focuser. Not sure if it's worth getting a proper focuser; correct in software instead.

How are you doing for d-shaped stars?

Cheers

 

[Greg Shaw]

Ah so its the CC thats causing the star collimation to look off at the edges?

I will post a picture I took the other night, I'm still currently processing it. One Thing I tried with this image was I locked the focuser with the large thumb screw. Up until now I haven't locked it.

Happy to look at a better focuser because I was to eventually move to a dedicated OSC and possible a mono and filter wheel set.

With the Hercules  focuser did you have to drill any new holes?

Also if I change the focuser I guess I won't need the Baader click lock that I currently have?

Thanks

Edited September 23, 2020 by Greg Shaw

[Greg Shaw]

Hi Alacant

 

I have been looking at focusers and was thinking of getting the Moonlite Cr2. I have heard that the Feathertouch  is the best but I am not sure which Feathertouch fits the 150PDS

 

 

Edited September 24, 2020 by Greg Shaw

[Greg Shaw]

Hi Alacant

Here is the image which I took the other night where I have locked the focuser with the large thumb screw. Up until now I haven't locked it. The stars look better but there is a slight deformation.

I have bit the bullet and orders a moonlight CR2 because I plan to go to a dedicate astro camera and possibly a mono with filter set.

[alacant]

Yeah, looks good, but no worries. You'll soon be able to take the focuser for granted. Not many PDS users can claim that!

If you're gonna get serious and be able to take the rest of the telescope for granted, there are a few other things you may want to consider.

To avoid having the camera so far away from the tube, you maybe better off with the GPU cc and add reinforcement -0.7mm aluminium sheet is good- around the tube where the focuser hangs. A longer, wide dovetail with the tube rings further apart will help with tube stability as will a rail along the top to tie the rings. The primary mirror springs need to be replaced with stronger versions with passive springs around the locking screws. To eliminate lateral movement also a good idea to 'glue' the primary mirror to the cell using silicone sealer.

It was @laser_jock99 who put me onto all this. I should take no credit. But I do have stable Newtonian reflectors which hold collimation and just work. If I've missed anything, I'm sure he'll check in;)

Cheers

Edited September 24, 2020 by alacant