SCT backfocus/focal length

[Ken82]

For a few evenings now I’ve been trying to get the optimal spacing for my edge hd + focal reducer. I was initially adding extra spacers to the optical train but recently I’ve been removing these. (The Baader delrings are a pain in the Bottom so it’s been slow progress). 
 

With the f7 reducer I should be at 1960mm but plate solve says I’m at 2000mm. Would it be correct to say the optical train is too long then and by removing spacers I should achieve the correct spacing ? My images appear to have quite bad astigmatism at the moment but centre of image is good. 
 

I know the edge hd white paper says an optimal spacing of 146mm but To achieve the f7 1960mm it appears to me I’ll have to be much closer than 146mm ?? 
 

ive also read that by moving the sensor closer I will trade astigmatism for field curvature. Is this true ? At the moment I’d just be pleased to get rid of the astigmatism! 
 

thanks for any help ! 
ken 

image shows centre of field is good but astigmatism is very clear. 

 

Edited October 20, 2019 by Ken82

[Davey-T]

Aren't Edge scopes supposed to be free of field curvature ?

SCT Focal length alters continuously when focusing.

Can you accommodate something like a Baader Variloc ? makes life easier.

Dave

[knobby]

Show us a pic of set up , I think the 146mm is without reducer ... Should he 105 mm with the celestron dedicated one.

Edit : just realised you have the 11" which as as you say correctly 146 mm 👍

Edited October 20, 2019 by knobby
Error

[Ken82]

27 minutes ago, Davey-T said:

Aren't Edge scopes supposed to be free of field curvature ?

SCT Focal length alters continuously when focusing.

Can you accommodate something like a Baader Variloc ? makes life easier.

Dave

I think free from field curvature to a certain extent depending on sensor size etc etc. The comment I made about reducing astigmatism and increasing field curvature was made from someone who also posted some technical drawings on cloudy nights. 
 

A Baader variloc wouldn’t work at the moment unless I completely re think the spacing and remove the edge hd t adapter. 

[Davey-T]

27 minutes ago, Ken82 said:

A Baader variloc wouldn’t work at the moment unless I completely re think the spacing and remove the edge hd t adapter. 

Might make life easier, I think you can get shorter SCT adapters if it's a standard thread.

Dave

[Kev M]

I have a 7mm SCT to T adapter on my SCT gets rid of the problem , you can use readily available T extension/packer rings to get the ideal length.

I think it was the Baader one....others exists.

[michael8554]

The spec of the Edge Reducer includes "Maintains the same back focus as when used at f/10". 

That suggests to me you should space your Reducer to place the imaging chip 146mm from the rear port, and then adjust the Edge focus. 

If that works I wouldn't worry too much if it isn't F7   🙂

Michael 

[Ken82]

16 hours ago, michael8554 said:

The spec of the Edge Reducer includes "Maintains the same back focus as when used at f/10". 

That suggests to me you should space your Reducer to place the imaging chip 146mm from the rear port, and then adjust the Edge focus. 

If that works I wouldn't worry too much if it isn't F7   🙂

Michael 

Yea thanks Michael, I have spaced the camera sensor to what I believe to be 146mm but still getting bad astigmatism. I have also tried moving the sensor further away with little success. 
 

My only option now is to move it closer which I hope will resolve the issue. The fact that platesolve tells me I’m at 2000mm focal length gives me some hope. 
 

[michael8554]

Two things strike me. 

Your galaxy image is a stack which may show all sorts of artefacts, in fact looks like Coma to me - can you post a stretched sub instead? 

The image of your setup, should the FR be mounted directly to the OTA, without the black gnurly adapter thingy? 

Third of two things, I believe you have to allow an amount for the diffraction of your filters?

Michael 

[kirkster501]

Focus with the mirror knob is not accurate enough for AP at such long focal length.  You need to be much more accurate and get a proper focuser onto the visual back.  This also adds to the FL.

https://agenaastro.com/articles/upgrading-the-focuser-on-your-sct.html

Until you do, use the "peanut butter jar lid" trick on the mirror focuser.

[Ken82]

6 hours ago, michael8554 said:

Two things strike me. 

Your galaxy image is a stack which may show all sorts of artefacts, in fact looks like Coma to me - can you post a stretched sub instead? 

The image of your setup, should the FR be mounted directly to the OTA, without the black gnurly adapter thingy? 

Third of two things, I believe you have to allow an amount for the diffraction of your filters?

Michael 

Thanks for your feedback Michael.

The official 11" edge celestron f7 focal reducer should be placed at the rear of the scope after removing the visual back.

Its interesting you think it may be coma as im certainly no expert at this but my single sub looks more like astigmatism to me from reading up on it?? 

I have currently allowed 2.5mm for the Idas d2 filter which i think should bring me to about 146mm. 

Ive included a recent image along with the m51 sub for comparison as this was taken with an aps-c sized sensor.  

 

[knobby]

Could it be tilt somewhere ? The last image doesn't have an even mix of star elongation, spacing would cause the far edges to smear away from center.

[michael8554]

1 hour ago, Ken82 said:

The official 11" edge celestron f7 focal reducer should be placed at the rear of the scope after removing the visual back

My bad, I didn't realise the gnarly bit was the Reducer. 

Michael