Good and bad results from last night

[RobertI]

Hi all,

Tried out the C8 with F3.3 FR  for the first time last night. See screen capture at end.

Overall I'm really please with the results as I have never seen M101 before and I'm really pleased with the level of detail I managed to capture and study. It was quite an experience seeing the image appear and then teasing out the detail using LLv10, which behaved perfectly.

This was a stack of 30 second exposures (can't remember - is this logged somewhere?) with 4 x 30 second darks from memory.

The good news is I appear to have successfully removed dirt blobs from the sensor - I used Stanwax Laser Polymer cleaner.

The bad news is as you can see I have significant distortion away from the centre of the field. I ensured that the lens to sensor distance was approximately 57mm although on the night this was probably + or - a few mm. I tried to rectify this by moving the lodestar inwards/outwards and refocussing each time but the problem seemed to remain. How critical is the lens to chip distance for removing distortion? I would have thought if it was critical then the situation would get worse the further away I got from the sweet spot, but it seemed to remain contant regardles of where the camera was. Is there anyway I can obtain the exact position during the day?

The other is focussing was a bit of a pain. Every time I moved to a new object I had to refocus - I am assuming this is mirror shift and is more prevalent at low focal ratios? Focussing is a pain with the C8 anyway and I always have to focus uphill (counterclockwise) to get best focus and the focussing is quite coarse.

However I think I will get used to the focussing issues, it's the distortion that annoys. Any thoughts on how to eliminate this would be appreciated.

Final note, there seems to be some trailing of stars, not unexpected, but should LLv10 have removed these? It was a bit windy on the night so the scope was probably all over the place.

Thanks

Rob

[Martin Meredith]

Hi Rob

Great detail in the arms of what is visually a real pain of an object! I can't offer any advice on the distortion (except to say that it doesn't detract from M101, although one wonders if even better results could be obtained if distortion is fixed). I sympathise with focussing issues. I have a single-speed R & P which is the opposite of smooth...

I wouldn't expect LL to be able to remove star trailing or other distortions e.g. coma if each sub has them.

Martin

[nytecam]

Hi - I've used f/3.3 and f/6.3 focal reducers for quite a while on my 30cm Meade LX200 SCT.  Mirror slop is a problem usually resolved by rotating the focus knob anti-clockwise until the knob feels slack then focus only in the clockwise direction in slow steps viewing results on the screen as resistance is felt.  Too big a step and you may overshoot good focus and need to start over again!   The actual focal ratio isn't as printed on the FR itself but may be a faster or more commonly slower f/ratio depending on precise separation of the FR from sensor - I just use trial and error. 

The f/3.3 FR is very sensitive to focusing errors and coma at the edge and will exaggerate any collimation issues of the secondary mirror.  Coma can be reduced to a minimum with careful focusing - see Don's excellent work here via Lodestar+f/3.3 FR.  Good luck   

[HiloDon]

Hi Rob,

First of all, excellent result! I think the issues you are concerned about are minor and can be easily resolved. I blew up your image and it does appear that you have both trailing error and coma that is near the edge. The trailing error is probably caused from poor polar alignment. How are you presently doing that? If you have a polar aligning scope, it's very simple to get an accurate polar alignment by finging the Polaris clock hour and then using the setting circle of RA to accurately position the Polaris circle in the scope. That's the way I do it now and it's good enough to get 60 sec without noticeable trail. You can check your drift error by using an eyepiece with a reticle or using the crosshairs in Lodestar Live. Just center a star and watch it for a minute to see how much it drifts. If you see it, it's too much. Pick a star near the celestial equater. There's a process called drift alignment that can get real accurate alignment, but I don't think that's necessary for the exposure times we have. As Martin indicated, you're probably getting the drift error in a 30 sec sub, so LL won't remove it. You could try shorter subs and get more of them. I tried stacking 10 sec subs and that seemed to work well.

The coma may be a more difficult issue. I think Nytecam had good advice there. The separation distance is more important in getting the proper focal ratio. More distance will reduce it to a point that you will get vignetting and less distance will increase it a give you a narrower FOV. The first thing to try is to do an accurate collimation of the scope. If you don't have Bob's knobs yet, get some. It's well worth the money. Also, use the collimation process that I gave in my thread the other day. If you need the link again, let me know. As pointed out in that article, you really need perfect collimation especially with the focal reducer in the train. It has to be done with high magnification and small defocusing. To center the star use LL again with the crosshairs. I think you can use the lodestar to do the whole process, but I haven't tried that yet. Maybe some more experience members can advise on that.

Thanks to Nytecam for his comments and advice on focusing. I learned sometime new, too.

Just let me know if you need more detailed information. I can post some good links on these things.

Don

[RobertI]

Thanks for the comments and advice guys. I will persevere and hopefully resolve the problems with practice.

[RobertI]

Don, thanks for the detailed response, really helpful. Re polar alignment, I normally set the mount to the hour angle of Polaris and use the polar scope to align - it usually works fine, but yesterday because of the wind I had the tripod at its lowest setting to shelter it behind the car and ended up on my knees to look through the polar scope. Obviously this did not work and is not something I shall be repeating!

Re collimating it sounds like I really need to look at this - I did start to read the article you provided me the link to (thanks again) but need to study this carefully as there is a lot of information. I don't have bob's knobs and will definitely be ordering some this week and trying to get it spot on. It looks like focussing and collimation are key. Thanks for the advice.

One thing I'm not totally clear on - does the FR lens to sensor distance also create coma or does it purely affect the FR (in which case I won't worry about it too much)?

It's good to know excellent results are possible with this set up judging by what you have posted.

Cheers

Rob

[HiloDon]

Rob,

Funny you should mention the issue of coma with different spacing. I was just researching that. I found one blog that said it did make a difference. I also found the Meade instructions that come with the 3.3 FR. The original FR came with two spacers that gave you two different focal reductions. You would think, therefore, that the spacing only changed the focal reduction and didn't introduce coma. If you don't have the instructions, I can give you that link. They don't specify any separation distances. The spacers are 15 and 30mm. The train includes a T adapter ring and they say that it's for the Meade camera. Other cameras may yield different focal ratios.

I think I'll run some tests on this and post the results on SGL in a new thread. It might take a while.

Don

[RobertI]

Thanks Don, I would appreciate the link if you have a moment. I bought the FR second hand so no instructions (perhaps the previous owner ran out of patience!).

Rob

[HiloDon]

Here's the link, Rob. The pdf's are in the sixth message down. I bought mine used, too, and didn't get the extension tubes either. I worked out some separation dimensions for both the 3.3 and 6.3 and will do some testing this afternoon. My 6.3 is an Antares, so the distance will be different for a Celestron or Meade because they have different focal lengths. Don't know how that affects quality.

http://www.iceinspace.com.au/forum/showthread.php?t=80743

[RobertI]

Cheers Don, look forward to the results of your testing.

Rob